LinuxLists.cc - [PATCH v4 0/9] File abstractions needed by Rust Binder

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Subject: [PATCH v4 1/9] rust: types: add `NotThreadSafe`

This introduces a new marker type for types that shouldn't be thread
safe. By adding a field of this type to a struct, it becomes non-Send
and non-Sync, which means that it cannot be accessed in any way from
threads other than the one it was created on.

This is useful for APIs that require globals such as `current` to remain
constant while the value exists.

We update two existing users in the Kernel to use this helper:

* `Task::current()` - moving the return type of this value to a
different thread would not be safe as you can no longer be guaranteed
that the `current` pointer remains valid.
* Lock guards. Mutexes and spinlocks should be unlocked on the same
thread as where they were locked, so we enforce this using the Send
trait.

There are also additional users in later patches of this patchset. See
[1] and [2] for the discussion that led to the introducion of this
patch.

Link: https://lore.kernel.org/all/nFDPJFnzE9Q5cqY7FwSMByRH2OAn_BpI4H53NQfWIlN6I2qfmAqnkp2wRqn0XjMO65OyZY4h6P4K2nAGKJpAOSzksYXaiAK_FoH_8QbgBI4=@proton.me/ [1]
Link: https://lore.kernel.org/all/nFDPJFnzE9Q5cqY7FwSMByRH2OAn_BpI4H53NQfWIlN6I2qfmAqnkp2wRqn0XjMO65OyZY4h6P4K2nAGKJpAOSzksYXaiAK_FoH_8QbgBI4=@proton.me/ [2]
Suggested-by: Benno Lossin <[email protected]>
Reviewed-by: Benno Lossin <[email protected]>
Signed-off-by: Alice Ryhl <[email protected]>
---
rust/kernel/sync/lock.rs | 15 +++++++++++----
rust/kernel/task.rs | 10 ++++++----
rust/kernel/types.rs | 17 +++++++++++++++++
3 files changed, 34 insertions(+), 8 deletions(-)

diff --git a/rust/kernel/sync/lock.rs b/rust/kernel/sync/lock.rs
index 149a5259d431..d08d9d32915a 100644
--- a/rust/kernel/sync/lock.rs
+++ b/rust/kernel/sync/lock.rs
@@ -6,8 +6,15 @@
//! spinlocks, raw spinlocks) to be provided with minimal effort.

use super::LockClassKey;
-use crate::{bindings, init::PinInit, pin_init, str::CStr, types::Opaque, types::ScopeGuard};
-use core::{cell::UnsafeCell, marker::PhantomData, marker::PhantomPinned};
+use crate::{
+ bindings,
+ init::PinInit,
+ pin_init,
+ str::CStr,
+ types::ScopeGuard,
+ types::{NotThreadSafe, Opaque},
+};
+use core::{cell::UnsafeCell, marker::PhantomPinned};
use macros::pin_data;

pub mod mutex;
@@ -132,7 +139,7 @@ pub fn lock(&self) -> Guard<'_, T, B> {
pub struct Guard<'a, T: ?Sized, B: Backend> {
pub(crate) lock: &'a Lock<T, B>,
pub(crate) state: B::GuardState,
- _not_send: PhantomData<*mut ()>,
+ _not_send: NotThreadSafe,
}

// SAFETY: `Guard` is sync when the data protected by the lock is also sync.
@@ -184,7 +191,7 @@ pub(crate) unsafe fn new(lock: &'a Lock<T, B>, state: B::GuardState) -> Self {
Self {
lock,
state,
- _not_send: PhantomData,
+ _not_send: NotThreadSafe,
}
}
}
diff --git a/rust/kernel/task.rs b/rust/kernel/task.rs
index a3a4007db682..148a4f4eb7a8 100644
--- a/rust/kernel/task.rs
+++ b/rust/kernel/task.rs
@@ -4,10 +4,12 @@
//!
//! C header: [`include/linux/sched.h`](srctree/include/linux/sched.h).

-use crate::{bindings, types::Opaque};
+use crate::{
+ bindings,
+ types::{NotThreadSafe, Opaque},
+};
use core::{
ffi::{c_int, c_long, c_uint},
- marker::PhantomData,
ops::Deref,
ptr,
};
@@ -106,7 +108,7 @@ impl Task {
pub unsafe fn current() -> impl Deref<Target = Task> {
struct TaskRef<'a> {
task: &'a Task,
- _not_send: PhantomData<*mut ()>,
+ _not_send: NotThreadSafe,
}

impl Deref for TaskRef<'_> {
@@ -125,7 +127,7 @@ fn deref(&self) -> &Self::Target {
// that `TaskRef` is not `Send`, we know it cannot be transferred to another thread
// (where it could potentially outlive the caller).
task: unsafe { &*ptr.cast() },
- _not_send: PhantomData,
+ _not_send: NotThreadSafe,
}
}

diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs
index fdb778e65d79..5841f7512971 100644
--- a/rust/kernel/types.rs
+++ b/rust/kernel/types.rs
@@ -387,3 +387,20 @@ pub enum Either<L, R> {
/// Constructs an instance of [`Either`] containing a value of type `R`.
Right(R),
}
+
+/// Zero-sized type to mark types not [`Send`].
+///
+/// Add this type as a field to your struct if your type should not be sent to a different task.
+/// Since [`Send`] is an auto trait, adding a single field that is `!Send` will ensure that the
+/// whole type is `!Send`.
+///
+/// If a type is `!Send` it is impossible to give control over an instance of the type to another
+/// task. This is useful when a type stores task-local information for example file descriptors.
+pub type NotThreadSafe = PhantomData<*mut ()>;
+
+/// Used to construct instances of type [`NotThreadSafe`] similar to how we construct
+/// `PhantomData`.
+///
+/// [`NotThreadSafe`]: type@NotThreadSafe
+#[allow(non_upper_case_globals)]
+pub const NotThreadSafe: NotThreadSafe = PhantomData;
--
2.43.0.594.gd9cf4e227d-goog

2024-02-02 10:56:40

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Subject: [PATCH v4 2/9] rust: task: add `Task::current_raw`

Introduces a safe function for getting a raw pointer to the current
task.

When writing bindings that need to access the current task, it is often
more convenient to call a method that directly returns a raw pointer
than to use the existing `Task::current` method. However, the only way
to do that is `bindings::get_current()` which is unsafe since it calls
into C. By introducing `Task::current_raw()`, it becomes possible to
obtain a pointer to the current task without using unsafe.

Link: https://lore.kernel.org/all/CAH5fLgjT48X-zYtidv31mox3C4_Ogoo_2cBOCmX0Ang3tAgGHA@mail.gmail.com/
Reviewed-by: Benno Lossin <[email protected]>
Signed-off-by: Alice Ryhl <[email protected]>
---
rust/kernel/task.rs | 15 +++++++++++----
1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/rust/kernel/task.rs b/rust/kernel/task.rs
index 148a4f4eb7a8..b579367fb923 100644
--- a/rust/kernel/task.rs
+++ b/rust/kernel/task.rs
@@ -97,6 +97,15 @@ unsafe impl Sync for Task {}
type Pid = bindings::pid_t;

impl Task {
+ /// Returns a raw pointer to the current task.
+ ///
+ /// It is up to the user to use the pointer correctly.
+ #[inline]
+ pub fn current_raw() -> *mut bindings::task_struct {
+ // SAFETY: Getting the current pointer is always safe.
+ unsafe { bindings::get_current() }
+ }
+
/// Returns a task reference for the currently executing task/thread.
///
/// The recommended way to get the current task/thread is to use the
@@ -119,14 +128,12 @@ pub unsafe fn current() -> impl Deref<Target = Task> {
}
}

- // SAFETY: Just an FFI call with no additional safety requirements.
- let ptr = unsafe { bindings::get_current() };
-
+ let current = Task::current_raw();
TaskRef {
// SAFETY: If the current thread is still running, the current task is valid. Given
// that `TaskRef` is not `Send`, we know it cannot be transferred to another thread
// (where it could potentially outlive the caller).
- task: unsafe { &*ptr.cast() },
+ task: unsafe { &*current.cast() },
_not_send: NotThreadSafe,
}
}
--
2.43.0.594.gd9cf4e227d-goog

2024-02-02 10:57:28

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Subject: [PATCH v4 5/9] rust: security: add abstraction for secctx

Adds an abstraction for viewing the string representation of a security
context.

This is needed by Rust Binder because it has feature where a process can
view the string representation of the security context for incoming
transactions. The process can use that to authenticate incoming
transactions, and since the feature is provided by the kernel, the
process can trust that the security context is legitimate.

Reviewed-by: Benno Lossin <[email protected]>
Signed-off-by: Alice Ryhl <[email protected]>
---
rust/bindings/bindings_helper.h | 1 +
rust/helpers.c | 21 ++++++++++
rust/kernel/cred.rs | 8 ++++
rust/kernel/lib.rs | 1 +
rust/kernel/security.rs | 72 +++++++++++++++++++++++++++++++++
5 files changed, 103 insertions(+)
create mode 100644 rust/kernel/security.rs

diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h
index 84a56e8b6b67..5ca497d786f0 100644
--- a/rust/bindings/bindings_helper.h
+++ b/rust/bindings/bindings_helper.h
@@ -15,6 +15,7 @@
#include <linux/jiffies.h>
#include <linux/mdio.h>
#include <linux/phy.h>
+#include <linux/security.h>
#include <linux/slab.h>
#include <linux/refcount.h>
#include <linux/wait.h>
diff --git a/rust/helpers.c b/rust/helpers.c
index 10ed69f76424..fd633d9db79a 100644
--- a/rust/helpers.c
+++ b/rust/helpers.c
@@ -30,6 +30,7 @@
#include <linux/mutex.h>
#include <linux/refcount.h>
#include <linux/sched/signal.h>
+#include <linux/security.h>
#include <linux/spinlock.h>
#include <linux/wait.h>
#include <linux/workqueue.h>
@@ -177,6 +178,26 @@ void rust_helper_put_cred(const struct cred *cred)
}
EXPORT_SYMBOL_GPL(rust_helper_put_cred);

+#ifndef CONFIG_SECURITY
+void rust_helper_security_cred_getsecid(const struct cred *c, u32 *secid)
+{
+ security_cred_getsecid(c, secid);
+}
+EXPORT_SYMBOL_GPL(rust_helper_security_cred_getsecid);
+
+int rust_helper_security_secid_to_secctx(u32 secid, char **secdata, u32 *seclen)
+{
+ return security_secid_to_secctx(secid, secdata, seclen);
+}
+EXPORT_SYMBOL_GPL(rust_helper_security_secid_to_secctx);
+
+void rust_helper_security_release_secctx(char *secdata, u32 seclen)
+{
+ security_release_secctx(secdata, seclen);
+}
+EXPORT_SYMBOL_GPL(rust_helper_security_release_secctx);
+#endif
+
/*
* `bindgen` binds the C `size_t` type as the Rust `usize` type, so we can
* use it in contexts where Rust expects a `usize` like slice (array) indices.
diff --git a/rust/kernel/cred.rs b/rust/kernel/cred.rs
index fabc50e48c9e..0640356a8c29 100644
--- a/rust/kernel/cred.rs
+++ b/rust/kernel/cred.rs
@@ -48,6 +48,14 @@ pub unsafe fn from_ptr<'a>(ptr: *const bindings::cred) -> &'a Credential {
unsafe { &*ptr.cast() }
}

+ /// Get the id for this security context.
+ pub fn get_secid(&self) -> u32 {
+ let mut secid = 0;
+ // SAFETY: The invariants of this type ensures that the pointer is valid.
+ unsafe { bindings::security_cred_getsecid(self.0.get(), &mut secid) };
+ secid
+ }
+
/// Returns the effective UID of the given credential.
pub fn euid(&self) -> bindings::kuid_t {
// SAFETY: By the type invariant, we know that `self.0` is valid. Furthermore, the `euid`
diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index f65e5978f807..d55d065642f0 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -44,6 +44,7 @@
pub mod net;
pub mod prelude;
pub mod print;
+pub mod security;
mod static_assert;
#[doc(hidden)]
pub mod std_vendor;
diff --git a/rust/kernel/security.rs b/rust/kernel/security.rs
new file mode 100644
index 000000000000..ee2ef0385bae
--- /dev/null
+++ b/rust/kernel/security.rs
@@ -0,0 +1,72 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Linux Security Modules (LSM).
+//!
+//! C header: [`include/linux/security.h`](srctree/include/linux/security.h).
+
+use crate::{
+ bindings,
+ error::{to_result, Result},
+};
+
+/// A security context string.
+///
+/// # Invariants
+///
+/// The `secdata` and `seclen` fields correspond to a valid security context as returned by a
+/// successful call to `security_secid_to_secctx`, that has not yet been destroyed by calling
+/// `security_release_secctx`.
+pub struct SecurityCtx {
+ secdata: *mut core::ffi::c_char,
+ seclen: usize,
+}
+
+impl SecurityCtx {
+ /// Get the security context given its id.
+ pub fn from_secid(secid: u32) -> Result<Self> {
+ let mut secdata = core::ptr::null_mut();
+ let mut seclen = 0u32;
+ // SAFETY: Just a C FFI call. The pointers are valid for writes.
+ to_result(unsafe { bindings::security_secid_to_secctx(secid, &mut secdata, &mut seclen) })?;
+
+ // INVARIANT: If the above call did not fail, then we have a valid security context.
+ Ok(Self {
+ secdata,
+ seclen: seclen as usize,
+ })
+ }
+
+ /// Returns whether the security context is empty.
+ pub fn is_empty(&self) -> bool {
+ self.seclen == 0
+ }
+
+ /// Returns the length of this security context.
+ pub fn len(&self) -> usize {
+ self.seclen
+ }
+
+ /// Returns the bytes for this security context.
+ pub fn as_bytes(&self) -> &[u8] {
+ let ptr = self.secdata;
+ if ptr.is_null() {
+ debug_assert_eq!(self.seclen, 0);
+ // We can't pass a null pointer to `slice::from_raw_parts` even if the length is zero.
+ return &[];
+ }
+
+ // SAFETY: The call to `security_secid_to_secctx` guarantees that the pointer is valid for
+ // `seclen` bytes. Furthermore, if the length is zero, then we have ensured that the
+ // pointer is not null.
+ unsafe { core::slice::from_raw_parts(ptr.cast(), self.seclen) }
+ }
+}
+
+impl Drop for SecurityCtx {
+ fn drop(&mut self) {
+ // SAFETY: By the invariant of `Self`, this frees a pointer that came from a successful
+ // call to `security_secid_to_secctx` and has not yet been destroyed by
+ // `security_release_secctx`.
+ unsafe { bindings::security_release_secctx(self.secdata, self.seclen as u32) };
+ }
+}
--
2.43.0.594.gd9cf4e227d-goog

2024-02-02 10:58:10

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Subject: [PATCH v4 7/9] rust: file: add `Kuid` wrapper

Adds a wrapper around `kuid_t` called `Kuid`. This allows us to define
various operations on kuids such as equality and current_euid. It also
lets us provide conversions from kuid into userspace values.

Rust Binder needs these operations because it needs to compare kuids for
equality, and it needs to tell userspace about the pid and uid of
incoming transactions.

To read kuids from a `struct task_struct`, you must currently use
various #defines that perform the appropriate field access under an RCU
read lock. Currently, we do not have a Rust wrapper for rcu_read_lock,
which means that for this patch, there are two ways forward:

1. Inline the methods into Rust code, and use __rcu_read_lock directly
rather than the rcu_read_lock wrapper. This gives up lockdep for
these usages of RCU.

2. Wrap the various #defines in helpers and call the helpers from Rust.

This patch uses the second option. One possible disadvantage of the
second option is the possible introduction of speculation gadgets, but
as discussed in [1], the risk appears to be acceptable.

Of course, once a wrapper for rcu_read_lock is available, it is
preferable to use that over either of the two above approaches.

Link: https://lore.kernel.org/all/202312080947.674CD2DC7@keescook/ [1]
Reviewed-by: Benno Lossin <[email protected]>
Signed-off-by: Alice Ryhl <[email protected]>
---
rust/bindings/bindings_helper.h | 1 +
rust/helpers.c | 45 +++++++++++++++++++++
rust/kernel/cred.rs | 5 ++-
rust/kernel/task.rs | 69 +++++++++++++++++++++++++++++++++
4 files changed, 118 insertions(+), 2 deletions(-)

diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h
index 5ca497d786f0..4194b057ef6b 100644
--- a/rust/bindings/bindings_helper.h
+++ b/rust/bindings/bindings_helper.h
@@ -15,6 +15,7 @@
#include <linux/jiffies.h>
#include <linux/mdio.h>
#include <linux/phy.h>
+#include <linux/pid_namespace.h>
#include <linux/security.h>
#include <linux/slab.h>
#include <linux/refcount.h>
diff --git a/rust/helpers.c b/rust/helpers.c
index fd633d9db79a..58e3a9dff349 100644
--- a/rust/helpers.c
+++ b/rust/helpers.c
@@ -142,6 +142,51 @@ void rust_helper_put_task_struct(struct task_struct *t)
}
EXPORT_SYMBOL_GPL(rust_helper_put_task_struct);

+kuid_t rust_helper_task_uid(struct task_struct *task)
+{
+ return task_uid(task);
+}
+EXPORT_SYMBOL_GPL(rust_helper_task_uid);
+
+kuid_t rust_helper_task_euid(struct task_struct *task)
+{
+ return task_euid(task);
+}
+EXPORT_SYMBOL_GPL(rust_helper_task_euid);
+
+#ifndef CONFIG_USER_NS
+uid_t rust_helper_from_kuid(struct user_namespace *to, kuid_t uid)
+{
+ return from_kuid(to, uid);
+}
+EXPORT_SYMBOL_GPL(rust_helper_from_kuid);
+#endif /* CONFIG_USER_NS */
+
+bool rust_helper_uid_eq(kuid_t left, kuid_t right)
+{
+ return uid_eq(left, right);
+}
+EXPORT_SYMBOL_GPL(rust_helper_uid_eq);
+
+kuid_t rust_helper_current_euid(void)
+{
+ return current_euid();
+}
+EXPORT_SYMBOL_GPL(rust_helper_current_euid);
+
+struct user_namespace *rust_helper_current_user_ns(void)
+{
+ return current_user_ns();
+}
+EXPORT_SYMBOL_GPL(rust_helper_current_user_ns);
+
+pid_t rust_helper_task_tgid_nr_ns(struct task_struct *tsk,
+ struct pid_namespace *ns)
+{
+ return task_tgid_nr_ns(tsk, ns);
+}
+EXPORT_SYMBOL_GPL(rust_helper_task_tgid_nr_ns);
+
struct kunit *rust_helper_kunit_get_current_test(void)
{
return kunit_get_current_test();
diff --git a/rust/kernel/cred.rs b/rust/kernel/cred.rs
index 0640356a8c29..b80276d68247 100644
--- a/rust/kernel/cred.rs
+++ b/rust/kernel/cred.rs
@@ -8,6 +8,7 @@

use crate::{
bindings,
+ task::Kuid,
types::{AlwaysRefCounted, Opaque},
};

@@ -57,11 +58,11 @@ pub fn get_secid(&self) -> u32 {
}

/// Returns the effective UID of the given credential.
- pub fn euid(&self) -> bindings::kuid_t {
+ pub fn euid(&self) -> Kuid {
// SAFETY: By the type invariant, we know that `self.0` is valid. Furthermore, the `euid`
// field of a credential is never changed after initialization, so there is no potential
// for data races.
- unsafe { (*self.0.get()).euid }
+ Kuid::from_raw(unsafe { (*self.0.get()).euid })
}
}

diff --git a/rust/kernel/task.rs b/rust/kernel/task.rs
index b579367fb923..7d59cf69ea8a 100644
--- a/rust/kernel/task.rs
+++ b/rust/kernel/task.rs
@@ -9,6 +9,7 @@
types::{NotThreadSafe, Opaque},
};
use core::{
+ cmp::{Eq, PartialEq},
ffi::{c_int, c_long, c_uint},
ops::Deref,
ptr,
@@ -96,6 +97,12 @@ unsafe impl Sync for Task {}
/// The type of process identifiers (PIDs).
type Pid = bindings::pid_t;

+/// The type of user identifiers (UIDs).
+#[derive(Copy, Clone)]
+pub struct Kuid {
+ kuid: bindings::kuid_t,
+}
+
impl Task {
/// Returns a raw pointer to the current task.
///
@@ -157,12 +164,34 @@ pub fn pid(&self) -> Pid {
unsafe { *ptr::addr_of!((*self.0.get()).pid) }
}

+ /// Returns the UID of the given task.
+ pub fn uid(&self) -> Kuid {
+ // SAFETY: By the type invariant, we know that `self.0` is valid.
+ Kuid::from_raw(unsafe { bindings::task_uid(self.0.get()) })
+ }
+
+ /// Returns the effective UID of the given task.
+ pub fn euid(&self) -> Kuid {
+ // SAFETY: By the type invariant, we know that `self.0` is valid.
+ Kuid::from_raw(unsafe { bindings::task_euid(self.0.get()) })
+ }
+
/// Determines whether the given task has pending signals.
pub fn signal_pending(&self) -> bool {
// SAFETY: By the type invariant, we know that `self.0` is valid.
unsafe { bindings::signal_pending(self.0.get()) != 0 }
}

+ /// Returns the given task's pid in the current pid namespace.
+ pub fn pid_in_current_ns(&self) -> Pid {
+ let current = Task::current_raw();
+ // SAFETY: Calling `task_active_pid_ns` with the current task is always safe.
+ let namespace = unsafe { bindings::task_active_pid_ns(current) };
+ // SAFETY: We know that `self.0.get()` is valid by the type invariant, and the namespace
+ // pointer is not dangling since it points at this task's namespace.
+ unsafe { bindings::task_tgid_nr_ns(self.0.get(), namespace) }
+ }
+
/// Wakes up the task.
pub fn wake_up(&self) {
// SAFETY: By the type invariant, we know that `self.0.get()` is non-null and valid.
@@ -172,6 +201,46 @@ pub fn wake_up(&self) {
}
}

+impl Kuid {
+ /// Get the current euid.
+ #[inline]
+ pub fn current_euid() -> Kuid {
+ // SAFETY: Just an FFI call.
+ Self::from_raw(unsafe { bindings::current_euid() })
+ }
+
+ /// Create a `Kuid` given the raw C type.
+ #[inline]
+ pub fn from_raw(kuid: bindings::kuid_t) -> Self {
+ Self { kuid }
+ }
+
+ /// Turn this kuid into the raw C type.
+ #[inline]
+ pub fn into_raw(self) -> bindings::kuid_t {
+ self.kuid
+ }
+
+ /// Converts this kernel UID into a userspace UID.
+ ///
+ /// Uses the namespace of the current task.
+ #[inline]
+ pub fn into_uid_in_current_ns(self) -> bindings::uid_t {
+ // SAFETY: Just an FFI call.
+ unsafe { bindings::from_kuid(bindings::current_user_ns(), self.kuid) }
+ }
+}
+
+impl PartialEq for Kuid {
+ #[inline]
+ fn eq(&self, other: &Kuid) -> bool {
+ // SAFETY: Just an FFI call.
+ unsafe { bindings::uid_eq(self.kuid, other.kuid) }
+ }
+}
+
+impl Eq for Kuid {}
+
// SAFETY: The type invariants guarantee that `Task` is always ref-counted.
unsafe impl crate::types::AlwaysRefCounted for Task {
fn inc_ref(&self) {
--
2.43.0.594.gd9cf4e227d-goog

2024-02-02 10:58:30

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Subject: [PATCH v4 3/9] rust: file: add Rust abstraction for `struct file`

From: Wedson Almeida Filho <[email protected]>

This abstraction makes it possible to manipulate the open files for a
process. The new `File` struct wraps the C `struct file`. When accessing
it using the smart pointer `ARef<File>`, the pointer will own a
reference count to the file. When accessing it as `&File`, then the
reference does not own a refcount, but the borrow checker will ensure
that the reference count does not hit zero while the `&File` is live.

Since this is intended to manipulate the open files of a process, we
introduce an `fget` constructor that corresponds to the C `fget`
method. In future patches, it will become possible to create a new fd in
a process and bind it to a `File`. Rust Binder will use these to send
fds from one process to another.

We also provide a method for accessing the file's flags. Rust Binder
will use this to access the flags of the Binder fd to check whether the
non-blocking flag is set, which affects what the Binder ioctl does.

This introduces a struct for the EBADF error type, rather than just
using the Error type directly. This has two advantages:
* `File::from_fd` returns a `Result<ARef<File>, BadFdError>`, which the
compiler will represent as a single pointer, with null being an error.
This is possible because the compiler understands that `BadFdError`
has only one possible value, and it also understands that the
`ARef<File>` smart pointer is guaranteed non-null.
* Additionally, we promise to users of the method that the method can
only fail with EBADF, which means that they can rely on this promise
without having to inspect its implementation.
That said, there are also two disadvantages:
* Defining additional error types involves boilerplate.
* The question mark operator will only utilize the `From` trait once,
which prevents you from using the question mark operator on
`BadFdError` in methods that return some third error type that the
kernel `Error` is convertible into. (However, it works fine in methods
that return `Error`.)

Signed-off-by: Wedson Almeida Filho <[email protected]>
Co-developed-by: Daniel Xu <[email protected]>
Signed-off-by: Daniel Xu <[email protected]>
Co-developed-by: Alice Ryhl <[email protected]>
Signed-off-by: Alice Ryhl <[email protected]>
---
fs/file.c | 7 +
rust/bindings/bindings_helper.h | 2 +
rust/helpers.c | 7 +
rust/kernel/file.rs | 249 ++++++++++++++++++++++++++++++++
rust/kernel/lib.rs | 1 +
5 files changed, 266 insertions(+)
create mode 100644 rust/kernel/file.rs

diff --git a/fs/file.c b/fs/file.c
index 3b683b9101d8..f2eab5fcb87f 100644
--- a/fs/file.c
+++ b/fs/file.c
@@ -1115,18 +1115,25 @@ EXPORT_SYMBOL(task_lookup_next_fdget_rcu);
/*
* Lightweight file lookup - no refcnt increment if fd table isn't shared.
*
* You can use this instead of fget if you satisfy all of the following
* conditions:
* 1) You must call fput_light before exiting the syscall and returning control
* to userspace (i.e. you cannot remember the returned struct file * after
* returning to userspace).
* 2) You must not call filp_close on the returned struct file * in between
* calls to fget_light and fput_light.
* 3) You must not clone the current task in between the calls to fget_light
* and fput_light.
*
* The fput_needed flag returned by fget_light should be passed to the
* corresponding fput_light.
+ *
+ * (As an exception to rule 2, you can call filp_close between fget_light and
+ * fput_light provided that you capture a real refcount with get_file before
+ * the call to filp_close, and ensure that this real refcount is fput *after*
+ * the fput_light call.)
+ *
+ * See also the documentation in rust/kernel/file.rs.
*/
static unsigned long __fget_light(unsigned int fd, fmode_t mask)
{
diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h
index 936651110c39..41fcd2905ed4 100644
--- a/rust/bindings/bindings_helper.h
+++ b/rust/bindings/bindings_helper.h
@@ -9,6 +9,8 @@
#include <kunit/test.h>
#include <linux/errname.h>
#include <linux/ethtool.h>
+#include <linux/file.h>
+#include <linux/fs.h>
#include <linux/jiffies.h>
#include <linux/mdio.h>
#include <linux/phy.h>
diff --git a/rust/helpers.c b/rust/helpers.c
index 70e59efd92bc..03141a3608a4 100644
--- a/rust/helpers.c
+++ b/rust/helpers.c
@@ -25,6 +25,7 @@
#include <linux/build_bug.h>
#include <linux/err.h>
#include <linux/errname.h>
+#include <linux/fs.h>
#include <linux/mutex.h>
#include <linux/refcount.h>
#include <linux/sched/signal.h>
@@ -157,6 +158,12 @@ void rust_helper_init_work_with_key(struct work_struct *work, work_func_t func,
}
EXPORT_SYMBOL_GPL(rust_helper_init_work_with_key);

+struct file *rust_helper_get_file(struct file *f)
+{
+ return get_file(f);
+}
+EXPORT_SYMBOL_GPL(rust_helper_get_file);
+
/*
* `bindgen` binds the C `size_t` type as the Rust `usize` type, so we can
* use it in contexts where Rust expects a `usize` like slice (array) indices.
diff --git a/rust/kernel/file.rs b/rust/kernel/file.rs
new file mode 100644
index 000000000000..0d6ef32009c6
--- /dev/null
+++ b/rust/kernel/file.rs
@@ -0,0 +1,249 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Files and file descriptors.
+//!
+//! C headers: [`include/linux/fs.h`](srctree/include/linux/fs.h) and
+//! [`include/linux/file.h`](srctree/include/linux/file.h)
+
+use crate::{
+ bindings,
+ error::{code::*, Error, Result},
+ types::{ARef, AlwaysRefCounted, Opaque},
+};
+use core::ptr;
+
+/// Flags associated with a [`File`].
+pub mod flags {
+ /// File is opened in append mode.
+ pub const O_APPEND: u32 = bindings::O_APPEND;
+
+ /// Signal-driven I/O is enabled.
+ pub const O_ASYNC: u32 = bindings::FASYNC;
+
+ /// Close-on-exec flag is set.
+ pub const O_CLOEXEC: u32 = bindings::O_CLOEXEC;
+
+ /// File was created if it didn't already exist.
+ pub const O_CREAT: u32 = bindings::O_CREAT;
+
+ /// Direct I/O is enabled for this file.
+ pub const O_DIRECT: u32 = bindings::O_DIRECT;
+
+ /// File must be a directory.
+ pub const O_DIRECTORY: u32 = bindings::O_DIRECTORY;
+
+ /// Like [`O_SYNC`] except metadata is not synced.
+ pub const O_DSYNC: u32 = bindings::O_DSYNC;
+
+ /// Ensure that this file is created with the `open(2)` call.
+ pub const O_EXCL: u32 = bindings::O_EXCL;
+
+ /// Large file size enabled (`off64_t` over `off_t`).
+ pub const O_LARGEFILE: u32 = bindings::O_LARGEFILE;
+
+ /// Do not update the file last access time.
+ pub const O_NOATIME: u32 = bindings::O_NOATIME;
+
+ /// File should not be used as process's controlling terminal.
+ pub const O_NOCTTY: u32 = bindings::O_NOCTTY;
+
+ /// If basename of path is a symbolic link, fail open.
+ pub const O_NOFOLLOW: u32 = bindings::O_NOFOLLOW;
+
+ /// File is using nonblocking I/O.
+ pub const O_NONBLOCK: u32 = bindings::O_NONBLOCK;
+
+ /// Also known as `O_NDELAY`.
+ ///
+ /// This is effectively the same flag as [`O_NONBLOCK`] on all architectures
+ /// except SPARC64.
+ pub const O_NDELAY: u32 = bindings::O_NDELAY;
+
+ /// Used to obtain a path file descriptor.
+ pub const O_PATH: u32 = bindings::O_PATH;
+
+ /// Write operations on this file will flush data and metadata.
+ pub const O_SYNC: u32 = bindings::O_SYNC;
+
+ /// This file is an unnamed temporary regular file.
+ pub const O_TMPFILE: u32 = bindings::O_TMPFILE;
+
+ /// File should be truncated to length 0.
+ pub const O_TRUNC: u32 = bindings::O_TRUNC;
+
+ /// Bitmask for access mode flags.
+ ///
+ /// # Examples
+ ///
+ /// ```
+ /// use kernel::file;
+ /// # fn do_something() {}
+ /// # let flags = 0;
+ /// if (flags & file::flags::O_ACCMODE) == file::flags::O_RDONLY {
+ /// do_something();
+ /// }
+ /// ```
+ pub const O_ACCMODE: u32 = bindings::O_ACCMODE;
+
+ /// File is read only.
+ pub const O_RDONLY: u32 = bindings::O_RDONLY;
+
+ /// File is write only.
+ pub const O_WRONLY: u32 = bindings::O_WRONLY;
+
+ /// File can be both read and written.
+ pub const O_RDWR: u32 = bindings::O_RDWR;
+}
+
+/// Wraps the kernel's `struct file`.
+///
+/// # Refcounting
+///
+/// Instances of this type are reference-counted. The reference count is incremented by the
+/// `fget`/`get_file` functions and decremented by `fput`. The Rust type `ARef<File>` represents a
+/// pointer that owns a reference count on the file.
+///
+/// Whenever a process opens a file descriptor (fd), it stores a pointer to the file in its `struct
+/// files_struct`. This pointer owns a reference count to the file, ensuring the file isn't
+/// prematurely deleted while the file descriptor is open. In Rust terminology, the pointers in
+/// `struct files_struct` are `ARef<File>` pointers.
+///
+/// ## Light refcounts
+///
+/// Whenever a process has an fd to a file, it may use something called a "light refcount" as a
+/// performance optimization. Light refcounts are acquired by calling `fdget` and released with
+/// `fdput`. The idea behind light refcounts is that if the fd is not closed between the calls to
+/// `fdget` and `fdput`, then the refcount cannot hit zero during that time, as the `struct
+/// files_struct` holds a reference until the fd is closed. This means that it's safe to access the
+/// file even if `fdget` does not increment the refcount.
+///
+/// The requirement that the fd is not closed during a light refcount applies globally across all
+/// threads - not just on the thread using the light refcount. For this reason, light refcounts are
+/// only used when the `struct files_struct` is not shared with other threads, since this ensures
+/// that other unrelated threads cannot suddenly start using the fd and close it. Therefore,
+/// calling `fdget` on a shared `struct files_struct` creates a normal refcount instead of a light
+/// refcount.
+///
+/// Light reference counts must be released with `fdput` before the system call returns to
+/// userspace. This means that if you wait until the current system call returns to userspace, then
+/// all light refcounts that existed at the time have gone away.
+///
+/// ## Rust references
+///
+/// The reference type `&File` is similar to light refcounts:
+///
+/// * `&File` references don't own a reference count. They can only exist as long as the reference
+/// count stays positive, and can only be created when there is some mechanism in place to ensure
+/// this.
+///
+/// * The Rust borrow-checker normally ensures this by enforcing that the `ARef<File>` from which
+/// a `&File` is created outlives the `&File`.
+///
+/// * Using the unsafe [`File::from_ptr`] means that it is up to the caller to ensure that the
+/// `&File` only exists while the reference count is positive.
+///
+/// * You can think of `fdget` as using an fd to look up an `ARef<File>` in the `struct
+/// files_struct` and create an `&File` from it. The "fd cannot be closed" rule is like the Rust
+/// rule "the `ARef<File>` must outlive the `&File`".
+///
+/// # Invariants
+///
+/// * Instances of this type are refcounted using the `f_count` field.
+/// * If an fd with active light refcounts is closed, then it must be the case that the file
+/// refcount is positive until all light refcounts of the fd have been dropped.
+/// * A light refcount must be dropped before returning to userspace.
+#[repr(transparent)]
+pub struct File(Opaque<bindings::file>);
+
+// SAFETY:
+// - `File::dec_ref` can be called from any thread.
+// - It is okay to send ownership of `File` across thread boundaries.
+unsafe impl Send for File {}
+
+// SAFETY: All methods defined on `File` that take `&self` are safe to call even if other threads
+// are concurrently accessing the same `struct file`, because those methods either access immutable
+// properties or have proper synchronization to ensure that such accesses are safe.
+unsafe impl Sync for File {}
+
+impl File {
+ /// Constructs a new `struct file` wrapper from a file descriptor.
+ ///
+ /// The file descriptor belongs to the current process.
+ pub fn fget(fd: u32) -> Result<ARef<Self>, BadFdError> {
+ // SAFETY: FFI call, there are no requirements on `fd`.
+ let ptr = ptr::NonNull::new(unsafe { bindings::fget(fd) }).ok_or(BadFdError)?;
+
+ // SAFETY: `bindings::fget` either returns null or a valid pointer to a file, and we
+ // checked for null above.
+ //
+ // INVARIANT: `bindings::fget` creates a refcount, and we pass ownership of the refcount to
+ // the new `ARef<File>`.
+ Ok(unsafe { ARef::from_raw(ptr.cast()) })
+ }
+
+ /// Creates a reference to a [`File`] from a valid pointer.
+ ///
+ /// # Safety
+ ///
+ /// The caller must ensure that `ptr` points at a valid file and that the file's refcount is
+ /// positive for the duration of 'a.
+ pub unsafe fn from_ptr<'a>(ptr: *const bindings::file) -> &'a File {
+ // SAFETY: The caller guarantees that the pointer is not dangling and stays valid for the
+ // duration of 'a. The cast is okay because `File` is `repr(transparent)`.
+ //
+ // INVARIANT: The safety requirements guarantee that the refcount does not hit zero during
+ // 'a.
+ unsafe { &*ptr.cast() }
+ }
+
+ /// Returns a raw pointer to the inner C struct.
+ #[inline]
+ pub fn as_ptr(&self) -> *mut bindings::file {
+ self.0.get()
+ }
+
+ /// Returns the flags associated with the file.
+ ///
+ /// The flags are a combination of the constants in [`flags`].
+ pub fn flags(&self) -> u32 {
+ // This `read_volatile` is intended to correspond to a READ_ONCE call.
+ //
+ // SAFETY: The file is valid because the shared reference guarantees a nonzero refcount.
+ //
+ // TODO: Replace with `read_once` when available on the Rust side.
+ unsafe { core::ptr::addr_of!((*self.as_ptr()).f_flags).read_volatile() }
+ }
+}
+
+// SAFETY: The type invariants guarantee that `File` is always ref-counted. This implementation
+// makes `ARef<File>` own a normal refcount.
+unsafe impl AlwaysRefCounted for File {
+ fn inc_ref(&self) {
+ // SAFETY: The existence of a shared reference means that the refcount is nonzero.
+ unsafe { bindings::get_file(self.as_ptr()) };
+ }
+
+ unsafe fn dec_ref(obj: ptr::NonNull<File>) {
+ // SAFETY: To call this method, the caller passes us ownership of a normal refcount, so we
+ // may drop it. The cast is okay since `File` has the same representation as `struct file`.
+ unsafe { bindings::fput(obj.cast().as_ptr()) }
+ }
+}
+
+/// Represents the `EBADF` error code.
+///
+/// Used for methods that can only fail with `EBADF`.
+#[derive(Copy, Clone, Eq, PartialEq)]
+pub struct BadFdError;
+
+impl From<BadFdError> for Error {
+ fn from(_: BadFdError) -> Error {
+ EBADF
+ }
+}
+
+impl core::fmt::Debug for BadFdError {
+ fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+ f.pad("EBADF")
+ }
+}
diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index b89ecf4e97a0..9353dd713a20 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -34,6 +34,7 @@
mod allocator;
mod build_assert;
pub mod error;
+pub mod file;
pub mod init;
pub mod ioctl;
#[cfg(CONFIG_KUNIT)]
--
2.43.0.594.gd9cf4e227d-goog

2024-02-02 10:58:47

[permalink] [raw]

Subject: [PATCH v4 9/9] rust: file: add abstraction for `poll_table`

The existing `CondVar` abstraction is a wrapper around
`wait_queue_head`, but it does not support all use-cases of the C
`wait_queue_head` type. To be specific, a `CondVar` cannot be registered
with a `struct poll_table`. This limitation has the advantage that you
do not need to call `synchronize_rcu` when destroying a `CondVar`.

However, we need the ability to register a `poll_table` with a
`wait_queue_head` in Rust Binder. To enable this, introduce a type
called `PollCondVar`, which is like `CondVar` except that you can
register a `poll_table`. We also introduce `PollTable`, which is a safe
wrapper around `poll_table` that is intended to be used with
`PollCondVar`.

The destructor of `PollCondVar` unconditionally calls `synchronize_rcu`
to ensure that the removal of epoll waiters has fully completed before
the `wait_queue_head` is destroyed.

That said, `synchronize_rcu` is rather expensive and is not needed in
all cases: If we have never registered a `poll_table` with the
`wait_queue_head`, then we don't need to call `synchronize_rcu`. (And
this is a common case in Binder - not all processes use Binder with
epoll.) The current implementation does not account for this, but if we
find that it is necessary to improve this, a future patch could store a
boolean next to the `wait_queue_head` to keep track of whether a
`poll_table` has ever been registered.

Signed-off-by: Alice Ryhl <[email protected]>
---
rust/bindings/bindings_helper.h | 1 +
rust/kernel/sync.rs | 1 +
rust/kernel/sync/poll.rs | 117 ++++++++++++++++++++++++++++++++
3 files changed, 119 insertions(+)
create mode 100644 rust/kernel/sync/poll.rs

diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h
index f4d9d04333c0..c651d38e5dd6 100644
--- a/rust/bindings/bindings_helper.h
+++ b/rust/bindings/bindings_helper.h
@@ -17,6 +17,7 @@
#include <linux/mdio.h>
#include <linux/phy.h>
#include <linux/pid_namespace.h>
+#include <linux/poll.h>
#include <linux/security.h>
#include <linux/slab.h>
#include <linux/refcount.h>
diff --git a/rust/kernel/sync.rs b/rust/kernel/sync.rs
index c1fb10fc64f4..84b69e337a55 100644
--- a/rust/kernel/sync.rs
+++ b/rust/kernel/sync.rs
@@ -11,6 +11,7 @@
mod condvar;
pub mod lock;
mod locked_by;
+pub mod poll;

pub use arc::{Arc, ArcBorrow, UniqueArc};
pub use condvar::{CondVar, CondVarTimeoutResult};
diff --git a/rust/kernel/sync/poll.rs b/rust/kernel/sync/poll.rs
new file mode 100644
index 000000000000..a0e4f3de109a
--- /dev/null
+++ b/rust/kernel/sync/poll.rs
@@ -0,0 +1,117 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Utilities for working with `struct poll_table`.
+
+use crate::{
+ bindings,
+ file::File,
+ prelude::*,
+ sync::{CondVar, LockClassKey},
+ types::Opaque,
+};
+use core::ops::Deref;
+
+/// Creates a [`PollCondVar`] initialiser with the given name and a newly-created lock class.
+#[macro_export]
+macro_rules! new_poll_condvar {
+ ($($name:literal)?) => {
+ $crate::sync::poll::PollCondVar::new($crate::optional_name!($($name)?), $crate::static_lock_class!())
+ };
+}
+
+/// Wraps the kernel's `struct poll_table`.
+///
+/// # Invariants
+///
+/// This struct contains a valid `struct poll_table`.
+///
+/// For a `struct poll_table` to be valid, its `_qproc` function must follow the safety
+/// requirements of `_qproc` functions:
+///
+/// * The `_qproc` function is given permission to enqueue a waiter to the provided `poll_table`
+/// during the call. Once the waiter is removed and an rcu grace period has passed, it must no
+/// longer access the `wait_queue_head`.
+#[repr(transparent)]
+pub struct PollTable(Opaque<bindings::poll_table>);
+
+impl PollTable {
+ /// Creates a reference to a [`PollTable`] from a valid pointer.
+ ///
+ /// # Safety
+ ///
+ /// The caller must ensure that for the duration of 'a, the pointer will point at a valid poll
+ /// table (as defined in the type invariants).
+ ///
+ /// The caller must also ensure that the `poll_table` is only accessed via the returned
+ /// reference for the duration of 'a.
+ pub unsafe fn from_ptr<'a>(ptr: *mut bindings::poll_table) -> &'a mut PollTable {
+ // SAFETY: The safety requirements guarantee the validity of the dereference, while the
+ // `PollTable` type being transparent makes the cast ok.
+ unsafe { &mut *ptr.cast() }
+ }
+
+ fn get_qproc(&self) -> bindings::poll_queue_proc {
+ let ptr = self.0.get();
+ // SAFETY: The `ptr` is valid because it originates from a reference, and the `_qproc`
+ // field is not modified concurrently with this call since we have an immutable reference.
+ unsafe { (*ptr)._qproc }
+ }
+
+ /// Register this [`PollTable`] with the provided [`PollCondVar`], so that it can be notified
+ /// using the condition variable.
+ pub fn register_wait(&mut self, file: &File, cv: &PollCondVar) {
+ if let Some(qproc) = self.get_qproc() {
+ // SAFETY: The pointers to `file` and `self` need to be valid for the duration of this
+ // call to `qproc`, which they are because they are references.
+ //
+ // The `cv.wait_queue_head` pointer must be valid until an rcu grace period after the
+ // waiter is removed. The `PollCondVar` is pinned, so before `cv.wait_queue_head` can
+ // be destroyed, the destructor must run. That destructor first removes all waiters,
+ // and then waits for an rcu grace period. Therefore, `cv.wait_queue_head` is valid for
+ // long enough.
+ unsafe { qproc(file.as_ptr() as _, cv.wait_queue_head.get(), self.0.get()) };
+ }
+ }
+}
+
+/// A wrapper around [`CondVar`] that makes it usable with [`PollTable`].
+///
+/// [`CondVar`]: crate::sync::CondVar
+#[pin_data(PinnedDrop)]
+pub struct PollCondVar {
+ #[pin]
+ inner: CondVar,
+}
+
+impl PollCondVar {
+ /// Constructs a new condvar initialiser.
+ pub fn new(name: &'static CStr, key: &'static LockClassKey) -> impl PinInit<Self> {
+ pin_init!(Self {
+ inner <- CondVar::new(name, key),
+ })
+ }
+}
+
+// Make the `CondVar` methods callable on `PollCondVar`.
+impl Deref for PollCondVar {
+ type Target = CondVar;
+
+ fn deref(&self) -> &CondVar {
+ &self.inner
+ }
+}
+
+#[pinned_drop]
+impl PinnedDrop for PollCondVar {
+ fn drop(self: Pin<&mut Self>) {
+ // Clear anything registered using `register_wait`.
+ //
+ // SAFETY: The pointer points at a valid `wait_queue_head`.
+ unsafe { bindings::__wake_up_pollfree(self.inner.wait_queue_head.get()) };
+
+ // Wait for epoll items to be properly removed.
+ //
+ // SAFETY: Just an FFI call.
+ unsafe { bindings::synchronize_rcu() };
+ }
+}
--
2.43.0.594.gd9cf4e227d-goog

2024-02-02 11:34:39

[permalink] [raw]

Subject: [PATCH v4 4/9] rust: cred: add Rust abstraction for `struct cred`

From: Wedson Almeida Filho <[email protected]>

Add a wrapper around `struct cred` called `Credential`, and provide
functionality to get the `Credential` associated with a `File`.

Rust Binder must check the credentials of processes when they attempt to
perform various operations, and these checks usually take a
`&Credential` as parameter. The security_binder_set_context_mgr function
would be one example. This patch is necessary to access these security_*
methods from Rust.

Signed-off-by: Wedson Almeida Filho <[email protected]>
Co-developed-by: Alice Ryhl <[email protected]>
Signed-off-by: Alice Ryhl <[email protected]>
---
rust/bindings/bindings_helper.h | 1 +
rust/helpers.c | 13 ++++++
rust/kernel/cred.rs | 72 +++++++++++++++++++++++++++++++++
rust/kernel/file.rs | 13 ++++++
rust/kernel/lib.rs | 1 +
5 files changed, 100 insertions(+)
create mode 100644 rust/kernel/cred.rs

diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h
index 41fcd2905ed4..84a56e8b6b67 100644
--- a/rust/bindings/bindings_helper.h
+++ b/rust/bindings/bindings_helper.h
@@ -7,6 +7,7 @@
*/

#include <kunit/test.h>
+#include <linux/cred.h>
#include <linux/errname.h>
#include <linux/ethtool.h>
#include <linux/file.h>
diff --git a/rust/helpers.c b/rust/helpers.c
index 03141a3608a4..10ed69f76424 100644
--- a/rust/helpers.c
+++ b/rust/helpers.c
@@ -23,6 +23,7 @@
#include <kunit/test-bug.h>
#include <linux/bug.h>
#include <linux/build_bug.h>
+#include <linux/cred.h>
#include <linux/err.h>
#include <linux/errname.h>
#include <linux/fs.h>
@@ -164,6 +165,18 @@ struct file *rust_helper_get_file(struct file *f)
}
EXPORT_SYMBOL_GPL(rust_helper_get_file);

+const struct cred *rust_helper_get_cred(const struct cred *cred)
+{
+ return get_cred(cred);
+}
+EXPORT_SYMBOL_GPL(rust_helper_get_cred);
+
+void rust_helper_put_cred(const struct cred *cred)
+{
+ put_cred(cred);
+}
+EXPORT_SYMBOL_GPL(rust_helper_put_cred);
+
/*
* `bindgen` binds the C `size_t` type as the Rust `usize` type, so we can
* use it in contexts where Rust expects a `usize` like slice (array) indices.
diff --git a/rust/kernel/cred.rs b/rust/kernel/cred.rs
new file mode 100644
index 000000000000..fabc50e48c9e
--- /dev/null
+++ b/rust/kernel/cred.rs
@@ -0,0 +1,72 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Credentials management.
+//!
+//! C header: [`include/linux/cred.h`](srctree/include/linux/cred.h).
+//!
+//! Reference: <https://www.kernel.org/doc/html/latest/security/credentials.html>
+
+use crate::{
+ bindings,
+ types::{AlwaysRefCounted, Opaque},
+};
+
+/// Wraps the kernel's `struct cred`.
+///
+/// Most fields of credentials are immutable. When things have their credentials changed, that
+/// happens by replacing the credential instad of changing an existing credential. See the [kernel
+/// documentation][ref] for more info on this.
+///
+/// # Invariants
+///
+/// Instances of this type are always ref-counted, that is, a call to `get_cred` ensures that the
+/// allocation remains valid at least until the matching call to `put_cred`.
+///
+/// [ref]: https://www.kernel.org/doc/html/latest/security/credentials.html
+#[repr(transparent)]
+pub struct Credential(Opaque<bindings::cred>);
+
+// SAFETY:
+// - `Credential::dec_ref` can be called from any thread.
+// - It is okay to send ownership of `Credential` across thread boundaries.
+unsafe impl Send for Credential {}
+
+// SAFETY: It's OK to access `Credential` through shared references from other threads because
+// we're either accessing properties that don't change or that are properly synchronised by C code.
+unsafe impl Sync for Credential {}
+
+impl Credential {
+ /// Creates a reference to a [`Credential`] from a valid pointer.
+ ///
+ /// # Safety
+ ///
+ /// The caller must ensure that `ptr` is valid and remains valid for the lifetime of the
+ /// returned [`Credential`] reference.
+ pub unsafe fn from_ptr<'a>(ptr: *const bindings::cred) -> &'a Credential {
+ // SAFETY: The safety requirements guarantee the validity of the dereference, while the
+ // `Credential` type being transparent makes the cast ok.
+ unsafe { &*ptr.cast() }
+ }
+
+ /// Returns the effective UID of the given credential.
+ pub fn euid(&self) -> bindings::kuid_t {
+ // SAFETY: By the type invariant, we know that `self.0` is valid. Furthermore, the `euid`
+ // field of a credential is never changed after initialization, so there is no potential
+ // for data races.
+ unsafe { (*self.0.get()).euid }
+ }
+}
+
+// SAFETY: The type invariants guarantee that `Credential` is always ref-counted.
+unsafe impl AlwaysRefCounted for Credential {
+ fn inc_ref(&self) {
+ // SAFETY: The existence of a shared reference means that the refcount is nonzero.
+ unsafe { bindings::get_cred(self.0.get()) };
+ }
+
+ unsafe fn dec_ref(obj: core::ptr::NonNull<Credential>) {
+ // SAFETY: The safety requirements guarantee that the refcount is nonzero. The cast is okay
+ // because `Credential` has the same representation as `struct cred`.
+ unsafe { bindings::put_cred(obj.cast().as_ptr()) };
+ }
+}
diff --git a/rust/kernel/file.rs b/rust/kernel/file.rs
index 0d6ef32009c6..095775411979 100644
--- a/rust/kernel/file.rs
+++ b/rust/kernel/file.rs
@@ -7,6 +7,7 @@

use crate::{
bindings,
+ cred::Credential,
error::{code::*, Error, Result},
types::{ARef, AlwaysRefCounted, Opaque},
};
@@ -202,6 +203,18 @@ pub fn as_ptr(&self) -> *mut bindings::file {
self.0.get()
}

+ /// Returns the credentials of the task that originally opened the file.
+ pub fn cred(&self) -> &Credential {
+ // SAFETY: It's okay to read the `f_cred` field without synchronization because `f_cred` is
+ // never changed after initialization of the file.
+ let ptr = unsafe { (*self.as_ptr()).f_cred };
+
+ // SAFETY: The signature of this function ensures that the caller will only access the
+ // returned credential while the file is still valid, and the C side ensures that the
+ // credential stays valid at least as long as the file.
+ unsafe { Credential::from_ptr(ptr) }
+ }
+
/// Returns the flags associated with the file.
///
/// The flags are a combination of the constants in [`flags`].
diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index 9353dd713a20..f65e5978f807 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -33,6 +33,7 @@
#[cfg(not(testlib))]
mod allocator;
mod build_assert;
+pub mod cred;
pub mod error;
pub mod file;
pub mod init;
--
2.43.0.594.gd9cf4e227d-goog

2024-02-02 11:35:25

[permalink] [raw]

Subject: [PATCH v4 6/9] rust: file: add `FileDescriptorReservation`

From: Wedson Almeida Filho <[email protected]>

Allow for the creation of a file descriptor in two steps: first, we
reserve a slot for it, then we commit or drop the reservation. The first
step may fail (e.g., the current process ran out of available slots),
but commit and drop never fail (and are mutually exclusive).

This is needed by Rust Binder when fds are sent from one process to
another. It has to be a two-step process to properly handle the case
where multiple fds are sent: The operation must fail or succeed
atomically, which we achieve by first reserving the fds we need, and
only installing the files once we have reserved enough fds to send the
files.

Fd reservations assume that the value of `current` does not change
between the call to get_unused_fd_flags and the call to fd_install (or
put_unused_fd). By not implementing the Send trait, this abstraction
ensures that the `FileDescriptorReservation` cannot be moved into a
different process.

Signed-off-by: Wedson Almeida Filho <[email protected]>
Co-developed-by: Alice Ryhl <[email protected]>
Reviewed-by: Benno Lossin <[email protected]>
Signed-off-by: Alice Ryhl <[email protected]>
---
rust/kernel/file.rs | 72 ++++++++++++++++++++++++++++++++++++++++++++-
1 file changed, 71 insertions(+), 1 deletion(-)

diff --git a/rust/kernel/file.rs b/rust/kernel/file.rs
index 095775411979..2004270a661c 100644
--- a/rust/kernel/file.rs
+++ b/rust/kernel/file.rs
@@ -9,7 +9,7 @@
bindings,
cred::Credential,
error::{code::*, Error, Result},
- types::{ARef, AlwaysRefCounted, Opaque},
+ types::{ARef, AlwaysRefCounted, NotThreadSafe, Opaque},
};
use core::ptr;

@@ -243,6 +243,76 @@ unsafe fn dec_ref(obj: ptr::NonNull<File>) {
}
}

+/// A file descriptor reservation.
+///
+/// This allows the creation of a file descriptor in two steps: first, we reserve a slot for it,
+/// then we commit or drop the reservation. The first step may fail (e.g., the current process ran
+/// out of available slots), but commit and drop never fail (and are mutually exclusive).
+///
+/// Dropping the reservation happens in the destructor of this type.
+///
+/// # Invariants
+///
+/// The fd stored in this struct must correspond to a reserved file descriptor of the current task.
+pub struct FileDescriptorReservation {
+ fd: u32,
+ /// Prevent values of this type from being moved to a different task.
+ ///
+ /// The `fd_install` and `put_unused_fd` functions assume that the value of `current` is
+ /// unchanged since the call to `get_unused_fd_flags`. By adding this marker to this type, we
+ /// prevent it from being moved across task boundaries, which ensures that `current` does not
+ /// change while this value exists.
+ _not_send: NotThreadSafe,
+}
+
+impl FileDescriptorReservation {
+ /// Creates a new file descriptor reservation.
+ pub fn get_unused_fd_flags(flags: u32) -> Result<Self> {
+ // SAFETY: FFI call, there are no safety requirements on `flags`.
+ let fd: i32 = unsafe { bindings::get_unused_fd_flags(flags) };
+ if fd < 0 {
+ return Err(Error::from_errno(fd));
+ }
+ Ok(Self {
+ fd: fd as u32,
+ _not_send: NotThreadSafe,
+ })
+ }
+
+ /// Returns the file descriptor number that was reserved.
+ pub fn reserved_fd(&self) -> u32 {
+ self.fd
+ }
+
+ /// Commits the reservation.
+ ///
+ /// The previously reserved file descriptor is bound to `file`. This method consumes the
+ /// [`FileDescriptorReservation`], so it will not be usable after this call.
+ pub fn fd_install(self, file: ARef<File>) {
+ // SAFETY: `self.fd` was previously returned by `get_unused_fd_flags`. We have not yet used
+ // the fd, so it is still valid, and `current` still refers to the same task, as this type
+ // cannot be moved across task boundaries.
+ //
+ // Furthermore, the file pointer is guaranteed to own a refcount by its type invariants,
+ // and we take ownership of that refcount by not running the destructor below.
+ unsafe { bindings::fd_install(self.fd, file.as_ptr()) };
+
+ // `fd_install` consumes both the file descriptor and the file reference, so we cannot run
+ // the destructors.
+ core::mem::forget(self);
+ core::mem::forget(file);
+ }
+}
+
+impl Drop for FileDescriptorReservation {
+ fn drop(&mut self) {
+ // SAFETY: By the type invariants of this type, `self.fd` was previously returned by
+ // `get_unused_fd_flags`. We have not yet used the fd, so it is still valid, and `current`
+ // still refers to the same task, as this type cannot be moved across task boundaries.
+ unsafe { bindings::put_unused_fd(self.fd) };
+ }
+}
+
/// Represents the `EBADF` error code.
///
/// Used for methods that can only fail with `EBADF`.
--
2.43.0.594.gd9cf4e227d-goog

2024-02-02 11:36:30

[permalink] [raw]

Subject: [PATCH v4 8/9] rust: file: add `DeferredFdCloser`

To close an fd from kernel space, we could call `ksys_close`. However,
if we do this to an fd that is held using `fdget`, then we may trigger a
use-after-free. Introduce a helper that can be used to close an fd even
if the fd is currently held with `fdget`. This is done by grabbing an
extra refcount to the file and dropping it in a task work once we return
to userspace.

This is necessary for Rust Binder because otherwise the user might try
to have Binder close its fd for /dev/binder, which would cause problems
as this happens inside an ioctl on /dev/binder, and ioctls hold the fd
using `fdget`.

Additional motivation can be found in commit 80cd795630d6 ("binder: fix
use-after-free due to ksys_close() during fdget()") and in the comments
on `binder_do_fd_close`.

If there is some way to detect whether an fd is currently held with
`fdget`, then this could be optimized to skip the allocation and task
work when this is not the case. Another possible optimization would be
to combine several fds into a single task work, since this is used with
fd arrays that might hold several fds.

That said, it might not be necessary to optimize it, because Rust Binder
has two ways to send fds: BINDER_TYPE_FD and BINDER_TYPE_FDA. With
BINDER_TYPE_FD, it is userspace's responsibility to close the fd, so
this mechanism is used only by BINDER_TYPE_FDA, but fd arrays are used
rarely these days.

Signed-off-by: Alice Ryhl <[email protected]>
---
rust/bindings/bindings_helper.h | 2 +
rust/helpers.c | 8 ++
rust/kernel/file.rs | 184 +++++++++++++++++++++++++++++++-
rust/kernel/task.rs | 14 +++
4 files changed, 207 insertions(+), 1 deletion(-)

diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h
index 4194b057ef6b..f4d9d04333c0 100644
--- a/rust/bindings/bindings_helper.h
+++ b/rust/bindings/bindings_helper.h
@@ -10,6 +10,7 @@
#include <linux/cred.h>
#include <linux/errname.h>
#include <linux/ethtool.h>
+#include <linux/fdtable.h>
#include <linux/file.h>
#include <linux/fs.h>
#include <linux/jiffies.h>
@@ -21,6 +22,7 @@
#include <linux/refcount.h>
#include <linux/wait.h>
#include <linux/sched.h>
+#include <linux/task_work.h>
#include <linux/workqueue.h>

/* `bindgen` gets confused at certain things. */
diff --git a/rust/helpers.c b/rust/helpers.c
index 58e3a9dff349..d146bbf25aec 100644
--- a/rust/helpers.c
+++ b/rust/helpers.c
@@ -32,6 +32,7 @@
#include <linux/sched/signal.h>
#include <linux/security.h>
#include <linux/spinlock.h>
+#include <linux/task_work.h>
#include <linux/wait.h>
#include <linux/workqueue.h>

@@ -243,6 +244,13 @@ void rust_helper_security_release_secctx(char *secdata, u32 seclen)
EXPORT_SYMBOL_GPL(rust_helper_security_release_secctx);
#endif

+void rust_helper_init_task_work(struct callback_head *twork,
+ task_work_func_t func)
+{
+ init_task_work(twork, func);
+}
+EXPORT_SYMBOL_GPL(rust_helper_init_task_work);
+
/*
* `bindgen` binds the C `size_t` type as the Rust `usize` type, so we can
* use it in contexts where Rust expects a `usize` like slice (array) indices.
diff --git a/rust/kernel/file.rs b/rust/kernel/file.rs
index 2004270a661c..e7628c574b57 100644
--- a/rust/kernel/file.rs
+++ b/rust/kernel/file.rs
@@ -11,7 +11,8 @@
error::{code::*, Error, Result},
types::{ARef, AlwaysRefCounted, NotThreadSafe, Opaque},
};
-use core::ptr;
+use alloc::boxed::Box;
+use core::{alloc::AllocError, mem, ptr};

/// Flags associated with a [`File`].
pub mod flags {
@@ -313,6 +314,187 @@ fn drop(&mut self) {
}
}

+/// Helper used for closing file descriptors in a way that is safe even if the file is currently
+/// held using `fdget`.
+///
+/// Additional motivation can be found in commit 80cd795630d6 ("binder: fix use-after-free due to
+/// ksys_close() during fdget()") and in the comments on `binder_do_fd_close`.
+pub struct DeferredFdCloser {
+ inner: Box<DeferredFdCloserInner>,
+}
+
+/// SAFETY: This just holds an allocation with no real content, so there's no safety issue with
+/// moving it across threads.
+unsafe impl Send for DeferredFdCloser {}
+unsafe impl Sync for DeferredFdCloser {}
+
+/// # Invariants
+///
+/// If the `file` pointer is non-null, then it points at a `struct file` and owns a refcount to
+/// that file.
+#[repr(C)]
+struct DeferredFdCloserInner {
+ twork: mem::MaybeUninit<bindings::callback_head>,
+ file: *mut bindings::file,
+}
+
+impl DeferredFdCloser {
+ /// Create a new [`DeferredFdCloser`].
+ pub fn new() -> Result<Self, AllocError> {
+ Ok(Self {
+ // INVARIANT: The `file` pointer is null, so the type invariant does not apply.
+ inner: Box::try_new(DeferredFdCloserInner {
+ twork: mem::MaybeUninit::uninit(),
+ file: core::ptr::null_mut(),
+ })?,
+ })
+ }
+
+ /// Schedule a task work that closes the file descriptor when this task returns to userspace.
+ ///
+ /// Fails if this is called from a context where we cannot run work when returning to
+ /// userspace. (E.g., from a kthread.)
+ pub fn close_fd(self, fd: u32) -> Result<(), DeferredFdCloseError> {
+ use bindings::task_work_notify_mode_TWA_RESUME as TWA_RESUME;
+
+ // In this method, we schedule the task work before closing the file. This is because
+ // scheduling a task work is fallible, and we need to know whether it will fail before we
+ // attempt to close the file.
+
+ // Task works are not available on kthreads.
+ let current = crate::current!();
+ if current.is_kthread() {
+ return Err(DeferredFdCloseError::TaskWorkUnavailable);
+ }
+
+ // Transfer ownership of the box's allocation to a raw pointer. This disables the
+ // destructor, so we must manually convert it back to a Box to drop it.
+ //
+ // Until we convert it back to a `Box`, there are no aliasing requirements on this
+ // pointer.
+ let inner = Box::into_raw(self.inner);
+
+ // The `callback_head` field is first in the struct, so this cast correctly gives us a
+ // pointer to the field.
+ let callback_head = inner.cast::<bindings::callback_head>();
+ // SAFETY: This pointer offset operation does not go out-of-bounds.
+ let file_field = unsafe { core::ptr::addr_of_mut!((*inner).file) };
+
+ let current = current.as_raw();
+
+ // SAFETY: This function currently has exclusive access to the `DeferredFdCloserInner`, so
+ // it is okay for us to perform unsynchronized writes to its `callback_head` field.
+ unsafe { bindings::init_task_work(callback_head, Some(Self::do_close_fd)) };
+
+ // SAFETY: This inserts the `DeferredFdCloserInner` into the task workqueue for the current
+ // task. If this operation is successful, then this transfers exclusive ownership of the
+ // `callback_head` field to the C side until it calls `do_close_fd`, and we don't touch or
+ // invalidate the field during that time.
+ //
+ // When the C side calls `do_close_fd`, the safety requirements of that method are
+ // satisfied because when a task work is executed, the callback is given ownership of the
+ // pointer.
+ //
+ // The file pointer is currently null. If it is changed to be non-null before `do_close_fd`
+ // is called, then that change happens due to the write at the end of this function, and
+ // that write has a safety comment that explains why the refcount can be dropped when
+ // `do_close_fd` runs.
+ let res = unsafe { bindings::task_work_add(current, callback_head, TWA_RESUME) };
+
+ if res != 0 {
+ // SAFETY: Scheduling the task work failed, so we still have ownership of the box, so
+ // we may destroy it.
+ unsafe { drop(Box::from_raw(inner)) };
+
+ return Err(DeferredFdCloseError::TaskWorkUnavailable);
+ }
+
+ // This removes the fd from the fd table in `current`. The file is not fully closed until
+ // `filp_close` is called. We are given ownership of one refcount to the file.
+ //
+ // SAFETY: This is safe no matter what `fd` is. If the `fd` is valid (that is, if the
+ // pointer is non-null), then we call `filp_close` on the returned pointer as required by
+ // `file_close_fd`.
+ let file = unsafe { bindings::file_close_fd(fd) };
+ if file.is_null() {
+ // We don't clean up the task work since that might be expensive if the task work queue
+ // is long. Just let it execute and let it clean up for itself.
+ return Err(DeferredFdCloseError::BadFd);
+ }
+
+ // Acquire a second refcount to the file.
+ //
+ // SAFETY: The `file` pointer points at a file with a non-zero refcount.
+ unsafe { bindings::get_file(file) };
+
+ // This method closes the fd, consuming one of our two refcounts. There could be active
+ // light refcounts created from that fd, so we must ensure that the file has a positive
+ // refcount for the duration of those active light refcounts. We do that by holding on to
+ // the second refcount until the current task returns to userspace.
+ //
+ // SAFETY: The `file` pointer is valid. Passing `current->files` as the file table to close
+ // it in is correct, since we just got the `fd` from `file_close_fd` which also uses
+ // `current->files`.
+ //
+ // Note: fl_owner_t is currently a void pointer.
+ unsafe { bindings::filp_close(file, (*current).files as bindings::fl_owner_t) };
+
+ // We update the file pointer that the task work is supposed to fput. This transfers
+ // ownership of our last refcount.
+ //
+ // INVARIANT: This changes the `file` field of a `DeferredFdCloserInner` from null to
+ // non-null. This doesn't break the type invariant for `DeferredFdCloserInner` because we
+ // still own a refcount to the file, so we can pass ownership of that refcount to the
+ // `DeferredFdCloserInner`.
+ //
+ // When `do_close_fd` runs, it must be safe for it to `fput` the refcount. However, this is
+ // the case because all light refcounts that are associated with the fd we closed
+ // previously must be dropped when `do_close_fd`, since light refcounts must be dropped
+ // before returning to userspace.
+ //
+ // SAFETY: Task works are executed on the current thread right before we return to
+ // userspace, so this write is guaranteed to happen before `do_close_fd` is called, which
+ // means that a race is not possible here.
+ unsafe { *file_field = file };
+
+ Ok(())
+ }
+
+ /// # Safety
+ ///
+ /// The provided pointer must point at the `twork` field of a `DeferredFdCloserInner` stored in
+ /// a `Box`, and the caller must pass exclusive ownership of that `Box`. Furthermore, if the
+ /// file pointer is non-null, then it must be okay to release the refcount by calling `fput`.
+ unsafe extern "C" fn do_close_fd(inner: *mut bindings::callback_head) {
+ // SAFETY: The caller just passed us ownership of this box.
+ let inner = unsafe { Box::from_raw(inner.cast::<DeferredFdCloserInner>()) };
+ if !inner.file.is_null() {
+ // SAFETY: By the type invariants, we own a refcount to this file, and the caller
+ // guarantees that dropping the refcount now is okay.
+ unsafe { bindings::fput(inner.file) };
+ }
+ // The allocation is freed when `inner` goes out of scope.
+ }
+}
+
+/// Represents a failure to close an fd in a deferred manner.
+#[derive(Copy, Clone, Debug, Eq, PartialEq)]
+pub enum DeferredFdCloseError {
+ /// Closing the fd failed because we were unable to schedule a task work.
+ TaskWorkUnavailable,
+ /// Closing the fd failed because the fd does not exist.
+ BadFd,
+}
+
+impl From<DeferredFdCloseError> for Error {
+ fn from(err: DeferredFdCloseError) -> Error {
+ match err {
+ DeferredFdCloseError::TaskWorkUnavailable => ESRCH,
+ DeferredFdCloseError::BadFd => EBADF,
+ }
+ }
+}
+
/// Represents the `EBADF` error code.
///
/// Used for methods that can only fail with `EBADF`.
diff --git a/rust/kernel/task.rs b/rust/kernel/task.rs
index 7d59cf69ea8a..438e467fb145 100644
--- a/rust/kernel/task.rs
+++ b/rust/kernel/task.rs
@@ -145,6 +145,12 @@ pub unsafe fn current() -> impl Deref<Target = Task> {
}
}

+ /// Returns a raw pointer to the task.
+ #[inline]
+ pub fn as_raw(&self) -> *mut bindings::task_struct {
+ self.0.get()
+ }
+
/// Returns the group leader of the given task.
pub fn group_leader(&self) -> &Task {
// SAFETY: By the type invariant, we know that `self.0` is a valid task. Valid tasks always
@@ -192,6 +198,14 @@ pub fn pid_in_current_ns(&self) -> Pid {
unsafe { bindings::task_tgid_nr_ns(self.0.get(), namespace) }
}

+ /// Returns whether this task corresponds to a kernel thread.
+ pub fn is_kthread(&self) -> bool {
+ // SAFETY: By the type invariant, we know that `self.0.get()` is non-null and valid. There
+ // are no further requirements to read the task's flags.
+ let flags = unsafe { (*self.0.get()).flags };
+ (flags & bindings::PF_KTHREAD) != 0
+ }
+
/// Wakes up the task.
pub fn wake_up(&self) {
// SAFETY: By the type invariant, we know that `self.0.get()` is non-null and valid.
--
2.43.0.594.gd9cf4e227d-goog

2024-02-02 15:36:35

by Greg Kroah-Hartman

[permalink] [raw]

Subject: Re: [PATCH v4 7/9] rust: file: add `Kuid` wrapper

On Fri, Feb 02, 2024 at 10:55:41AM +0000, Alice Ryhl wrote:
> + /// Returns the given task's pid in the current pid namespace.
> + pub fn pid_in_current_ns(&self) -> Pid {
> + let current = Task::current_raw();
> + // SAFETY: Calling `task_active_pid_ns` with the current task is always safe.
> + let namespace = unsafe { bindings::task_active_pid_ns(current) };
> + // SAFETY: We know that `self.0.get()` is valid by the type invariant, and the namespace
> + // pointer is not dangling since it points at this task's namespace.
> + unsafe { bindings::task_tgid_nr_ns(self.0.get(), namespace) }
> + }

pids are reference counted in the kernel, how does this deal with that?
Are they just ignored somehow? Where is the reference count given back?

thanks,

greg k-h

2024-02-02 15:56:40

[permalink] [raw]

Subject: Re: [PATCH v4 7/9] rust: file: add `Kuid` wrapper

On Fri, Feb 2, 2024 at 4:36 PM Greg Kroah-Hartman
<[email protected]> wrote:
>
> On Fri, Feb 02, 2024 at 10:55:41AM +0000, Alice Ryhl wrote:
> > + /// Returns the given task's pid in the current pid namespace.
> > + pub fn pid_in_current_ns(&self) -> Pid {
> > + let current = Task::current_raw();
> > + // SAFETY: Calling `task_active_pid_ns` with the current task is always safe.
> > + let namespace = unsafe { bindings::task_active_pid_ns(current) };
> > + // SAFETY: We know that `self.0.get()` is valid by the type invariant, and the namespace
> > + // pointer is not dangling since it points at this task's namespace.
> > + unsafe { bindings::task_tgid_nr_ns(self.0.get(), namespace) }
> > + }
>
> pids are reference counted in the kernel, how does this deal with that?
> Are they just ignored somehow? Where is the reference count given back?

The intention is that it will be used to replicate the following line
of code from C binder:

trd->sender_pid = task_tgid_nr_ns(sender, task_active_pid_ns(current));

The context of this is an ioctl where the `trd` struct contains what
will be copied into userspace as the output of the ioctl. So, the pid
here is just a number that is given to userspace immediately, and
userspace can then do with it as it likes. It is true that the pid is
stale immediately, as the remote process could die and the pid could
get reused. But it is up to userspace to handle that properly. Binder
has a different mechanism than pids that userspace can use for a
trusted way of verifying credentials (see the 5th patch).

If this implementation of pid_in_current_ns actually takes a refcount
on the pid, then it is incorrect because it will leak the refcount in
that case.

Alice

2024-02-05 12:19:21

[permalink] [raw]

Subject: Re: [PATCH v4 8/9] rust: file: add `DeferredFdCloser`

On 2/2/24 11:55, Alice Ryhl wrote:
> To close an fd from kernel space, we could call `ksys_close`. However,
> if we do this to an fd that is held using `fdget`, then we may trigger a
> use-after-free. Introduce a helper that can be used to close an fd even
> if the fd is currently held with `fdget`. This is done by grabbing an
> extra refcount to the file and dropping it in a task work once we return
> to userspace.
>
> This is necessary for Rust Binder because otherwise the user might try
> to have Binder close its fd for /dev/binder, which would cause problems
> as this happens inside an ioctl on /dev/binder, and ioctls hold the fd
> using `fdget`.
>
> Additional motivation can be found in commit 80cd795630d6 ("binder: fix
> use-after-free due to ksys_close() during fdget()") and in the comments
> on `binder_do_fd_close`.
>
> If there is some way to detect whether an fd is currently held with
> `fdget`, then this could be optimized to skip the allocation and task
> work when this is not the case. Another possible optimization would be
> to combine several fds into a single task work, since this is used with
> fd arrays that might hold several fds.
>
> That said, it might not be necessary to optimize it, because Rust Binder
> has two ways to send fds: BINDER_TYPE_FD and BINDER_TYPE_FDA. With
> BINDER_TYPE_FD, it is userspace's responsibility to close the fd, so
> this mechanism is used only by BINDER_TYPE_FDA, but fd arrays are used
> rarely these days.
>
> Signed-off-by: Alice Ryhl <[email protected]>
> ---
> rust/bindings/bindings_helper.h | 2 +
> rust/helpers.c | 8 ++
> rust/kernel/file.rs | 184 +++++++++++++++++++++++++++++++-
> rust/kernel/task.rs | 14 +++
> 4 files changed, 207 insertions(+), 1 deletion(-)
>

Reviewed-by: Benno Lossin <[email protected]>

--
Cheers,
Benno

2024-02-05 12:21:14

[permalink] [raw]

Subject: Re: [PATCH v4 4/9] rust: cred: add Rust abstraction for `struct cred`

On 2/2/24 11:55, Alice Ryhl wrote:
> From: Wedson Almeida Filho <[email protected]>
>
> Add a wrapper around `struct cred` called `Credential`, and provide
> functionality to get the `Credential` associated with a `File`.
>
> Rust Binder must check the credentials of processes when they attempt to
> perform various operations, and these checks usually take a
> `&Credential` as parameter. The security_binder_set_context_mgr function
> would be one example. This patch is necessary to access these security_*
> methods from Rust.
>
> Signed-off-by: Wedson Almeida Filho <[email protected]>
> Co-developed-by: Alice Ryhl <[email protected]>
> Signed-off-by: Alice Ryhl <[email protected]>
> ---
> rust/bindings/bindings_helper.h | 1 +
> rust/helpers.c | 13 ++++++
> rust/kernel/cred.rs | 72 +++++++++++++++++++++++++++++++++
> rust/kernel/file.rs | 13 ++++++
> rust/kernel/lib.rs | 1 +
> 5 files changed, 100 insertions(+)
> create mode 100644 rust/kernel/cred.rs

Reviewed-by: Benno Lossin <[email protected]>

--
Cheers,
Benno

2024-02-05 12:28:34

[permalink] [raw]

Subject: Re: [PATCH v4 9/9] rust: file: add abstraction for `poll_table`

On 2/2/24 11:55, Alice Ryhl wrote:
> The existing `CondVar` abstraction is a wrapper around
> `wait_queue_head`, but it does not support all use-cases of the C
> `wait_queue_head` type. To be specific, a `CondVar` cannot be registered
> with a `struct poll_table`. This limitation has the advantage that you
> do not need to call `synchronize_rcu` when destroying a `CondVar`.
>
> However, we need the ability to register a `poll_table` with a
> `wait_queue_head` in Rust Binder. To enable this, introduce a type
> called `PollCondVar`, which is like `CondVar` except that you can
> register a `poll_table`. We also introduce `PollTable`, which is a safe
> wrapper around `poll_table` that is intended to be used with
> `PollCondVar`.
>
> The destructor of `PollCondVar` unconditionally calls `synchronize_rcu`
> to ensure that the removal of epoll waiters has fully completed before
> the `wait_queue_head` is destroyed.
>
> That said, `synchronize_rcu` is rather expensive and is not needed in
> all cases: If we have never registered a `poll_table` with the
> `wait_queue_head`, then we don't need to call `synchronize_rcu`. (And
> this is a common case in Binder - not all processes use Binder with
> epoll.) The current implementation does not account for this, but if we
> find that it is necessary to improve this, a future patch could store a
> boolean next to the `wait_queue_head` to keep track of whether a
> `poll_table` has ever been registered.
>
> Signed-off-by: Alice Ryhl <[email protected]>
> ---
> rust/bindings/bindings_helper.h | 1 +
> rust/kernel/sync.rs | 1 +
> rust/kernel/sync/poll.rs | 117 ++++++++++++++++++++++++++++++++
> 3 files changed, 119 insertions(+)
> create mode 100644 rust/kernel/sync/poll.rs

Reviewed-by: Benno Lossin <[email protected]>

--
Cheers,
Benno

2024-02-05 12:34:04

[permalink] [raw]

Subject: Re: [PATCH v4 3/9] rust: file: add Rust abstraction for `struct file`

On 2/2/24 11:55, Alice Ryhl wrote:
> From: Wedson Almeida Filho <[email protected]>
>
> This abstraction makes it possible to manipulate the open files for a
> process. The new `File` struct wraps the C `struct file`. When accessing
> it using the smart pointer `ARef<File>`, the pointer will own a
> reference count to the file. When accessing it as `&File`, then the
> reference does not own a refcount, but the borrow checker will ensure
> that the reference count does not hit zero while the `&File` is live.
>
> Since this is intended to manipulate the open files of a process, we
> introduce an `fget` constructor that corresponds to the C `fget`
> method. In future patches, it will become possible to create a new fd in
> a process and bind it to a `File`. Rust Binder will use these to send
> fds from one process to another.
>
> We also provide a method for accessing the file's flags. Rust Binder
> will use this to access the flags of the Binder fd to check whether the
> non-blocking flag is set, which affects what the Binder ioctl does.
>
> This introduces a struct for the EBADF error type, rather than just
> using the Error type directly. This has two advantages:
> * `File::from_fd` returns a `Result<ARef<File>, BadFdError>`, which the
> compiler will represent as a single pointer, with null being an error.
> This is possible because the compiler understands that `BadFdError`
> has only one possible value, and it also understands that the
> `ARef<File>` smart pointer is guaranteed non-null.
> * Additionally, we promise to users of the method that the method can
> only fail with EBADF, which means that they can rely on this promise
> without having to inspect its implementation.
> That said, there are also two disadvantages:
> * Defining additional error types involves boilerplate.
> * The question mark operator will only utilize the `From` trait once,
> which prevents you from using the question mark operator on
> `BadFdError` in methods that return some third error type that the
> kernel `Error` is convertible into. (However, it works fine in methods
> that return `Error`.)
>
> Signed-off-by: Wedson Almeida Filho <[email protected]>
> Co-developed-by: Daniel Xu <[email protected]>
> Signed-off-by: Daniel Xu <[email protected]>
> Co-developed-by: Alice Ryhl <[email protected]>
> Signed-off-by: Alice Ryhl <[email protected]>
> ---
> fs/file.c | 7 +
> rust/bindings/bindings_helper.h | 2 +
> rust/helpers.c | 7 +
> rust/kernel/file.rs | 249 ++++++++++++++++++++++++++++++++
> rust/kernel/lib.rs | 1 +
> 5 files changed, 266 insertions(+)
> create mode 100644 rust/kernel/file.rs

Reviewed-by: Benno Lossin <[email protected]>

--
Cheers,
Benno

2024-02-05 22:24:20

by Valentin Obst

[permalink] [raw]

Subject: Re: [PATCH v4 7/9] rust: file: add `Kuid` wrapper

> > + /// Returns the given task's pid in the current pid namespace.
> > + pub fn pid_in_current_ns(&self) -> Pid {
> > + let current = Task::current_raw();
> > + // SAFETY: Calling `task_active_pid_ns` with the current task is always safe.
> > + let namespace = unsafe { bindings::task_active_pid_ns(current) };
> > + // SAFETY: We know that `self.0.get()` is valid by the type invariant, and the namespace
> > + // pointer is not dangling since it points at this task's namespace.
> > + unsafe { bindings::task_tgid_nr_ns(self.0.get(), namespace) }
> > + }
>
> pids are reference counted in the kernel, how does this deal with that?
> Are they just ignored somehow? Where is the reference count given back?
>
> thanks,
>
> greg k-h

As far as I can see, neither `task_active_pid_ns` nor `task_active_pid_ns`
return with an incremented refcount. However, looking at the above code,
it looks like it could be simplified to:

```rust
pub fn pid_in_current_ns(&self) -> Pid {
// SAFETY: We know that `self.0.get()` is valid by the type invariant.
// Passing a null pointer in the second argument defaults to the current ns.
unsafe { bindings::task_tgid_nr_ns(self.0.get(), ptr::null_mut()) }
}
```

Since:

```C
static inline pid_t task_tgid_nr_ns(struct task_struct *tsk, struct pid_namespace *ns)
{
return __task_pid_nr_ns(tsk, PIDTYPE_TGID, ns);
}
..
pid_t __task_pid_nr_ns(struct task_struct *task, enum pid_type type,
struct pid_namespace *ns)
{
pid_t nr = 0;

rcu_read_lock();
if (!ns)
ns = task_active_pid_ns(current);
nr = pid_nr_ns(rcu_dereference(*task_pid_ptr(task, type)), ns);
rcu_read_unlock();

return nr;
}
EXPORT_SYMBOL(__task_pid_nr_ns);
```

anyway defaults to current's pid ns (plus some RCU lock protection, not sure if
that is relevant here).

- Best Valentin

2024-02-06 02:14:45

[permalink] [raw]

Subject: Re: [PATCH v4 1/9] rust: types: add `NotThreadSafe`

On Fri, Feb 2, 2024 at 5:56 AM Alice Ryhl <[email protected]> wrote:
>
> This introduces a new marker type for types that shouldn't be thread
> safe. By adding a field of this type to a struct, it becomes non-Send
> and non-Sync, which means that it cannot be accessed in any way from
> threads other than the one it was created on.
>
> This is useful for APIs that require globals such as `current` to remain
> constant while the value exists.
>
> We update two existing users in the Kernel to use this helper:
>
> * `Task::current()` - moving the return type of this value to a
> different thread would not be safe as you can no longer be guaranteed
> that the `current` pointer remains valid.
> * Lock guards. Mutexes and spinlocks should be unlocked on the same
> thread as where they were locked, so we enforce this using the Send
> trait.
>
> There are also additional users in later patches of this patchset. See
> [1] and [2] for the discussion that led to the introducion of this

s/introducion/introduction

> patch.
>
> Link: https://lore.kernel.org/all/nFDPJFnzE9Q5cqY7FwSMByRH2OAn_BpI4H53NQfWIlN6I2qfmAqnkp2wRqn0XjMO65OyZY4h6P4K2nAGKJpAOSzksYXaiAK_FoH_8QbgBI4=@proton.me/ [1]
> Link: https://lore.kernel.org/all/nFDPJFnzE9Q5cqY7FwSMByRH2OAn_BpI4H53NQfWIlN6I2qfmAqnkp2wRqn0XjMO65OyZY4h6P4K2nAGKJpAOSzksYXaiAK_FoH_8QbgBI4=@proton.me/ [2]
> Suggested-by: Benno Lossin <[email protected]>
> Reviewed-by: Benno Lossin <[email protected]>
> Signed-off-by: Alice Ryhl <[email protected]>
> ---
> rust/kernel/sync/lock.rs | 15 +++++++++++----
> rust/kernel/task.rs | 10 ++++++----
> rust/kernel/types.rs | 17 +++++++++++++++++
> 3 files changed, 34 insertions(+), 8 deletions(-)
>
> diff --git a/rust/kernel/sync/lock.rs b/rust/kernel/sync/lock.rs
> index 149a5259d431..d08d9d32915a 100644
> --- a/rust/kernel/sync/lock.rs
> +++ b/rust/kernel/sync/lock.rs
> @@ -6,8 +6,15 @@
> //! spinlocks, raw spinlocks) to be provided with minimal effort.
>
> use super::LockClassKey;
> -use crate::{bindings, init::PinInit, pin_init, str::CStr, types::Opaque, types::ScopeGuard};
> -use core::{cell::UnsafeCell, marker::PhantomData, marker::PhantomPinned};
> +use crate::{
> + bindings,
> + init::PinInit,
> + pin_init,
> + str::CStr,
> + types::ScopeGuard,
> + types::{NotThreadSafe, Opaque},

Formatting nit: ScopeGuard could probably be placed in the same group
as NotThreadSafe & Opaque

> [...]
> +
> +/// Zero-sized type to mark types not [`Send`].
> +///
> +/// Add this type as a field to your struct if your type should not be sent to a different task.
> +/// Since [`Send`] is an auto trait, adding a single field that is `!Send` will ensure that the
> +/// whole type is `!Send`.
> +///
> +/// If a type is `!Send` it is impossible to give control over an instance of the type to another
> +/// task. This is useful when a type stores task-local information for example file descriptors.

I initially read this thinking it meant to include this type if your
struct also had a FD rather than being part of the FD. Maybe

This is useful to include in types that store or reference task-local
information. A file descriptor is an example of one such type.

> +pub type NotThreadSafe = PhantomData<*mut ()>;
> +
> +/// Used to construct instances of type [`NotThreadSafe`] similar to how we construct
> +/// `PhantomData`.

I think it sounds slightly better reworded from personal to passive, i.e.

... similar to how `PhantomData` is constructed.

> +/// [`NotThreadSafe`]: type@NotThreadSafe
> +#[allow(non_upper_case_globals)]
> +pub const NotThreadSafe: NotThreadSafe = PhantomData;
> --
> 2.43.0.594.gd9cf4e227d-goog

This looks good, sounds nice to make the intent more clear. Nothing
that isn't optional, so

Reviewed-by: Trevor Gross <[email protected]>

2024-02-06 02:18:45

[permalink] [raw]

Subject: Re: [PATCH v4 2/9] rust: task: add `Task::current_raw`

On Fri, Feb 2, 2024 at 5:56 AM Alice Ryhl <[email protected]> wrote:
>
> Introduces a safe function for getting a raw pointer to the current
> task.
>
> When writing bindings that need to access the current task, it is often
> more convenient to call a method that directly returns a raw pointer
> than to use the existing `Task::current` method. However, the only way
> to do that is `bindings::get_current()` which is unsafe since it calls
> into C. By introducing `Task::current_raw()`, it becomes possible to
> obtain a pointer to the current task without using unsafe.
>
> Link: https://lore.kernel.org/all/CAH5fLgjT48X-zYtidv31mox3C4_Ogoo_2cBOCmX0Ang3tAgGHA@mail.gmail.com/
> Reviewed-by: Benno Lossin <[email protected]>
> Signed-off-by: Alice Ryhl <[email protected]>
> ---

Reviewed-by: Trevor Gross <[email protected]>

2024-02-06 02:48:42

[permalink] [raw]

Subject: Re: [PATCH v4 3/9] rust: file: add Rust abstraction for `struct file`

On Fri, Feb 2, 2024 at 5:56 AM Alice Ryhl <[email protected]> wrote:
>
> From: Wedson Almeida Filho <[email protected]>
>
> This abstraction makes it possible to manipulate the open files for a
> process. The new `File` struct wraps the C `struct file`. When accessing
> it using the smart pointer `ARef<File>`, the pointer will own a
> reference count to the file. When accessing it as `&File`, then the
> reference does not own a refcount, but the borrow checker will ensure
> that the reference count does not hit zero while the `&File` is live.
>
> Since this is intended to manipulate the open files of a process, we
> introduce an `fget` constructor that corresponds to the C `fget`
> method. In future patches, it will become possible to create a new fd in
> a process and bind it to a `File`. Rust Binder will use these to send
> fds from one process to another.
>
> We also provide a method for accessing the file's flags. Rust Binder
> will use this to access the flags of the Binder fd to check whether the
> non-blocking flag is set, which affects what the Binder ioctl does.
>
> This introduces a struct for the EBADF error type, rather than just
> using the Error type directly. This has two advantages:
> * `File::from_fd` returns a `Result<ARef<File>, BadFdError>`, which the
> compiler will represent as a single pointer, with null being an error.
> This is possible because the compiler understands that `BadFdError`
> has only one possible value, and it also understands that the
> `ARef<File>` smart pointer is guaranteed non-null.
> * Additionally, we promise to users of the method that the method can
> only fail with EBADF, which means that they can rely on this promise
> without having to inspect its implementation.
> That said, there are also two disadvantages:
> * Defining additional error types involves boilerplate.
> * The question mark operator will only utilize the `From` trait once,
> which prevents you from using the question mark operator on
> `BadFdError` in methods that return some third error type that the
> kernel `Error` is convertible into. (However, it works fine in methods
> that return `Error`.)
>
> Signed-off-by: Wedson Almeida Filho <[email protected]>
> Co-developed-by: Daniel Xu <[email protected]>
> Signed-off-by: Daniel Xu <[email protected]>
> Co-developed-by: Alice Ryhl <[email protected]>
> Signed-off-by: Alice Ryhl <[email protected]>
> ---
> fs/file.c | 7 +
> rust/bindings/bindings_helper.h | 2 +
> rust/helpers.c | 7 +
> rust/kernel/file.rs | 249 ++++++++++++++++++++++++++++++++
> rust/kernel/lib.rs | 1 +
> 5 files changed, 266 insertions(+)
> create mode 100644 rust/kernel/file.rs
>
> diff --git a/fs/file.c b/fs/file.c
> index 3b683b9101d8..f2eab5fcb87f 100644
> --- a/fs/file.c
> +++ b/fs/file.c
> @@ -1115,18 +1115,25 @@ EXPORT_SYMBOL(task_lookup_next_fdget_rcu);
> /*
> * Lightweight file lookup - no refcnt increment if fd table isn't shared.
> *
> * You can use this instead of fget if you satisfy all of the following
> * conditions:
> * 1) You must call fput_light before exiting the syscall and returning control
> * to userspace (i.e. you cannot remember the returned struct file * after
> * returning to userspace).
> * 2) You must not call filp_close on the returned struct file * in between
> * calls to fget_light and fput_light.
> * 3) You must not clone the current task in between the calls to fget_light
> * and fput_light.
> *
> * The fput_needed flag returned by fget_light should be passed to the
> * corresponding fput_light.
> + *
> + * (As an exception to rule 2, you can call filp_close between fget_light and
> + * fput_light provided that you capture a real refcount with get_file before
> + * the call to filp_close, and ensure that this real refcount is fput *after*
> + * the fput_light call.)
> + *
> + * See also the documentation in rust/kernel/file.rs.
> */
> static unsigned long __fget_light(unsigned int fd, fmode_t mask)
> {

Should this be split to its own patch so it can be applied separately if needed?

> [...]
> + /// Also known as `O_NDELAY`.
> + ///
> + /// This is effectively the same flag as [`O_NONBLOCK`] on all architectures
> + /// except SPARC64.
> + pub const O_NDELAY: u32 = bindings::O_NDELAY;

This is O_NDELAY, should the AKA say O_NONBLOCK?
> [...]
> +/// Wraps the kernel's `struct file`.

It is probably better to say what it does for the summary, and mention
what it wraps later.

> +/// # Refcounting
> +///
> +/// Instances of this type are reference-counted. The reference count is incremented by the
> +/// `fget`/`get_file` functions and decremented by `fput`. The Rust type `ARef<File>` represents a
> +/// pointer that owns a reference count on the file.
> +///
> +/// Whenever a process opens a file descriptor (fd), it stores a pointer to the file in its `struct
> +/// files_struct`. This pointer owns a reference count to the file, ensuring the file isn't
> +/// prematurely deleted while the file descriptor is open. In Rust terminology, the pointers in
> +/// `struct files_struct` are `ARef<File>` pointers.
> +///
> +/// ## Light refcounts
> +///
> +/// Whenever a process has an fd to a file, it may use something called a "light refcount" as a
> +/// performance optimization. Light refcounts are acquired by calling `fdget` and released with
> +/// `fdput`. The idea behind light refcounts is that if the fd is not closed between the calls to
> +/// `fdget` and `fdput`, then the refcount cannot hit zero during that time, as the `struct
> +/// files_struct` holds a reference until the fd is closed. This means that it's safe to access the
> +/// file even if `fdget` does not increment the refcount.
> +///
> +/// The requirement that the fd is not closed during a light refcount applies globally across all
> +/// threads - not just on the thread using the light refcount. For this reason, light refcounts are
> +/// only used when the `struct files_struct` is not shared with other threads, since this ensures
> +/// that other unrelated threads cannot suddenly start using the fd and close it. Therefore,
> +/// calling `fdget` on a shared `struct files_struct` creates a normal refcount instead of a light
> +/// refcount.
> +///
> +/// Light reference counts must be released with `fdput` before the system call returns to
> +/// userspace. This means that if you wait until the current system call returns to userspace, then
> +/// all light refcounts that existed at the time have gone away.
> +///
> +/// ## Rust references
> +///
> +/// The reference type `&File` is similar to light refcounts:
> +///
> +/// * `&File` references don't own a reference count. They can only exist as long as the reference
> +/// count stays positive, and can only be created when there is some mechanism in place to ensure
> +/// this.
> +///
> +/// * The Rust borrow-checker normally ensures this by enforcing that the `ARef<File>` from which
> +/// a `&File` is created outlives the `&File`.
> +///
> +/// * Using the unsafe [`File::from_ptr`] means that it is up to the caller to ensure that the
> +/// `&File` only exists while the reference count is positive.
> +///
> +/// * You can think of `fdget` as using an fd to look up an `ARef<File>` in the `struct
> +/// files_struct` and create an `&File` from it. The "fd cannot be closed" rule is like the Rust
> +/// rule "the `ARef<File>` must outlive the `&File`".
> +///
> +/// # Invariants
> +///
> +/// * Instances of this type are refcounted using the `f_count` field.
> +/// * If an fd with active light refcounts is closed, then it must be the case that the file
> +/// refcount is positive until all light refcounts of the fd have been dropped.
> +/// * A light refcount must be dropped before returning to userspace.
> +#[repr(transparent)]
> +pub struct File(Opaque<bindings::file>);
> +
> +// SAFETY:
> +// - `File::dec_ref` can be called from any thread.
> +// - It is okay to send ownership of `File` across thread boundaries.

Shouldn't this be lowecase `file` because it is referring to the
underlying C object?

> +unsafe impl Send for File {}
> [...]
> + /// Returns the flags associated with the file.
> + ///
> + /// The flags are a combination of the constants in [`flags`].
> + pub fn flags(&self) -> u32 {

A typedef used here and in the constants module could be useful

type FileFlags = u32;

> + // This `read_volatile` is intended to correspond to a READ_ONCE call.
> + //
> + // SAFETY: The file is valid because the shared reference guarantees a nonzero refcount.
> + //
> + // TODO: Replace with `read_once` when available on the Rust side.

Shouldn't the TODO become a `FIXME(read_once): ...` since it is going
into the codebase?

> + unsafe { core::ptr::addr_of!((*self.as_ptr()).f_flags).read_volatile() }
> + }
> +}

Some suggestions but nothing blocking

Reviewed-by: Trevor Gross <[email protected]>

2024-02-06 03:00:49

[permalink] [raw]

Subject: Re: [PATCH v4 4/9] rust: cred: add Rust abstraction for `struct cred`

On Fri, Feb 2, 2024 at 5:57 AM Alice Ryhl <[email protected]> wrote:
>
> From: Wedson Almeida Filho <[email protected]>
>
> Add a wrapper around `struct cred` called `Credential`, and provide
> functionality to get the `Credential` associated with a `File`.
>
> Rust Binder must check the credentials of processes when they attempt to
> perform various operations, and these checks usually take a
> `&Credential` as parameter. The security_binder_set_context_mgr function
> would be one example. This patch is necessary to access these security_*
> methods from Rust.
>
> Signed-off-by: Wedson Almeida Filho <[email protected]>
> Co-developed-by: Alice Ryhl <[email protected]>
> Signed-off-by: Alice Ryhl <[email protected]>
> ---
> [...]
> diff --git a/rust/kernel/cred.rs b/rust/kernel/cred.rs
> new file mode 100644
> index 000000000000..fabc50e48c9e
> --- /dev/null
> +++ b/rust/kernel/cred.rs
> @@ -0,0 +1,72 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +//! Credentials management.
> +//!
> +//! C header: [`include/linux/cred.h`](srctree/include/linux/cred.h).
> +//!
> +//! Reference: <https://www.kernel.org/doc/html/latest/security/credentials.html>
> +
> +use crate::{
> + bindings,
> + types::{AlwaysRefCounted, Opaque},
> +};
> +
> +/// Wraps the kernel's `struct cred`.

Could you add a brief description to the summary line?

> +/// Most fields of credentials are immutable. When things have their credentials changed, that
> +/// happens by replacing the credential instad of changing an existing credential. See the [kernel

s/instad/instead

> [...]

Reviewed-by: Trevor Gross <[email protected]>

2024-02-06 03:05:35

[permalink] [raw]

Subject: Re: [PATCH v4 5/9] rust: security: add abstraction for secctx

On Fri, Feb 2, 2024 at 5:57 AM Alice Ryhl <[email protected]> wrote:
>
> Adds an abstraction for viewing the string representation of a security
> context.

Adds -> add

> This is needed by Rust Binder because it has feature where a process can

has feature -> has a feature

> view the string representation of the security context for incoming
> transactions. The process can use that to authenticate incoming
> transactions, and since the feature is provided by the kernel, the
> process can trust that the security context is legitimate.
>
> Reviewed-by: Benno Lossin <[email protected]>
> Signed-off-by: Alice Ryhl <[email protected]>
> ---
> [...]
> diff --git a/rust/kernel/cred.rs b/rust/kernel/cred.rs
> index fabc50e48c9e..0640356a8c29 100644
> --- a/rust/kernel/cred.rs
> +++ b/rust/kernel/cred.rs
> @@ -48,6 +48,14 @@ pub unsafe fn from_ptr<'a>(ptr: *const bindings::cred) -> &'a Credential {
> unsafe { &*ptr.cast() }
> }
>
> + /// Get the id for this security context.
> + pub fn get_secid(&self) -> u32 {

Since this is used in multiple places, would a typedef be useful?

type SecurityId = u32;

> [...]

Reviewed-by: Trevor Gross <[email protected]>

2024-02-08 00:35:14

[permalink] [raw]

Subject: Re: [PATCH v4 1/9] rust: types: add `NotThreadSafe`

On 2/2/24 07:55, Alice Ryhl wrote:
> This introduces a new marker type for types that shouldn't be thread
> safe. By adding a field of this type to a struct, it becomes non-Send
> and non-Sync, which means that it cannot be accessed in any way from
> threads other than the one it was created on.
>
> This is useful for APIs that require globals such as `current` to remain
> constant while the value exists.
>
> We update two existing users in the Kernel to use this helper:
>
> * `Task::current()` - moving the return type of this value to a
> different thread would not be safe as you can no longer be guaranteed
> that the `current` pointer remains valid.
> * Lock guards. Mutexes and spinlocks should be unlocked on the same
> thread as where they were locked, so we enforce this using the Send
> trait.
>
> There are also additional users in later patches of this patchset. See
> [1] and [2] for the discussion that led to the introducion of this
> patch.
>
> Link: https://lore.kernel.org/all/nFDPJFnzE9Q5cqY7FwSMByRH2OAn_BpI4H53NQfWIlN6I2qfmAqnkp2wRqn0XjMO65OyZY4h6P4K2nAGKJpAOSzksYXaiAK_FoH_8QbgBI4=@proton.me/ [1]
> Link: https://lore.kernel.org/all/nFDPJFnzE9Q5cqY7FwSMByRH2OAn_BpI4H53NQfWIlN6I2qfmAqnkp2wRqn0XjMO65OyZY4h6P4K2nAGKJpAOSzksYXaiAK_FoH_8QbgBI4=@proton.me/ [2]
> Suggested-by: Benno Lossin <[email protected]>
> Reviewed-by: Benno Lossin <[email protected]>
> Signed-off-by: Alice Ryhl <[email protected]>
> ---
> [...]

Reviewed-by: Martin Rodriguez Reboredo <[email protected]>

2024-02-08 00:36:59

[permalink] [raw]

Subject: Re: [PATCH v4 6/9] rust: file: add `FileDescriptorReservation`

On 2/2/24 07:55, Alice Ryhl wrote:
> From: Wedson Almeida Filho <[email protected]>
>
> Allow for the creation of a file descriptor in two steps: first, we
> reserve a slot for it, then we commit or drop the reservation. The first
> step may fail (e.g., the current process ran out of available slots),
> but commit and drop never fail (and are mutually exclusive).
>
> This is needed by Rust Binder when fds are sent from one process to
> another. It has to be a two-step process to properly handle the case
> where multiple fds are sent: The operation must fail or succeed
> atomically, which we achieve by first reserving the fds we need, and
> only installing the files once we have reserved enough fds to send the
> files.
>
> Fd reservations assume that the value of `current` does not change
> between the call to get_unused_fd_flags and the call to fd_install (or
> put_unused_fd). By not implementing the Send trait, this abstraction
> ensures that the `FileDescriptorReservation` cannot be moved into a
> different process.
>
> Signed-off-by: Wedson Almeida Filho <[email protected]>
> Co-developed-by: Alice Ryhl <[email protected]>
> Reviewed-by: Benno Lossin <[email protected]>
> Signed-off-by: Alice Ryhl <[email protected]>
> ---
> [...]

Reviewed-by: Martin Rodriguez Reboredo <[email protected]>

2024-02-08 00:37:13

[permalink] [raw]

Subject: Re: [PATCH v4 2/9] rust: task: add `Task::current_raw`

On 2/2/24 07:55, Alice Ryhl wrote:
> Introduces a safe function for getting a raw pointer to the current
> task.
>
> When writing bindings that need to access the current task, it is often
> more convenient to call a method that directly returns a raw pointer
> than to use the existing `Task::current` method. However, the only way
> to do that is `bindings::get_current()` which is unsafe since it calls
> into C. By introducing `Task::current_raw()`, it becomes possible to
> obtain a pointer to the current task without using unsafe.
>
> Link: https://lore.kernel.org/all/CAH5fLgjT48X-zYtidv31mox3C4_Ogoo_2cBOCmX0Ang3tAgGHA@mail.gmail.com/
> Reviewed-by: Benno Lossin <[email protected]>
> Signed-off-by: Alice Ryhl <[email protected]>
> ---
> [...]

Reviewed-by: Martin Rodriguez Reboredo <[email protected]>

2024-02-08 00:37:23

[permalink] [raw]

Subject: Re: [PATCH v4 7/9] rust: file: add `Kuid` wrapper

On 2/2/24 07:55, Alice Ryhl wrote:
> Adds a wrapper around `kuid_t` called `Kuid`. This allows us to define
> various operations on kuids such as equality and current_euid. It also
> lets us provide conversions from kuid into userspace values.
>
> Rust Binder needs these operations because it needs to compare kuids for
> equality, and it needs to tell userspace about the pid and uid of
> incoming transactions.
>
> To read kuids from a `struct task_struct`, you must currently use
> various #defines that perform the appropriate field access under an RCU
> read lock. Currently, we do not have a Rust wrapper for rcu_read_lock,
> which means that for this patch, there are two ways forward:
>
> 1. Inline the methods into Rust code, and use __rcu_read_lock directly
> rather than the rcu_read_lock wrapper. This gives up lockdep for
> these usages of RCU.
>
> 2. Wrap the various #defines in helpers and call the helpers from Rust.
>
> This patch uses the second option. One possible disadvantage of the
> second option is the possible introduction of speculation gadgets, but
> as discussed in [1], the risk appears to be acceptable.
>
> Of course, once a wrapper for rcu_read_lock is available, it is
> preferable to use that over either of the two above approaches.
>
> Link: https://lore.kernel.org/all/202312080947.674CD2DC7@keescook/ [1]
> Reviewed-by: Benno Lossin <[email protected]>
> Signed-off-by: Alice Ryhl <[email protected]>
> ---
> [...]

Reviewed-by: Martin Rodriguez Reboredo <[email protected]>

2024-02-08 00:37:38

[permalink] [raw]

Subject: Re: [PATCH v4 3/9] rust: file: add Rust abstraction for `struct file`

On 2/2/24 07:55, Alice Ryhl wrote:
> From: Wedson Almeida Filho <[email protected]>
>
> This abstraction makes it possible to manipulate the open files for a
> process. The new `File` struct wraps the C `struct file`. When accessing
> it using the smart pointer `ARef<File>`, the pointer will own a
> reference count to the file. When accessing it as `&File`, then the
> reference does not own a refcount, but the borrow checker will ensure
> that the reference count does not hit zero while the `&File` is live.
>
> Since this is intended to manipulate the open files of a process, we
> introduce an `fget` constructor that corresponds to the C `fget`
> method. In future patches, it will become possible to create a new fd in
> a process and bind it to a `File`. Rust Binder will use these to send
> fds from one process to another.
>
> We also provide a method for accessing the file's flags. Rust Binder
> will use this to access the flags of the Binder fd to check whether the
> non-blocking flag is set, which affects what the Binder ioctl does.
>
> This introduces a struct for the EBADF error type, rather than just
> using the Error type directly. This has two advantages:
> * `File::from_fd` returns a `Result<ARef<File>, BadFdError>`, which the
> compiler will represent as a single pointer, with null being an error.
> This is possible because the compiler understands that `BadFdError`
> has only one possible value, and it also understands that the
> `ARef<File>` smart pointer is guaranteed non-null.
> * Additionally, we promise to users of the method that the method can
> only fail with EBADF, which means that they can rely on this promise
> without having to inspect its implementation.
> That said, there are also two disadvantages:
> * Defining additional error types involves boilerplate.
> * The question mark operator will only utilize the `From` trait once,
> which prevents you from using the question mark operator on
> `BadFdError` in methods that return some third error type that the
> kernel `Error` is convertible into. (However, it works fine in methods
> that return `Error`.)
>
> Signed-off-by: Wedson Almeida Filho <[email protected]>
> Co-developed-by: Daniel Xu <[email protected]>
> Signed-off-by: Daniel Xu <[email protected]>
> Co-developed-by: Alice Ryhl <[email protected]>
> Signed-off-by: Alice Ryhl <[email protected]>
> ---
> [...]
> +/// ## Rust references
> +///
> +/// The reference type `&File` is similar to light refcounts:
> +///
> +/// * `&File` references don't own a reference count. They can only exist as long as the reference
> +/// count stays positive, and can only be created when there is some mechanism in place to ensure
> +/// this.
> +///
> +/// * The Rust borrow-checker normally ensures this by enforcing that the `ARef<File>` from which
> +/// a `&File` is created outlives the `&File`.
> +///
> +/// * Using the unsafe [`File::from_ptr`] means that it is up to the caller to ensure that the
> +/// `&File` only exists while the reference count is positive.
> +///
> +/// * You can think of `fdget` as using an fd to look up an `ARef<File>` in the `struct
> +/// files_struct` and create an `&File` from it. The "fd cannot be closed" rule is like the Rust
> +/// rule "the `ARef<File>` must outlive the `&File`".

I find it kinda odd that this unordered list interspaces elements with
blank lines as opposed to the following one, though, I don't see it as
rather a big deal.

> +///
> +/// # Invariants
> +///
> +/// * Instances of this type are refcounted using the `f_count` field.
> +/// * If an fd with active light refcounts is closed, then it must be the case that the file
> +/// refcount is positive until all light refcounts of the fd have been dropped.
> +/// * A light refcount must be dropped before returning to userspace.
> [...]

Reviewed-by: Martin Rodriguez Reboredo <[email protected]>

2024-02-08 00:37:45

[permalink] [raw]

Subject: Re: [PATCH v4 8/9] rust: file: add `DeferredFdCloser`

On 2/2/24 07:55, Alice Ryhl wrote:
> To close an fd from kernel space, we could call `ksys_close`. However,
> if we do this to an fd that is held using `fdget`, then we may trigger a
> use-after-free. Introduce a helper that can be used to close an fd even
> if the fd is currently held with `fdget`. This is done by grabbing an
> extra refcount to the file and dropping it in a task work once we return
> to userspace.
>
> This is necessary for Rust Binder because otherwise the user might try
> to have Binder close its fd for /dev/binder, which would cause problems
> as this happens inside an ioctl on /dev/binder, and ioctls hold the fd
> using `fdget`.
>
> Additional motivation can be found in commit 80cd795630d6 ("binder: fix
> use-after-free due to ksys_close() during fdget()") and in the comments
> on `binder_do_fd_close`.
>
> If there is some way to detect whether an fd is currently held with
> `fdget`, then this could be optimized to skip the allocation and task
> work when this is not the case. Another possible optimization would be
> to combine several fds into a single task work, since this is used with
> fd arrays that might hold several fds.
>
> That said, it might not be necessary to optimize it, because Rust Binder
> has two ways to send fds: BINDER_TYPE_FD and BINDER_TYPE_FDA. With
> BINDER_TYPE_FD, it is userspace's responsibility to close the fd, so
> this mechanism is used only by BINDER_TYPE_FDA, but fd arrays are used
> rarely these days.
>
> Signed-off-by: Alice Ryhl <[email protected]>
> ---
> [...]

Reviewed-by: Martin Rodriguez Reboredo <[email protected]>

2024-02-08 00:37:59

[permalink] [raw]

Subject: Re: [PATCH v4 4/9] rust: cred: add Rust abstraction for `struct cred`

On 2/2/24 07:55, Alice Ryhl wrote:
> From: Wedson Almeida Filho <[email protected]>
>
> Add a wrapper around `struct cred` called `Credential`, and provide
> functionality to get the `Credential` associated with a `File`.
>
> Rust Binder must check the credentials of processes when they attempt to
> perform various operations, and these checks usually take a
> `&Credential` as parameter. The security_binder_set_context_mgr function
> would be one example. This patch is necessary to access these security_*
> methods from Rust.
>
> Signed-off-by: Wedson Almeida Filho <[email protected]>
> Co-developed-by: Alice Ryhl <[email protected]>
> Signed-off-by: Alice Ryhl <[email protected]>
> ---
> [...]
> + /// Returns the credentials of the task that originally opened the file.
> + pub fn cred(&self) -> &Credential {
> + // SAFETY: It's okay to read the `f_cred` field without synchronization because `f_cred` is
> + // never changed after initialization of the file.
> + let ptr = unsafe { (*self.as_ptr()).f_cred };

From what I can see `file.f_cred` is valid from the moment `file` was
initialized, worth to notice too IMO.

> +
> + // SAFETY: The signature of this function ensures that the caller will only access the
> + // returned credential while the file is still valid, and the C side ensures that the
> + // credential stays valid at least as long as the file.
> + unsafe { Credential::from_ptr(ptr) }
> + }
> [...]

Reviewed-by: Martin Rodriguez Reboredo <[email protected]>

2024-02-08 00:38:11

[permalink] [raw]

Subject: Re: [PATCH v4 9/9] rust: file: add abstraction for `poll_table`

On 2/2/24 07:55, Alice Ryhl wrote:
> The existing `CondVar` abstraction is a wrapper around
> `wait_queue_head`, but it does not support all use-cases of the C
> `wait_queue_head` type. To be specific, a `CondVar` cannot be registered
> with a `struct poll_table`. This limitation has the advantage that you
> do not need to call `synchronize_rcu` when destroying a `CondVar`.
>
> However, we need the ability to register a `poll_table` with a
> `wait_queue_head` in Rust Binder. To enable this, introduce a type
> called `PollCondVar`, which is like `CondVar` except that you can
> register a `poll_table`. We also introduce `PollTable`, which is a safe
> wrapper around `poll_table` that is intended to be used with
> `PollCondVar`.
>
> The destructor of `PollCondVar` unconditionally calls `synchronize_rcu`
> to ensure that the removal of epoll waiters has fully completed before
> the `wait_queue_head` is destroyed.
>
> That said, `synchronize_rcu` is rather expensive and is not needed in
> all cases: If we have never registered a `poll_table` with the
> `wait_queue_head`, then we don't need to call `synchronize_rcu`. (And
> this is a common case in Binder - not all processes use Binder with
> epoll.) The current implementation does not account for this, but if we
> find that it is necessary to improve this, a future patch could store a
> boolean next to the `wait_queue_head` to keep track of whether a
> `poll_table` has ever been registered.
>
> Signed-off-by: Alice Ryhl <[email protected]>
> ---
> [...]

Reviewed-by: Martin Rodriguez Reboredo <[email protected]>

2024-02-08 00:38:20

[permalink] [raw]

Subject: Re: [PATCH v4 5/9] rust: security: add abstraction for secctx

On 2/2/24 07:55, Alice Ryhl wrote:
> Adds an abstraction for viewing the string representation of a security
> context.
>
> This is needed by Rust Binder because it has feature where a process can
> view the string representation of the security context for incoming
> transactions. The process can use that to authenticate incoming
> transactions, and since the feature is provided by the kernel, the
> process can trust that the security context is legitimate.
>
> Reviewed-by: Benno Lossin <[email protected]>
> Signed-off-by: Alice Ryhl <[email protected]>
> ---
> [...]

Reviewed-by: Martin Rodriguez Reboredo <[email protected]>

2024-02-08 15:05:59

[permalink] [raw]

Subject: Re: [PATCH v4 3/9] rust: file: add Rust abstraction for `struct file`

On Tue, Feb 6, 2024 at 3:48 AM Trevor Gross <[email protected]> wrote:
>
> On Fri, Feb 2, 2024 at 5:56 AM Alice Ryhl <[email protected]> wrote:
> >
> > From: Wedson Almeida Filho <[email protected]>
> >
> > This abstraction makes it possible to manipulate the open files for a
> > process. The new `File` struct wraps the C `struct file`. When accessing
> > it using the smart pointer `ARef<File>`, the pointer will own a
> > reference count to the file. When accessing it as `&File`, then the
> > reference does not own a refcount, but the borrow checker will ensure
> > that the reference count does not hit zero while the `&File` is live.
> >
> > Since this is intended to manipulate the open files of a process, we
> > introduce an `fget` constructor that corresponds to the C `fget`
> > method. In future patches, it will become possible to create a new fd in
> > a process and bind it to a `File`. Rust Binder will use these to send
> > fds from one process to another.
> >
> > We also provide a method for accessing the file's flags. Rust Binder
> > will use this to access the flags of the Binder fd to check whether the
> > non-blocking flag is set, which affects what the Binder ioctl does.
> >
> > This introduces a struct for the EBADF error type, rather than just
> > using the Error type directly. This has two advantages:
> > * `File::from_fd` returns a `Result<ARef<File>, BadFdError>`, which the
> > compiler will represent as a single pointer, with null being an error.
> > This is possible because the compiler understands that `BadFdError`
> > has only one possible value, and it also understands that the
> > `ARef<File>` smart pointer is guaranteed non-null.
> > * Additionally, we promise to users of the method that the method can
> > only fail with EBADF, which means that they can rely on this promise
> > without having to inspect its implementation.
> > That said, there are also two disadvantages:
> > * Defining additional error types involves boilerplate.
> > * The question mark operator will only utilize the `From` trait once,
> > which prevents you from using the question mark operator on
> > `BadFdError` in methods that return some third error type that the
> > kernel `Error` is convertible into. (However, it works fine in methods
> > that return `Error`.)
> >
> > Signed-off-by: Wedson Almeida Filho <[email protected]>
> > Co-developed-by: Daniel Xu <[email protected]>
> > Signed-off-by: Daniel Xu <[email protected]>
> > Co-developed-by: Alice Ryhl <[email protected]>
> > Signed-off-by: Alice Ryhl <[email protected]>
> > ---
> > fs/file.c | 7 +
> > rust/bindings/bindings_helper.h | 2 +
> > rust/helpers.c | 7 +
> > rust/kernel/file.rs | 249 ++++++++++++++++++++++++++++++++
> > rust/kernel/lib.rs | 1 +
> > 5 files changed, 266 insertions(+)
> > create mode 100644 rust/kernel/file.rs
> >
> > diff --git a/fs/file.c b/fs/file.c
> > index 3b683b9101d8..f2eab5fcb87f 100644
> > --- a/fs/file.c
> > +++ b/fs/file.c
> > @@ -1115,18 +1115,25 @@ EXPORT_SYMBOL(task_lookup_next_fdget_rcu);
> > /*
> > * Lightweight file lookup - no refcnt increment if fd table isn't shared.
> > *
> > * You can use this instead of fget if you satisfy all of the following
> > * conditions:
> > * 1) You must call fput_light before exiting the syscall and returning control
> > * to userspace (i.e. you cannot remember the returned struct file * after
> > * returning to userspace).
> > * 2) You must not call filp_close on the returned struct file * in between
> > * calls to fget_light and fput_light.
> > * 3) You must not clone the current task in between the calls to fget_light
> > * and fput_light.
> > *
> > * The fput_needed flag returned by fget_light should be passed to the
> > * corresponding fput_light.
> > + *
> > + * (As an exception to rule 2, you can call filp_close between fget_light and
> > + * fput_light provided that you capture a real refcount with get_file before
> > + * the call to filp_close, and ensure that this real refcount is fput *after*
> > + * the fput_light call.)
> > + *
> > + * See also the documentation in rust/kernel/file.rs.
> > */
> > static unsigned long __fget_light(unsigned int fd, fmode_t mask)
> > {
>
> Should this be split to its own patch so it can be applied separately if needed?

I don't think that's necessary.

> > [...]
> > + /// Also known as `O_NDELAY`.
> > + ///
> > + /// This is effectively the same flag as [`O_NONBLOCK`] on all architectures
> > + /// except SPARC64.
> > + pub const O_NDELAY: u32 = bindings::O_NDELAY;
>
> This is O_NDELAY, should the AKA say O_NONBLOCK?
> > [...]
> > +/// Wraps the kernel's `struct file`.
>
> It is probably better to say what it does for the summary, and mention
> what it wraps later.

I added a bit more info.

> > +/// # Refcounting
> > +///
> > +/// Instances of this type are reference-counted. The reference count is incremented by the
> > +/// `fget`/`get_file` functions and decremented by `fput`. The Rust type `ARef<File>` represents a
> > +/// pointer that owns a reference count on the file.
> > +///
> > +/// Whenever a process opens a file descriptor (fd), it stores a pointer to the file in its `struct
> > +/// files_struct`. This pointer owns a reference count to the file, ensuring the file isn't
> > +/// prematurely deleted while the file descriptor is open. In Rust terminology, the pointers in
> > +/// `struct files_struct` are `ARef<File>` pointers.
> > +///
> > +/// ## Light refcounts
> > +///
> > +/// Whenever a process has an fd to a file, it may use something called a "light refcount" as a
> > +/// performance optimization. Light refcounts are acquired by calling `fdget` and released with
> > +/// `fdput`. The idea behind light refcounts is that if the fd is not closed between the calls to
> > +/// `fdget` and `fdput`, then the refcount cannot hit zero during that time, as the `struct
> > +/// files_struct` holds a reference until the fd is closed. This means that it's safe to access the
> > +/// file even if `fdget` does not increment the refcount.
> > +///
> > +/// The requirement that the fd is not closed during a light refcount applies globally across all
> > +/// threads - not just on the thread using the light refcount. For this reason, light refcounts are
> > +/// only used when the `struct files_struct` is not shared with other threads, since this ensures
> > +/// that other unrelated threads cannot suddenly start using the fd and close it. Therefore,
> > +/// calling `fdget` on a shared `struct files_struct` creates a normal refcount instead of a light
> > +/// refcount.
> > +///
> > +/// Light reference counts must be released with `fdput` before the system call returns to
> > +/// userspace. This means that if you wait until the current system call returns to userspace, then
> > +/// all light refcounts that existed at the time have gone away.
> > +///
> > +/// ## Rust references
> > +///
> > +/// The reference type `&File` is similar to light refcounts:
> > +///
> > +/// * `&File` references don't own a reference count. They can only exist as long as the reference
> > +/// count stays positive, and can only be created when there is some mechanism in place to ensure
> > +/// this.
> > +///
> > +/// * The Rust borrow-checker normally ensures this by enforcing that the `ARef<File>` from which
> > +/// a `&File` is created outlives the `&File`.
> > +///
> > +/// * Using the unsafe [`File::from_ptr`] means that it is up to the caller to ensure that the
> > +/// `&File` only exists while the reference count is positive.
> > +///
> > +/// * You can think of `fdget` as using an fd to look up an `ARef<File>` in the `struct
> > +/// files_struct` and create an `&File` from it. The "fd cannot be closed" rule is like the Rust
> > +/// rule "the `ARef<File>` must outlive the `&File`".
> > +///
> > +/// # Invariants
> > +///
> > +/// * Instances of this type are refcounted using the `f_count` field.
> > +/// * If an fd with active light refcounts is closed, then it must be the case that the file
> > +/// refcount is positive until all light refcounts of the fd have been dropped.
> > +/// * A light refcount must be dropped before returning to userspace.
> > +#[repr(transparent)]
> > +pub struct File(Opaque<bindings::file>);
> > +
> > +// SAFETY:
> > +// - `File::dec_ref` can be called from any thread.
> > +// - It is okay to send ownership of `File` across thread boundaries.
>
> Shouldn't this be lowecase `file` because it is referring to the
> underlying C object?

Done.

> > +unsafe impl Send for File {}
> > [...]
> > + /// Returns the flags associated with the file.
> > + ///
> > + /// The flags are a combination of the constants in [`flags`].
> > + pub fn flags(&self) -> u32 {
>
> A typedef used here and in the constants module could be useful
>
> type FileFlags = u32;

Maybe, but it doesn't actually do anything as far as the type checker
is concerned, so meh.

> > + // This `read_volatile` is intended to correspond to a READ_ONCE call.
> > + //
> > + // SAFETY: The file is valid because the shared reference guarantees a nonzero refcount.
> > + //
> > + // TODO: Replace with `read_once` when available on the Rust side.
>
> Shouldn't the TODO become a `FIXME(read_once): ...` since it is going
> into the codebase?

I'm not sure we necessarily have a convention for that? But I'll change it.

>
> Some suggestions but nothing blocking
>
> Reviewed-by: Trevor Gross <[email protected]>

2024-02-08 15:07:02

[permalink] [raw]

Subject: Re: [PATCH v4 3/9] rust: file: add Rust abstraction for `struct file`

On Thu, Feb 8, 2024 at 1:35 AM Martin Rodriguez Reboredo
<[email protected]> wrote:
>
> On 2/2/24 07:55, Alice Ryhl wrote:
> > From: Wedson Almeida Filho <[email protected]>
> >
> > This abstraction makes it possible to manipulate the open files for a
> > process. The new `File` struct wraps the C `struct file`. When accessing
> > it using the smart pointer `ARef<File>`, the pointer will own a
> > reference count to the file. When accessing it as `&File`, then the
> > reference does not own a refcount, but the borrow checker will ensure
> > that the reference count does not hit zero while the `&File` is live.
> >
> > Since this is intended to manipulate the open files of a process, we
> > introduce an `fget` constructor that corresponds to the C `fget`
> > method. In future patches, it will become possible to create a new fd in
> > a process and bind it to a `File`. Rust Binder will use these to send
> > fds from one process to another.
> >
> > We also provide a method for accessing the file's flags. Rust Binder
> > will use this to access the flags of the Binder fd to check whether the
> > non-blocking flag is set, which affects what the Binder ioctl does.
> >
> > This introduces a struct for the EBADF error type, rather than just
> > using the Error type directly. This has two advantages:
> > * `File::from_fd` returns a `Result<ARef<File>, BadFdError>`, which the
> > compiler will represent as a single pointer, with null being an error.
> > This is possible because the compiler understands that `BadFdError`
> > has only one possible value, and it also understands that the
> > `ARef<File>` smart pointer is guaranteed non-null.
> > * Additionally, we promise to users of the method that the method can
> > only fail with EBADF, which means that they can rely on this promise
> > without having to inspect its implementation.
> > That said, there are also two disadvantages:
> > * Defining additional error types involves boilerplate.
> > * The question mark operator will only utilize the `From` trait once,
> > which prevents you from using the question mark operator on
> > `BadFdError` in methods that return some third error type that the
> > kernel `Error` is convertible into. (However, it works fine in methods
> > that return `Error`.)
> >
> > Signed-off-by: Wedson Almeida Filho <[email protected]>
> > Co-developed-by: Daniel Xu <[email protected]>
> > Signed-off-by: Daniel Xu <[email protected]>
> > Co-developed-by: Alice Ryhl <[email protected]>
> > Signed-off-by: Alice Ryhl <[email protected]>
> > ---
> > [...]
> > +/// ## Rust references
> > +///
> > +/// The reference type `&File` is similar to light refcounts:
> > +///
> > +/// * `&File` references don't own a reference count. They can only exist as long as the reference
> > +/// count stays positive, and can only be created when there is some mechanism in place to ensure
> > +/// this.
> > +///
> > +/// * The Rust borrow-checker normally ensures this by enforcing that the `ARef<File>` from which
> > +/// a `&File` is created outlives the `&File`.
> > +///
> > +/// * Using the unsafe [`File::from_ptr`] means that it is up to the caller to ensure that the
> > +/// `&File` only exists while the reference count is positive.
> > +///
> > +/// * You can think of `fdget` as using an fd to look up an `ARef<File>` in the `struct
> > +/// files_struct` and create an `&File` from it. The "fd cannot be closed" rule is like the Rust
> > +/// rule "the `ARef<File>` must outlive the `&File`".
>
> I find it kinda odd that this unordered list interspaces elements with
> blank lines as opposed to the following one, though, I don't see it as
> rather a big deal.

Shrug. I did it here because I found it more readable.

> > +///
> > +/// # Invariants
> > +///
> > +/// * Instances of this type are refcounted using the `f_count` field.
> > +/// * If an fd with active light refcounts is closed, then it must be the case that the file
> > +/// refcount is positive until all light refcounts of the fd have been dropped.
> > +/// * A light refcount must be dropped before returning to userspace.
> > [...]
>
> Reviewed-by: Martin Rodriguez Reboredo <[email protected]>

2024-02-08 15:08:24