This patchset contains some abstractions needed by the Rust
implementation of the Binder driver for passing data between userspace,
kernelspace, and directly into other processes.
These abstractions do not exactly match what was included in the Rust
Binder RFC - I have made various improvements and simplifications since
then. Nonetheless, please see the Rust Binder RFC [1] to get an
understanding for how this will be used:
Users of "rust: add userspace pointers"
and "rust: add typed accessors for userspace pointers":
rust_binder: add binderfs support to Rust binder
rust_binder: add threading support
rust_binder: add nodes and context managers
rust_binder: add oneway transactions
rust_binder: add death notifications
rust_binder: send nodes in transactions
rust_binder: add BINDER_TYPE_PTR support
rust_binder: add BINDER_TYPE_FDA support
rust_binder: add process freezing
Users of "rust: add abstraction for `struct page`":
rust_binder: add oneway transactions
rust_binder: add vma shrinker
Links: https://lore.kernel.org/rust-for-linux/[email protected]/ [1]
Signed-off-by: Alice Ryhl <[email protected]>
---
Changes in v4:
- Rephrase when we fail with EFAULT.
- Remove `pub` from examples.
- Use slices for raw uaccess methods.
- Fix PAGE_MASK constant.
- Rephrase most safety comments in Page abstraction.
- Make with_pointer_into_page and with_page_mapped private.
- Explain how raw pointers into pages are used correctly.
- Other minor doc improvements.
- Link to v3: https://lore.kernel.org/rust-for-linux/[email protected]/
Changes in v3:
- Fix bug in read_all.
- Add missing `#include <linux/nospec.h>`.
- Mention that the second patch passes CONFIG_TEST_USER_COPY.
- Add gfp flags for Page.
- Minor documentation adjustments.
- Link to v2: https://lore.kernel.org/rust-for-linux/[email protected]/
Changes in v2:
- Rename user_ptr module to uaccess.
- Use srctree-relative links.
- Improve documentation.
- Rename UserSlicePtr to UserSlice.
- Make read_to_end append to the buffer.
- Use named fields for uaccess types.
- Add examples.
- Use _copy_from/to_user to skip check_object_size.
- Rename traits and move to kernel::types.
- Remove PAGE_MASK constant.
- Rename page methods to say _raw.
- Link to v1: https://lore.kernel.org/rust-for-linux/[email protected]/
---
Alice Ryhl (2):
rust: uaccess: add typed accessors for userspace pointers
rust: add abstraction for `struct page`
Arnd Bergmann (1):
uaccess: always export _copy_[from|to]_user with CONFIG_RUST
Wedson Almeida Filho (1):
rust: uaccess: add userspace pointers
include/linux/uaccess.h | 38 ++--
lib/usercopy.c | 30 +---
rust/bindings/bindings_helper.h | 2 +
rust/helpers.c | 34 ++++
rust/kernel/lib.rs | 2 +
rust/kernel/page.rs | 259 +++++++++++++++++++++++++++
rust/kernel/types.rs | 67 +++++++
rust/kernel/uaccess.rs | 383 ++++++++++++++++++++++++++++++++++++++++
8 files changed, 775 insertions(+), 40 deletions(-)
---
base-commit: 4cece764965020c22cff7665b18a012006359095
change-id: 20231128-alice-mm-bc533456cee8
Best regards,
--
Alice Ryhl <[email protected]>
From: Arnd Bergmann <[email protected]>
Rust code needs to be able to access _copy_from_user and _copy_to_user
so that it can skip the check_copy_size check in cases where the length
is known at compile-time, mirroring the logic for when C code will skip
check_copy_size. To do this, we ensure that exported versions of these
methods are available when CONFIG_RUST is enabled.
Alice has verified that this patch passes the CONFIG_TEST_USER_COPY test
on x86 using the Android cuttlefish emulator.
Signed-off-by: Arnd Bergmann <[email protected]>
Tested-by: Alice Ryhl <[email protected]>
Reviewed-by: Boqun Feng <[email protected]>
Signed-off-by: Alice Ryhl <[email protected]>
---
include/linux/uaccess.h | 38 ++++++++++++++++++++++++--------------
lib/usercopy.c | 30 ++++--------------------------
2 files changed, 28 insertions(+), 40 deletions(-)
diff --git a/include/linux/uaccess.h b/include/linux/uaccess.h
index 3064314f4832..2ebfce98b5cc 100644
--- a/include/linux/uaccess.h
+++ b/include/linux/uaccess.h
@@ -5,6 +5,7 @@
#include <linux/fault-inject-usercopy.h>
#include <linux/instrumented.h>
#include <linux/minmax.h>
+#include <linux/nospec.h>
#include <linux/sched.h>
#include <linux/thread_info.h>
@@ -138,13 +139,18 @@ __copy_to_user(void __user *to, const void *from, unsigned long n)
return raw_copy_to_user(to, from, n);
}
-#ifdef INLINE_COPY_FROM_USER
static inline __must_check unsigned long
-_copy_from_user(void *to, const void __user *from, unsigned long n)
+_inline_copy_from_user(void *to, const void __user *from, unsigned long n)
{
unsigned long res = n;
might_fault();
if (!should_fail_usercopy() && likely(access_ok(from, n))) {
+ /*
+ * Ensure that bad access_ok() speculation will not
+ * lead to nasty side effects *after* the copy is
+ * finished:
+ */
+ barrier_nospec();
instrument_copy_from_user_before(to, from, n);
res = raw_copy_from_user(to, from, n);
instrument_copy_from_user_after(to, from, n, res);
@@ -153,14 +159,11 @@ _copy_from_user(void *to, const void __user *from, unsigned long n)
memset(to + (n - res), 0, res);
return res;
}
-#else
extern __must_check unsigned long
_copy_from_user(void *, const void __user *, unsigned long);
-#endif
-#ifdef INLINE_COPY_TO_USER
static inline __must_check unsigned long
-_copy_to_user(void __user *to, const void *from, unsigned long n)
+_inline_copy_to_user(void __user *to, const void *from, unsigned long n)
{
might_fault();
if (should_fail_usercopy())
@@ -171,25 +174,32 @@ _copy_to_user(void __user *to, const void *from, unsigned long n)
}
return n;
}
-#else
extern __must_check unsigned long
_copy_to_user(void __user *, const void *, unsigned long);
-#endif
static __always_inline unsigned long __must_check
copy_from_user(void *to, const void __user *from, unsigned long n)
{
- if (check_copy_size(to, n, false))
- n = _copy_from_user(to, from, n);
- return n;
+ if (!check_copy_size(to, n, false))
+ return n;
+#ifdef INLINE_COPY_FROM_USER
+ return _inline_copy_from_user(to, from, n);
+#else
+ return _copy_from_user(to, from, n);
+#endif
}
static __always_inline unsigned long __must_check
copy_to_user(void __user *to, const void *from, unsigned long n)
{
- if (check_copy_size(from, n, true))
- n = _copy_to_user(to, from, n);
- return n;
+ if (!check_copy_size(from, n, true))
+ return n;
+
+#ifdef INLINE_COPY_TO_USER
+ return _inline_copy_to_user(to, from, n);
+#else
+ return _copy_to_user(to, from, n);
+#endif
}
#ifndef copy_mc_to_kernel
diff --git a/lib/usercopy.c b/lib/usercopy.c
index d29fe29c6849..de7f30618293 100644
--- a/lib/usercopy.c
+++ b/lib/usercopy.c
@@ -7,40 +7,18 @@
/* out-of-line parts */
-#ifndef INLINE_COPY_FROM_USER
+#if !defined(INLINE_COPY_FROM_USER) || defined(CONFIG_RUST)
unsigned long _copy_from_user(void *to, const void __user *from, unsigned long n)
{
- unsigned long res = n;
- might_fault();
- if (!should_fail_usercopy() && likely(access_ok(from, n))) {
- /*
- * Ensure that bad access_ok() speculation will not
- * lead to nasty side effects *after* the copy is
- * finished:
- */
- barrier_nospec();
- instrument_copy_from_user_before(to, from, n);
- res = raw_copy_from_user(to, from, n);
- instrument_copy_from_user_after(to, from, n, res);
- }
- if (unlikely(res))
- memset(to + (n - res), 0, res);
- return res;
+ return _inline_copy_from_user(to, from, n);
}
EXPORT_SYMBOL(_copy_from_user);
#endif
-#ifndef INLINE_COPY_TO_USER
+#if !defined(INLINE_COPY_TO_USER) || defined(CONFIG_RUST)
unsigned long _copy_to_user(void __user *to, const void *from, unsigned long n)
{
- might_fault();
- if (should_fail_usercopy())
- return n;
- if (likely(access_ok(to, n))) {
- instrument_copy_to_user(to, from, n);
- n = raw_copy_to_user(to, from, n);
- }
- return n;
+ return _inline_copy_to_user(to, from, n);
}
EXPORT_SYMBOL(_copy_to_user);
#endif
--
2.44.0.478.gd926399ef9-goog
From: Wedson Almeida Filho <[email protected]>
A pointer to an area in userspace memory, which can be either read-only
or read-write.
All methods on this struct are safe: attempting to read or write on bad
addresses (either out of the bound of the slice or unmapped addresses)
will return `EFAULT`. Concurrent access, *including data races to/from
userspace memory*, is permitted, because fundamentally another userspace
thread/process could always be modifying memory at the same time (in the
same way that userspace Rust's `std::io` permits data races with the
contents of files on disk). In the presence of a race, the exact byte
values read/written are unspecified but the operation is well-defined.
Kernelspace code should validate its copy of data after completing a
read, and not expect that multiple reads of the same address will return
the same value.
These APIs are designed to make it difficult to accidentally write
TOCTOU bugs. Every time you read from a memory location, the pointer is
advanced by the length so that you cannot use that reader to read the
same memory location twice. Preventing double-fetches avoids TOCTOU
bugs. This is accomplished by taking `self` by value to prevent
obtaining multiple readers on a given `UserSlicePtr`, and the readers
only permitting forward reads. If double-fetching a memory location is
necessary for some reason, then that is done by creating multiple
readers to the same memory location.
Constructing a `UserSlicePtr` performs no checks on the provided
address and length, it can safely be constructed inside a kernel thread
with no current userspace process. Reads and writes wrap the kernel APIs
`copy_from_user` and `copy_to_user`, which check the memory map of the
current process and enforce that the address range is within the user
range (no additional calls to `access_ok` are needed).
This code is based on something that was originally written by Wedson on
the old rust branch. It was modified by Alice by removing the
`IoBufferReader` and `IoBufferWriter` traits, and various other changes.
Signed-off-by: Wedson Almeida Filho <[email protected]>
Co-developed-by: Alice Ryhl <[email protected]>
Signed-off-by: Alice Ryhl <[email protected]>
---
rust/helpers.c | 14 +++
rust/kernel/lib.rs | 1 +
rust/kernel/uaccess.rs | 311 +++++++++++++++++++++++++++++++++++++++++++++++++
3 files changed, 326 insertions(+)
diff --git a/rust/helpers.c b/rust/helpers.c
index 70e59efd92bc..312b6fcb49d5 100644
--- a/rust/helpers.c
+++ b/rust/helpers.c
@@ -38,6 +38,20 @@ __noreturn void rust_helper_BUG(void)
}
EXPORT_SYMBOL_GPL(rust_helper_BUG);
+unsigned long rust_helper_copy_from_user(void *to, const void __user *from,
+ unsigned long n)
+{
+ return copy_from_user(to, from, n);
+}
+EXPORT_SYMBOL_GPL(rust_helper_copy_from_user);
+
+unsigned long rust_helper_copy_to_user(void __user *to, const void *from,
+ unsigned long n)
+{
+ return copy_to_user(to, from, n);
+}
+EXPORT_SYMBOL_GPL(rust_helper_copy_to_user);
+
void rust_helper_mutex_lock(struct mutex *lock)
{
mutex_lock(lock);
diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index be68d5e567b1..37f84223b83f 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -49,6 +49,7 @@
pub mod task;
pub mod time;
pub mod types;
+pub mod uaccess;
pub mod workqueue;
#[doc(hidden)]
diff --git a/rust/kernel/uaccess.rs b/rust/kernel/uaccess.rs
new file mode 100644
index 000000000000..3f8ad4dc13c4
--- /dev/null
+++ b/rust/kernel/uaccess.rs
@@ -0,0 +1,311 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Slices to user space memory regions.
+//!
+//! C header: [`include/linux/uaccess.h`](srctree/include/linux/uaccess.h)
+
+use crate::{bindings, error::code::*, error::Result};
+use alloc::vec::Vec;
+use core::ffi::{c_ulong, c_void};
+use core::mem::MaybeUninit;
+
+/// A pointer to an area in userspace memory, which can be either read-only or
+/// read-write.
+///
+/// All methods on this struct are safe: attempting to read or write on bad
+/// addresses (either out of the bound of the slice or unmapped addresses) will
+/// return `EFAULT`. Concurrent access, *including data races to/from userspace
+/// memory*, is permitted, because fundamentally another userspace
+/// thread/process could always be modifying memory at the same time (in the
+/// same way that userspace Rust's [`std::io`] permits data races with the
+/// contents of files on disk). In the presence of a race, the exact byte values
+/// read/written are unspecified but the operation is well-defined. Kernelspace
+/// code should validate its copy of data after completing a read, and not
+/// expect that multiple reads of the same address will return the same value.
+///
+/// These APIs are designed to make it difficult to accidentally write TOCTOU
+/// (time-of-check to time-of-use) bugs. Every time a memory location is read,
+/// the reader's position is advanced by the read length and the next read will
+/// start from there. This helps prevent accidentally reading the same location
+/// twice and causing a TOCTOU bug.
+///
+/// Creating a [`UserSliceReader`] and/or [`UserSliceWriter`] consumes the
+/// `UserSlice`, helping ensure that there aren't multiple readers or writers to
+/// the same location.
+///
+/// If double-fetching a memory location is necessary for some reason, then that
+/// is done by creating multiple readers to the same memory location, e.g. using
+/// [`clone_reader`].
+///
+/// # Examples
+///
+/// Takes a region of userspace memory from the current process, and modify it
+/// by adding one to every byte in the region.
+///
+/// ```no_run
+/// use alloc::vec::Vec;
+/// use core::ffi::c_void;
+/// use kernel::error::Result;
+/// use kernel::uaccess::UserSlice;
+///
+/// fn bytes_add_one(uptr: *mut c_void, len: usize) -> Result<()> {
+/// let (read, mut write) = UserSlice::new(uptr, len).reader_writer();
+///
+/// let mut buf = Vec::new();
+/// read.read_all(&mut buf)?;
+///
+/// for b in &mut buf {
+/// *b = b.wrapping_add(1);
+/// }
+///
+/// write.write_slice(&buf)?;
+/// Ok(())
+/// }
+/// ```
+///
+/// Example illustrating a TOCTOU (time-of-check to time-of-use) bug.
+///
+/// ```no_run
+/// use alloc::vec::Vec;
+/// use core::ffi::c_void;
+/// use kernel::error::{code::EINVAL, Result};
+/// use kernel::uaccess::UserSlice;
+///
+/// /// Returns whether the data in this region is valid.
+/// fn is_valid(uptr: *mut c_void, len: usize) -> Result<bool> {
+/// let read = UserSlice::new(uptr, len).reader();
+///
+/// let mut buf = Vec::new();
+/// read.read_all(&mut buf)?;
+///
+/// todo!()
+/// }
+///
+/// /// Returns the bytes behind this user pointer if they are valid.
+/// fn get_bytes_if_valid(uptr: *mut c_void, len: usize) -> Result<Vec<u8>> {
+/// if !is_valid(uptr, len)? {
+/// return Err(EINVAL);
+/// }
+///
+/// let read = UserSlice::new(uptr, len).reader();
+///
+/// let mut buf = Vec::new();
+/// read.read_all(&mut buf)?;
+///
+/// // THIS IS A BUG! The bytes could have changed since we checked them.
+/// //
+/// // To avoid this kind of bug, don't call `UserSlice::new` multiple
+/// // times with the same address.
+/// Ok(buf)
+/// }
+/// ```
+///
+/// [`std::io`]: https://doc.rust-lang.org/std/io/index.html
+/// [`clone_reader`]: UserSliceReader::clone_reader
+pub struct UserSlice {
+ ptr: *mut c_void,
+ length: usize,
+}
+
+impl UserSlice {
+ /// Constructs a user slice from a raw pointer and a length in bytes.
+ ///
+ /// Constructing a [`UserSlice`] performs no checks on the provided address
+ /// and length, it can safely be constructed inside a kernel thread with no
+ /// current userspace process. Reads and writes wrap the kernel APIs
+ /// `copy_from_user` and `copy_to_user`, which check the memory map of the
+ /// current process and enforce that the address range is within the user
+ /// range (no additional calls to `access_ok` are needed).
+ ///
+ /// Callers must be careful to avoid time-of-check-time-of-use
+ /// (TOCTOU) issues. The simplest way is to create a single instance of
+ /// [`UserSlice`] per user memory block as it reads each byte at
+ /// most once.
+ pub fn new(ptr: *mut c_void, length: usize) -> Self {
+ UserSlice { ptr, length }
+ }
+
+ /// Reads the entirety of the user slice, appending it to the end of the
+ /// provided buffer.
+ ///
+ /// Fails with `EFAULT` if the read happens on a bad address.
+ pub fn read_all(self, buf: &mut Vec<u8>) -> Result {
+ self.reader().read_all(buf)
+ }
+
+ /// Constructs a [`UserSliceReader`].
+ pub fn reader(self) -> UserSliceReader {
+ UserSliceReader {
+ ptr: self.ptr,
+ length: self.length,
+ }
+ }
+
+ /// Constructs a [`UserSliceWriter`].
+ pub fn writer(self) -> UserSliceWriter {
+ UserSliceWriter {
+ ptr: self.ptr,
+ length: self.length,
+ }
+ }
+
+ /// Constructs both a [`UserSliceReader`] and a [`UserSliceWriter`].
+ ///
+ /// Usually when this is used, you will first read the data, and then
+ /// overwrite it afterwards.
+ pub fn reader_writer(self) -> (UserSliceReader, UserSliceWriter) {
+ (
+ UserSliceReader {
+ ptr: self.ptr,
+ length: self.length,
+ },
+ UserSliceWriter {
+ ptr: self.ptr,
+ length: self.length,
+ },
+ )
+ }
+}
+
+/// A reader for [`UserSlice`].
+///
+/// Used to incrementally read from the user slice.
+pub struct UserSliceReader {
+ ptr: *mut c_void,
+ length: usize,
+}
+
+impl UserSliceReader {
+ /// Skip the provided number of bytes.
+ ///
+ /// Returns an error if skipping more than the length of the buffer.
+ pub fn skip(&mut self, num_skip: usize) -> Result {
+ // Update `self.length` first since that's the fallible part of this
+ // operation.
+ self.length = self.length.checked_sub(num_skip).ok_or(EFAULT)?;
+ self.ptr = self.ptr.wrapping_byte_add(num_skip);
+ Ok(())
+ }
+
+ /// Create a reader that can access the same range of data.
+ ///
+ /// Reading from the clone does not advance the current reader.
+ ///
+ /// The caller should take care to not introduce TOCTOU issues, as described
+ /// in the documentation for [`UserSlice`].
+ pub fn clone_reader(&self) -> UserSliceReader {
+ UserSliceReader {
+ ptr: self.ptr,
+ length: self.length,
+ }
+ }
+
+ /// Returns the number of bytes left to be read from this reader.
+ ///
+ /// Note that even reading less than this number of bytes may fail.
+ pub fn len(&self) -> usize {
+ self.length
+ }
+
+ /// Returns `true` if no data is available in the io buffer.
+ pub fn is_empty(&self) -> bool {
+ self.length == 0
+ }
+
+ /// Reads raw data from the user slice into a kernel buffer.
+ ///
+ /// Fails with `EFAULT` if the read happens on a bad address.
+ pub fn read_raw(&mut self, out: &mut [MaybeUninit<u8>]) -> Result {
+ let len = out.len();
+ let out_ptr = out.as_mut_ptr().cast::<c_void>();
+ if len > self.length {
+ return Err(EFAULT);
+ }
+ let Ok(len_ulong) = c_ulong::try_from(len) else {
+ return Err(EFAULT);
+ };
+ // SAFETY: The caller promises that `out` is valid for writing `len` bytes.
+ let res = unsafe { bindings::copy_from_user(out_ptr, self.ptr, len_ulong) };
+ if res != 0 {
+ return Err(EFAULT);
+ }
+ // Userspace pointers are not directly dereferencable by the kernel, so
+ // we cannot use `add`, which has C-style rules for defined behavior.
+ self.ptr = self.ptr.wrapping_byte_add(len);
+ self.length -= len;
+ Ok(())
+ }
+
+ /// Reads raw data from the user slice into a kernel buffer.
+ ///
+ /// Fails with `EFAULT` if the read happens on a bad address.
+ pub fn read_slice(&mut self, out: &mut [u8]) -> Result {
+ // SAFETY: The types are compatible and `read_raw` doesn't write
+ // uninitialized bytes to `out`.
+ let out = unsafe { &mut *(out as *mut [u8] as *mut [MaybeUninit<u8>]) };
+ self.read_raw(out)
+ }
+
+ /// Reads the entirety of the user slice, appending it to the end of the
+ /// provided buffer.
+ ///
+ /// Fails with `EFAULT` if the read happens on a bad address.
+ pub fn read_all(mut self, buf: &mut Vec<u8>) -> Result {
+ let len = self.length;
+ buf.try_reserve(len)?;
+
+ // The call to `try_reserve` was successful, so the spare capacity is at
+ // least `len` bytes long.
+ self.read_raw(&mut buf.spare_capacity_mut()[..len])?;
+
+ // SAFETY: Since the call to `read_raw` was successful, so the next
+ // `len` bytes of the vector have been initialized.
+ unsafe { buf.set_len(buf.len() + len) };
+ Ok(())
+ }
+}
+
+/// A writer for [`UserSlice`].
+///
+/// Used to incrementally write into the user slice.
+pub struct UserSliceWriter {
+ ptr: *mut c_void,
+ length: usize,
+}
+
+impl UserSliceWriter {
+ /// Returns the amount of space remaining in this buffer.
+ ///
+ /// Note that even writing less than this number of bytes may fail.
+ pub fn len(&self) -> usize {
+ self.length
+ }
+
+ /// Returns `true` if no more data can be written to this buffer.
+ pub fn is_empty(&self) -> bool {
+ self.length == 0
+ }
+
+ /// Writes raw data to this user pointer from a kernel buffer.
+ ///
+ /// Fails with `EFAULT` if the write happens on a bad address.
+ pub fn write_slice(&mut self, data: &[u8]) -> Result {
+ let len = data.len();
+ let data_ptr = data.as_ptr().cast::<c_void>();
+ if len > self.length {
+ return Err(EFAULT);
+ }
+ let Ok(len_ulong) = c_ulong::try_from(len) else {
+ return Err(EFAULT);
+ };
+ let res = unsafe { bindings::copy_to_user(self.ptr, data_ptr, len_ulong) };
+ if res != 0 {
+ return Err(EFAULT);
+ }
+ // Userspace pointers are not directly dereferencable by the kernel, so
+ // we cannot use `add`, which has C-style rules for defined behavior.
+ self.ptr = self.ptr.wrapping_byte_add(len);
+ self.length -= len;
+ Ok(())
+ }
+}
--
2.44.0.478.gd926399ef9-goog
Adds a new struct called `Page` that wraps a pointer to `struct page`.
This struct is assumed to hold ownership over the page, so that Rust
code can allocate and manage pages directly.
The page type has various methods for reading and writing into the page.
These methods will temporarily map the page to allow the operation. All
of these methods use a helper that takes an offset and length, performs
bounds checks, and returns a pointer to the given offset in the page.
This patch only adds support for pages of order zero, as that is all
Rust Binder needs. However, it is written to make it easy to add support
for higher-order pages in the future. To do that, you would add a const
generic parameter to `Page` that specifies the order. Most of the
methods do not need to be adjusted, as the logic for dealing with
mapping multiple pages at once can be isolated to just the
`with_pointer_into_page` method. Finally, the struct can be renamed to
`Pages<ORDER>`, and the type alias `Page = Pages<0>` can be introduced.
Rust Binder needs to manage pages directly as that is how transactions
are delivered: Each process has an mmap'd region for incoming
transactions. When an incoming transaction arrives, the Binder driver
will choose a region in the mmap, allocate and map the relevant pages
manually, and copy the incoming transaction directly into the page. This
architecture allows the driver to copy transactions directly from the
address space of one process to another, without an intermediate copy
to a kernel buffer.
This code is based on Wedson's page abstractions from the old rust
branch, but it has been modified by Alice by removing the incomplete
support for higher-order pages, by introducing the `with_*` helpers
to consolidate the bounds checking logic into a single place, and by
introducing gfp flags.
Co-developed-by: Wedson Almeida Filho <[email protected]>
Signed-off-by: Wedson Almeida Filho <[email protected]>
Signed-off-by: Alice Ryhl <[email protected]>
---
rust/bindings/bindings_helper.h | 2 +
rust/helpers.c | 20 ++++
rust/kernel/lib.rs | 1 +
rust/kernel/page.rs | 259 ++++++++++++++++++++++++++++++++++++++++
4 files changed, 282 insertions(+)
diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h
index 65b98831b975..da1e97871419 100644
--- a/rust/bindings/bindings_helper.h
+++ b/rust/bindings/bindings_helper.h
@@ -20,5 +20,7 @@
/* `bindgen` gets confused at certain things. */
const size_t RUST_CONST_HELPER_ARCH_SLAB_MINALIGN = ARCH_SLAB_MINALIGN;
+const size_t RUST_CONST_HELPER_PAGE_SIZE = PAGE_SIZE;
const gfp_t RUST_CONST_HELPER_GFP_KERNEL = GFP_KERNEL;
const gfp_t RUST_CONST_HELPER___GFP_ZERO = __GFP_ZERO;
+const gfp_t RUST_CONST_HELPER___GFP_HIGHMEM = ___GFP_HIGHMEM;
diff --git a/rust/helpers.c b/rust/helpers.c
index 312b6fcb49d5..72361003ba91 100644
--- a/rust/helpers.c
+++ b/rust/helpers.c
@@ -25,6 +25,8 @@
#include <linux/build_bug.h>
#include <linux/err.h>
#include <linux/errname.h>
+#include <linux/gfp.h>
+#include <linux/highmem.h>
#include <linux/mutex.h>
#include <linux/refcount.h>
#include <linux/sched/signal.h>
@@ -93,6 +95,24 @@ int rust_helper_signal_pending(struct task_struct *t)
}
EXPORT_SYMBOL_GPL(rust_helper_signal_pending);
+struct page *rust_helper_alloc_pages(gfp_t gfp_mask, unsigned int order)
+{
+ return alloc_pages(gfp_mask, order);
+}
+EXPORT_SYMBOL_GPL(rust_helper_alloc_pages);
+
+void *rust_helper_kmap_local_page(struct page *page)
+{
+ return kmap_local_page(page);
+}
+EXPORT_SYMBOL_GPL(rust_helper_kmap_local_page);
+
+void rust_helper_kunmap_local(const void *addr)
+{
+ kunmap_local(addr);
+}
+EXPORT_SYMBOL_GPL(rust_helper_kunmap_local);
+
refcount_t rust_helper_REFCOUNT_INIT(int n)
{
return (refcount_t)REFCOUNT_INIT(n);
diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index 37f84223b83f..667fc67fa24f 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -39,6 +39,7 @@
pub mod kunit;
#[cfg(CONFIG_NET)]
pub mod net;
+pub mod page;
pub mod prelude;
pub mod print;
mod static_assert;
diff --git a/rust/kernel/page.rs b/rust/kernel/page.rs
new file mode 100644
index 000000000000..5aba0261242d
--- /dev/null
+++ b/rust/kernel/page.rs
@@ -0,0 +1,259 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Kernel page allocation and management.
+
+use crate::{bindings, error::code::*, error::Result, uaccess::UserSliceReader};
+use core::{
+ alloc::AllocError,
+ ptr::{self, NonNull},
+};
+
+/// A bitwise shift for the page size.
+#[allow(clippy::unnecessary_cast)]
+pub const PAGE_SHIFT: usize = bindings::PAGE_SHIFT as usize;
+
+/// The number of bytes in a page.
+#[allow(clippy::unnecessary_cast)]
+pub const PAGE_SIZE: usize = bindings::PAGE_SIZE as usize;
+
+/// A bitmask that gives the page containing a given address.
+pub const PAGE_MASK: usize = !(PAGE_SIZE-1);
+
+/// Flags for the "get free page" function that underlies all memory allocations.
+pub mod flags {
+ /// gfp flags.
+ #[allow(non_camel_case_types)]
+ pub type gfp_t = bindings::gfp_t;
+
+ /// `GFP_KERNEL` is typical for kernel-internal allocations. The caller requires `ZONE_NORMAL`
+ /// or a lower zone for direct access but can direct reclaim.
+ pub const GFP_KERNEL: gfp_t = bindings::GFP_KERNEL;
+ /// `GFP_ZERO` returns a zeroed page on success.
+ pub const __GFP_ZERO: gfp_t = bindings::__GFP_ZERO;
+ /// `GFP_HIGHMEM` indicates that the allocated memory may be located in high memory.
+ pub const __GFP_HIGHMEM: gfp_t = bindings::__GFP_HIGHMEM;
+}
+
+/// A pointer to a page that owns the page allocation.
+///
+/// # Invariants
+///
+/// The pointer is valid, and has ownership over the page.
+pub struct Page {
+ page: NonNull<bindings::page>,
+}
+
+// SAFETY: Pages have no logic that relies on them staying on a given thread, so
+// moving them across threads is safe.
+unsafe impl Send for Page {}
+
+// SAFETY: Pages have no logic that relies on them not being accessed
+// concurrently, so accessing them concurrently is safe.
+unsafe impl Sync for Page {}
+
+impl Page {
+ /// Allocates a new page.
+ pub fn alloc_page(gfp_flags: flags::gfp_t) -> Result<Self, AllocError> {
+ // SAFETY: Depending on the value of `gfp_flags`, this call may sleep.
+ // Other than that, it is always safe to call this method.
+ let page = unsafe { bindings::alloc_pages(gfp_flags, 0) };
+ let page = NonNull::new(page).ok_or(AllocError)?;
+ // INVARIANT: We just successfully allocated a page, so we now have
+ // ownership of the newly allocated page. We transfer that ownership to
+ // the new `Page` object.
+ Ok(Self { page })
+ }
+
+ /// Returns a raw pointer to the page.
+ pub fn as_ptr(&self) -> *mut bindings::page {
+ self.page.as_ptr()
+ }
+
+ /// Runs a piece of code with this page mapped to an address.
+ ///
+ /// The page is unmapped when this call returns.
+ ///
+ /// # Using the raw pointer
+ ///
+ /// It is up to the caller to use the provided raw pointer correctly. The
+ /// pointer is valid for `PAGE_SIZE` bytes and for the duration in which the
+ /// closure is called. The pointer might only be mapped on the current
+ /// thread, and when that is the case, dereferencing it on other threads is
+ /// UB. Other than that, the usual rules for dereferencing a raw pointer
+ /// apply: don't cause data races, the memory may be uninitialized, and so
+ /// on.
+ ///
+ /// If multiple threads map the same page at the same time, then they may
+ /// reference with different addresses. However, even if the addresses are
+ /// different, the underlying memory is still the same for these purposes
+ /// (e.g., it's still a data race if they both write to the same underlying
+ /// byte at the same time).
+ fn with_page_mapped<T>(&self, f: impl FnOnce(*mut u8) -> T) -> T {
+ // SAFETY: `page` is valid due to the type invariants on `Page`.
+ let mapped_addr = unsafe { bindings::kmap_local_page(self.as_ptr()) };
+
+ let res = f(mapped_addr.cast());
+
+ // This unmaps the page mapped above.
+ //
+ // SAFETY: Since this API takes the user code as a closure, it can only
+ // be used in a manner where the pages are unmapped in reverse order.
+ // This is as required by `kunmap_local`.
+ //
+ // In other words, if this call to `kunmap_local` happens when a
+ // different page should be unmapped first, then there must necessarily
+ // be a call to `kmap_local_page` other than the call just above in
+ // `with_page_mapped` that made that possible. In this case, it is the
+ // unsafe block that wraps that other call that is incorrect.
+ unsafe { bindings::kunmap_local(mapped_addr) };
+
+ res
+ }
+
+ /// Runs a piece of code with a raw pointer to a slice of this page, with
+ /// bounds checking.
+ ///
+ /// If `f` is called, then it will be called with a pointer that points at
+ /// `off` bytes into the page, and the pointer will be valid for at least
+ /// `len` bytes. The pointer is only valid on this task, as this method uses
+ /// a local mapping.
+ ///
+ /// If `off` and `len` refers to a region outside of this page, then this
+ /// method returns `EINVAL` and does not call `f`.
+ ///
+ /// # Using the raw pointer
+ ///
+ /// It is up to the caller to use the provided raw pointer correctly. The
+ /// pointer is valid for `len` bytes and for the duration in which the
+ /// closure is called. The pointer might only be mapped on the current
+ /// thread, and when that is the case, dereferencing it on other threads is
+ /// UB. Other than that, the usual rules for dereferencing a raw pointer
+ /// apply: don't cause data races, the memory may be uninitialized, and so
+ /// on.
+ ///
+ /// If multiple threads map the same page at the same time, then they may
+ /// reference with different addresses. However, even if the addresses are
+ /// different, the underlying memory is still the same for these purposes
+ /// (e.g., it's still a data race if they both write to the same underlying
+ /// byte at the same time).
+ fn with_pointer_into_page<T>(
+ &self,
+ off: usize,
+ len: usize,
+ f: impl FnOnce(*mut u8) -> Result<T>,
+ ) -> Result<T> {
+ let bounds_ok = off <= PAGE_SIZE && len <= PAGE_SIZE && (off + len) <= PAGE_SIZE;
+
+ if bounds_ok {
+ self.with_page_mapped(move |page_addr| {
+ // SAFETY: The `off` integer is at most `PAGE_SIZE`, so this
+ // pointer offset will result in a pointer that is in bounds or
+ // one off the end of the page.
+ f(unsafe { page_addr.add(off) })
+ })
+ } else {
+ Err(EINVAL)
+ }
+ }
+
+ /// Maps the page and reads from it into the given buffer.
+ ///
+ /// This method will perform bounds checks on the page offset. If `offset ..
+ /// offset+len` goes outside ot the page, then this call returns `EINVAL`.
+ ///
+ /// # Safety
+ ///
+ /// * Callers must ensure that `dst` is valid for writing `len` bytes.
+ /// * Callers must ensure that this call does not race with a write to the
+ /// same page that overlaps with this read.
+ pub unsafe fn read_raw(&self, dst: *mut u8, offset: usize, len: usize) -> Result {
+ self.with_pointer_into_page(offset, len, move |src| {
+ // SAFETY: If `with_pointer_into_page` calls into this closure, then
+ // it has performed a bounds check and guarantees that `src` is
+ // valid for `len` bytes.
+ //
+ // There caller guarantees that there is no data race.
+ unsafe { ptr::copy_nonoverlapping(src, dst, len) };
+ Ok(())
+ })
+ }
+
+ /// Maps the page and writes into it from the given buffer.
+ ///
+ /// This method will perform bounds checks on the page offset. If `offset ..
+ /// offset+len` goes outside ot the page, then this call returns `EINVAL`.
+ ///
+ /// # Safety
+ ///
+ /// * Callers must ensure that `src` is valid for reading `len` bytes.
+ /// * Callers must ensure that this call does not race with a read or write
+ /// to the same page that overlaps with this write.
+ pub unsafe fn write_raw(&self, src: *const u8, offset: usize, len: usize) -> Result {
+ self.with_pointer_into_page(offset, len, move |dst| {
+ // SAFETY: If `with_pointer_into_page` calls into this closure, then
+ // it has performed a bounds check and guarantees that `dst` is
+ // valid for `len` bytes.
+ //
+ // There caller guarantees that there is no data race.
+ unsafe { ptr::copy_nonoverlapping(src, dst, len) };
+ Ok(())
+ })
+ }
+
+ /// Maps the page and zeroes the given slice.
+ ///
+ /// This method will perform bounds checks on the page offset. If `offset ..
+ /// offset+len` goes outside ot the page, then this call returns `EINVAL`.
+ ///
+ /// # Safety
+ ///
+ /// Callers must ensure that this call does not race with a read or write to
+ /// the same page that overlaps with this write.
+ pub unsafe fn fill_zero(&self, offset: usize, len: usize) -> Result {
+ self.with_pointer_into_page(offset, len, move |dst| {
+ // SAFETY: If `with_pointer_into_page` calls into this closure, then
+ // it has performed a bounds check and guarantees that `dst` is
+ // valid for `len` bytes.
+ //
+ // There caller guarantees that there is no data race.
+ unsafe { ptr::write_bytes(dst, 0u8, len) };
+ Ok(())
+ })
+ }
+
+ /// Copies data from userspace into this page.
+ ///
+ /// This method will perform bounds checks on the page offset. If `offset ..
+ /// offset+len` goes outside ot the page, then this call returns `EINVAL`.
+ ///
+ /// Like the other `UserSliceReader` methods, data races are allowed on the
+ /// userspace address. However, they are not allowed on the page you are
+ /// copying into.
+ ///
+ /// # Safety
+ ///
+ /// Callers must ensure that this call does not race with a read or write to
+ /// the same page that overlaps with this write.
+ pub unsafe fn copy_from_user_slice(
+ &self,
+ reader: &mut UserSliceReader,
+ offset: usize,
+ len: usize,
+ ) -> Result {
+ self.with_pointer_into_page(offset, len, move |dst| {
+ // SAFETY: If `with_pointer_into_page` calls into this closure, then
+ // it has performed a bounds check and guarantees that `dst` is
+ // valid for `len` bytes. Furthermore, we have exclusive access to
+ // the slice since the caller guarantees that there are no races.
+ reader.read_raw(unsafe { core::slice::from_raw_parts_mut(dst.cast(), len) })
+ })
+ }
+}
+
+impl Drop for Page {
+ fn drop(&mut self) {
+ // SAFETY: By the type invariants, we have ownership of the page and can
+ // free it.
+ unsafe { bindings::__free_pages(self.page.as_ptr(), 0) };
+ }
+}
--
2.44.0.478.gd926399ef9-goog
On 04.04.24 14:31, Alice Ryhl wrote:
> diff --git a/rust/kernel/uaccess.rs b/rust/kernel/uaccess.rs
> new file mode 100644
> index 000000000000..3f8ad4dc13c4
> --- /dev/null
> +++ b/rust/kernel/uaccess.rs
> @@ -0,0 +1,311 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +//! Slices to user space memory regions.
> +//!
> +//! C header: [`include/linux/uaccess.h`](srctree/include/linux/uaccess.h)
> +
> +use crate::{bindings, error::code::*, error::Result};
> +use alloc::vec::Vec;
> +use core::ffi::{c_ulong, c_void};
> +use core::mem::MaybeUninit;
> +
> +/// A pointer to an area in userspace memory, which can be either read-only or
> +/// read-write.
> +///
> +/// All methods on this struct are safe: attempting to read or write on bad
> +/// addresses (either out of the bound of the slice or unmapped addresses) will
> +/// return `EFAULT`. Concurrent access, *including data races to/from userspace
> +/// memory*, is permitted, because fundamentally another userspace
> +/// thread/process could always be modifying memory at the same time (in the
> +/// same way that userspace Rust's [`std::io`] permits data races with the
> +/// contents of files on disk). In the presence of a race, the exact byte values
> +/// read/written are unspecified but the operation is well-defined. Kernelspace
> +/// code should validate its copy of data after completing a read, and not
> +/// expect that multiple reads of the same address will return the same value.
> +///
> +/// These APIs are designed to make it difficult to accidentally write TOCTOU
> +/// (time-of-check to time-of-use) bugs. Every time a memory location is read,
> +/// the reader's position is advanced by the read length and the next read will
> +/// start from there. This helps prevent accidentally reading the same location
> +/// twice and causing a TOCTOU bug.
> +///
> +/// Creating a [`UserSliceReader`] and/or [`UserSliceWriter`] consumes the
> +/// `UserSlice`, helping ensure that there aren't multiple readers or writers to
> +/// the same location.
> +///
> +/// If double-fetching a memory location is necessary for some reason, then that
> +/// is done by creating multiple readers to the same memory location, e.g. using
> +/// [`clone_reader`].
I think we should have consistent 100 column formatting. And not
something less.
> +///
> +/// # Examples
[...]
> + /// Reads raw data from the user slice into a kernel buffer.
> + ///
> + /// Fails with `EFAULT` if the read happens on a bad address.
> + pub fn read_raw(&mut self, out: &mut [MaybeUninit<u8>]) -> Result {
> + let len = out.len();
> + let out_ptr = out.as_mut_ptr().cast::<c_void>();
> + if len > self.length {
> + return Err(EFAULT);
> + }
> + let Ok(len_ulong) = c_ulong::try_from(len) else {
> + return Err(EFAULT);
> + };
> + // SAFETY: The caller promises that `out` is valid for writing `len` bytes.
This comment needs updating.
> + let res = unsafe { bindings::copy_from_user(out_ptr, self.ptr, len_ulong) };
> + if res != 0 {
> + return Err(EFAULT);
> + }
> + // Userspace pointers are not directly dereferencable by the kernel, so
> + // we cannot use `add`, which has C-style rules for defined behavior.
> + self.ptr = self.ptr.wrapping_byte_add(len);
> + self.length -= len;
> + Ok(())
> + }
> +
> + /// Reads raw data from the user slice into a kernel buffer.
> + ///
> + /// Fails with `EFAULT` if the read happens on a bad address.
> + pub fn read_slice(&mut self, out: &mut [u8]) -> Result {
> + // SAFETY: The types are compatible and `read_raw` doesn't write
> + // uninitialized bytes to `out`.
Can you add this as a guarantee to `read_raw`?
> + let out = unsafe { &mut *(out as *mut [u8] as *mut [MaybeUninit<u8>]) };
> + self.read_raw(out)
> + }
> +
> + /// Reads the entirety of the user slice, appending it to the end of the
> + /// provided buffer.
> + ///
> + /// Fails with `EFAULT` if the read happens on a bad address.
> + pub fn read_all(mut self, buf: &mut Vec<u8>) -> Result {
> + let len = self.length;
> + buf.try_reserve(len)?;
> +
> + // The call to `try_reserve` was successful, so the spare capacity is at
> + // least `len` bytes long.
> + self.read_raw(&mut buf.spare_capacity_mut()[..len])?;
> +
> + // SAFETY: Since the call to `read_raw` was successful, so the next
> + // `len` bytes of the vector have been initialized.
> + unsafe { buf.set_len(buf.len() + len) };
> + Ok(())
> + }
> +}
> +
> +/// A writer for [`UserSlice`].
> +///
> +/// Used to incrementally write into the user slice.
> +pub struct UserSliceWriter {
> + ptr: *mut c_void,
> + length: usize,
> +}
> +
> +impl UserSliceWriter {
> + /// Returns the amount of space remaining in this buffer.
> + ///
> + /// Note that even writing less than this number of bytes may fail.
> + pub fn len(&self) -> usize {
> + self.length
> + }
> +
> + /// Returns `true` if no more data can be written to this buffer.
> + pub fn is_empty(&self) -> bool {
> + self.length == 0
> + }
> +
> + /// Writes raw data to this user pointer from a kernel buffer.
> + ///
> + /// Fails with `EFAULT` if the write happens on a bad address.
> + pub fn write_slice(&mut self, data: &[u8]) -> Result {
> + let len = data.len();
> + let data_ptr = data.as_ptr().cast::<c_void>();
> + if len > self.length {
> + return Err(EFAULT);
> + }
> + let Ok(len_ulong) = c_ulong::try_from(len) else {
> + return Err(EFAULT);
> + };
> + let res = unsafe { bindings::copy_to_user(self.ptr, data_ptr, len_ulong) };
Missing SAFETY comment.
--
Cheers,
Benno
> + if res != 0 {
> + return Err(EFAULT);
> + }
> + // Userspace pointers are not directly dereferencable by the kernel, so
> + // we cannot use `add`, which has C-style rules for defined behavior.
> + self.ptr = self.ptr.wrapping_byte_add(len);
> + self.length -= len;
> + Ok(())
> + }
> +}
>
> --
> 2.44.0.478.gd926399ef9-goog
>
On 04.04.24 14:31, Alice Ryhl wrote:
> Adds a new struct called `Page` that wraps a pointer to `struct page`.
> This struct is assumed to hold ownership over the page, so that Rust
> code can allocate and manage pages directly.
>
> The page type has various methods for reading and writing into the page.
> These methods will temporarily map the page to allow the operation. All
> of these methods use a helper that takes an offset and length, performs
> bounds checks, and returns a pointer to the given offset in the page.
>
> This patch only adds support for pages of order zero, as that is all
> Rust Binder needs. However, it is written to make it easy to add support
> for higher-order pages in the future. To do that, you would add a const
> generic parameter to `Page` that specifies the order. Most of the
> methods do not need to be adjusted, as the logic for dealing with
> mapping multiple pages at once can be isolated to just the
> `with_pointer_into_page` method. Finally, the struct can be renamed to
> `Pages<ORDER>`, and the type alias `Page = Pages<0>` can be introduced.
This part seems outdated, I think we probably make `ORDER` default to 0.
>
> Rust Binder needs to manage pages directly as that is how transactions
> are delivered: Each process has an mmap'd region for incoming
> transactions. When an incoming transaction arrives, the Binder driver
> will choose a region in the mmap, allocate and map the relevant pages
> manually, and copy the incoming transaction directly into the page. This
> architecture allows the driver to copy transactions directly from the
> address space of one process to another, without an intermediate copy
> to a kernel buffer.
[...]
> diff --git a/rust/kernel/page.rs b/rust/kernel/page.rs
> new file mode 100644
> index 000000000000..5aba0261242d
> --- /dev/null
> +++ b/rust/kernel/page.rs
> @@ -0,0 +1,259 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +//! Kernel page allocation and management.
> +
> +use crate::{bindings, error::code::*, error::Result, uaccess::UserSliceReader};
> +use core::{
> + alloc::AllocError,
> + ptr::{self, NonNull},
> +};
> +
> +/// A bitwise shift for the page size.
> +#[allow(clippy::unnecessary_cast)]
Why can't you remove the cast?
> +pub const PAGE_SHIFT: usize = bindings::PAGE_SHIFT as usize;
> +
> +/// The number of bytes in a page.
> +#[allow(clippy::unnecessary_cast)]
> +pub const PAGE_SIZE: usize = bindings::PAGE_SIZE as usize;
> +
> +/// A bitmask that gives the page containing a given address.
> +pub const PAGE_MASK: usize = !(PAGE_SIZE-1);
This line doesn't seem to be correctly formatted.
> +
> +/// Flags for the "get free page" function that underlies all memory allocations.
> +pub mod flags {
> + /// gfp flags.
> + #[allow(non_camel_case_types)]
> + pub type gfp_t = bindings::gfp_t;
> +
> + /// `GFP_KERNEL` is typical for kernel-internal allocations. The caller requires `ZONE_NORMAL`
> + /// or a lower zone for direct access but can direct reclaim.
> + pub const GFP_KERNEL: gfp_t = bindings::GFP_KERNEL;
> + /// `GFP_ZERO` returns a zeroed page on success.
> + pub const __GFP_ZERO: gfp_t = bindings::__GFP_ZERO;
> + /// `GFP_HIGHMEM` indicates that the allocated memory may be located in high memory.
> + pub const __GFP_HIGHMEM: gfp_t = bindings::__GFP_HIGHMEM;
> +}
> +
> +/// A pointer to a page that owns the page allocation.
> +///
> +/// # Invariants
> +///
> +/// The pointer is valid, and has ownership over the page.
> +pub struct Page {
> + page: NonNull<bindings::page>,
> +}
> +
> +// SAFETY: Pages have no logic that relies on them staying on a given thread, so
> +// moving them across threads is safe.
> +unsafe impl Send for Page {}
> +
> +// SAFETY: Pages have no logic that relies on them not being accessed
> +// concurrently, so accessing them concurrently is safe.
> +unsafe impl Sync for Page {}
> +
> +impl Page {
> + /// Allocates a new page.
> + pub fn alloc_page(gfp_flags: flags::gfp_t) -> Result<Self, AllocError> {
> + // SAFETY: Depending on the value of `gfp_flags`, this call may sleep.
> + // Other than that, it is always safe to call this method.
> + let page = unsafe { bindings::alloc_pages(gfp_flags, 0) };
> + let page = NonNull::new(page).ok_or(AllocError)?;
> + // INVARIANT: We just successfully allocated a page, so we now have
> + // ownership of the newly allocated page. We transfer that ownership to
> + // the new `Page` object.
> + Ok(Self { page })
> + }
> +
> + /// Returns a raw pointer to the page.
> + pub fn as_ptr(&self) -> *mut bindings::page {
> + self.page.as_ptr()
> + }
> +
> + /// Runs a piece of code with this page mapped to an address.
> + ///
> + /// The page is unmapped when this call returns.
> + ///
> + /// # Using the raw pointer
> + ///
> + /// It is up to the caller to use the provided raw pointer correctly The
> + /// pointer is valid for `PAGE_SIZE` bytes and for the duration in which the
> + /// closure is called. The pointer might only be mapped on the current
> + /// thread, and when that is the case, dereferencing it on other threads is
> + /// UB. Other than that, the usual rules for dereferencing a raw pointer
> + /// apply: don't cause data races, the memory may be uninitialized, and so
> + /// on.
> + ///
> + /// If multiple threads map the same page at the same time, then they may
> + /// reference with different addresses. However, even if the addresses are
> + /// different, the underlying memory is still the same for these purposes
> + /// (e.g., it's still a data race if they both write to the same underlying
> + /// byte at the same time).
This is nice.
--
Cheers,
Benno
> + fn with_page_mapped<T>(&self, f: impl FnOnce(*mut u8) -> T) -> T {
> + // SAFETY: `page` is valid due to the type invariants on `Page`.
> + let mapped_addr = unsafe { bindings::kmap_local_page(self.as_ptr()) };
> +
> + let res = f(mapped_addr.cast());
> +
> + // This unmaps the page mapped above.
> + //
> + // SAFETY: Since this API takes the user code as a closure, it can only
> + // be used in a manner where the pages are unmapped in reverse order.
> + // This is as required by `kunmap_local`.
> + //
> + // In other words, if this call to `kunmap_local` happens when a
> + // different page should be unmapped first, then there must necessarily
> + // be a call to `kmap_local_page` other than the call just above in
> + // `with_page_mapped` that made that possible. In this case, it is the
> + // unsafe block that wraps that other call that is incorrect.
> + unsafe { bindings::kunmap_local(mapped_addr) };
> +
> + res
> + }
On Fri, Apr 5, 2024 at 12:33 AM Benno Lossin <[email protected]> wrote:
>
> On 04.04.24 14:31, Alice Ryhl wrote:
> > +/// A bitwise shift for the page size.
> > +#[allow(clippy::unnecessary_cast)]
>
> Why can't you remove the cast?
Bindgen could decide to use a different type in the future or on
different platforms.
Alice
On Sun, Apr 7, 2024 at 10:59 AM Benno Lossin <[email protected]> wrote:
>
> On 05.04.24 09:44, Alice Ryhl wrote:
> > On Fri, Apr 5, 2024 at 12:33 AM Benno Lossin <[email protected]> wrote:
> >>
> >> On 04.04.24 14:31, Alice Ryhl wrote:
> >>> +/// A bitwise shift for the page size.
> >>> +#[allow(clippy::unnecessary_cast)]
> >>
> >> Why can't you remove the cast?
> >
> > Bindgen could decide to use a different type in the future or on
> > different platforms.
>
> Did that already happen?
>
> I think that we might want to know if the type changes, since then the
> value might change?
I mean, it's quite unlikely that the page size will not fit in an
usize, even if it changes?
From bindgen's point of view, this constant is just an integer literal
with no type information. So I don't see how we can expect it to
always be generated as a usize?
Alice
On Mon, Apr 8, 2024 at 9:54 AM Alice Ryhl <[email protected]> wrote:
>
> From bindgen's point of view, this constant is just an integer literal
> with no type information. So I don't see how we can expect it to
> always be generated as a usize?
In the case of `PAGE_SIZE`, there is type information (`size_t`),
since it comes from the constant helper:
const size_t RUST_CONST_HELPER_PAGE_SIZE = PAGE_SIZE;
For the other one, `PAGE_SHIFT`, there is also type information
(`int`), but bindgen currently picks a type based on the value for
those (but ideally/eventually bindgen should respect it instead).
So for the former, the allow and the cast are not needed since we are
already handling it explicitly. And for the latter, if we want to have
it as `usize`, we should have the cast but not the allow, because now
it does `u32`, but if it respected the type, it would be `c_int` or
`i32`. So either way we would need the cast.
Cheers,
Miguel
On Mon, Apr 8, 2024 at 11:19 AM Miguel Ojeda
<[email protected]> wrote:
>
> On Mon, Apr 8, 2024 at 9:54 AM Alice Ryhl <[email protected]> wrote:
> >
> > From bindgen's point of view, this constant is just an integer literal
> > with no type information. So I don't see how we can expect it to
> > always be generated as a usize?
>
> In the case of `PAGE_SIZE`, there is type information (`size_t`),
> since it comes from the constant helper:
>
> const size_t RUST_CONST_HELPER_PAGE_SIZE = PAGE_SIZE;
>
> For the other one, `PAGE_SHIFT`, there is also type information
> (`int`), but bindgen currently picks a type based on the value for
> those (but ideally/eventually bindgen should respect it instead).
>
> So for the former, the allow and the cast are not needed since we are
> already handling it explicitly. And for the latter, if we want to have
> it as `usize`, we should have the cast but not the allow, because now
> it does `u32`, but if it respected the type, it would be `c_int` or
> `i32`. So either way we would need the cast.
Good point. Will fix this for next version.
Alice
On 05.04.24 09:44, Alice Ryhl wrote:
> On Fri, Apr 5, 2024 at 12:33 AM Benno Lossin <benno.lossin@protonme> wrote:
>>
>> On 04.04.24 14:31, Alice Ryhl wrote:
>>> +/// A bitwise shift for the page size.
>>> +#[allow(clippy::unnecessary_cast)]
>>
>> Why can't you remove the cast?
>
> Bindgen could decide to use a different type in the future or on
> different platforms.
Did that already happen?
I think that we might want to know if the type changes, since then the
value might change?
--
Cheers,
Benno