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Date:   Fri, 31 Mar 2023 03:47:01 +0100
From:   Gary Guo <gary@garyguo.net>
To:     Wedson Almeida Filho <wedsonaf@gmail.com>
Cc:     rust-for-linux@vger.kernel.org, Miguel Ojeda <ojeda@kernel.org>,
        Alex Gaynor <alex.gaynor@gmail.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        =?UTF-8?B?QmrDtnJu?= Roy Baron <bjorn3_gh@protonmail.com>,
        linux-kernel@vger.kernel.org,
        Wedson Almeida Filho <walmeida@microsoft.com>,
        Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH 10/13] rust: introduce `Task::current`
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On Thu, 30 Mar 2023 01:39:51 -0300
Wedson Almeida Filho <wedsonaf@gmail.com> wrote:

> From: Wedson Almeida Filho <walmeida@microsoft.com>
> 
> This allows Rust code to get a reference to the current task without
> having to increment the refcount, but still guaranteeing memory safety.
> 
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Signed-off-by: Wedson Almeida Filho <walmeida@microsoft.com>
> ---
>  rust/helpers.c      |  6 ++++
>  rust/kernel/task.rs | 83 ++++++++++++++++++++++++++++++++++++++++++++-
>  2 files changed, 88 insertions(+), 1 deletion(-)
> 
> diff --git a/rust/helpers.c b/rust/helpers.c
> index 58a194042c86..96441744030e 100644
> --- a/rust/helpers.c
> +++ b/rust/helpers.c
> @@ -100,6 +100,12 @@ bool rust_helper_refcount_dec_and_test(refcount_t *r)
>  }
>  EXPORT_SYMBOL_GPL(rust_helper_refcount_dec_and_test);
>  
> +struct task_struct *rust_helper_get_current(void)
> +{
> +	return current;
> +}
> +EXPORT_SYMBOL_GPL(rust_helper_get_current);
> +
>  void rust_helper_get_task_struct(struct task_struct *t)
>  {
>  	get_task_struct(t);
> diff --git a/rust/kernel/task.rs b/rust/kernel/task.rs
> index 8d7a8222990f..8b2b56ba9c6d 100644
> --- a/rust/kernel/task.rs
> +++ b/rust/kernel/task.rs
> @@ -5,7 +5,7 @@
>  //! C header: [`include/linux/sched.h`](../../../../include/linux/sched.h).
>  
>  use crate::bindings;
> -use core::{cell::UnsafeCell, ptr};
> +use core::{cell::UnsafeCell, marker::PhantomData, ops::Deref, ptr};
>  
>  /// Wraps the kernel's `struct task_struct`.
>  ///
> @@ -13,6 +13,46 @@ use core::{cell::UnsafeCell, ptr};
>  ///
>  /// Instances of this type are always ref-counted, that is, a call to `get_task_struct` ensures
>  /// that the allocation remains valid at least until the matching call to `put_task_struct`.
> +///
> +/// # Examples
> +///
> +/// The following is an example of getting the PID of the current thread with zero additional cost
> +/// when compared to the C version:
> +///
> +/// ```
> +/// use kernel::task::Task;
> +///
> +/// let pid = Task::current().pid();
> +/// ```
> +///
> +/// Getting the PID of the current process, also zero additional cost:
> +///
> +/// ```
> +/// use kernel::task::Task;
> +///
> +/// let pid = Task::current().group_leader().pid();
> +/// ```
> +///
> +/// Getting the current task and storing it in some struct. The reference count is automatically
> +/// incremented when creating `State` and decremented when it is dropped:
> +///
> +/// ```
> +/// use kernel::{task::Task, ARef};
> +///
> +/// struct State {
> +///     creator: ARef<Task>,
> +///     index: u32,
> +/// }
> +///
> +/// impl State {
> +///     fn new() -> Self {
> +///         Self {
> +///             creator: Task::current().into(),
> +///             index: 0,
> +///         }
> +///     }
> +/// }
> +/// ```
>  #[repr(transparent)]
>  pub struct Task(pub(crate) UnsafeCell<bindings::task_struct>);
>  
> @@ -25,6 +65,20 @@ unsafe impl Sync for Task {}
>  type Pid = bindings::pid_t;
>  
>  impl Task {
> +    /// Returns a task reference for the currently executing task/thread.
> +    pub fn current<'a>() -> TaskRef<'a> {
> +        // SAFETY: Just an FFI call with no additional safety requirements.
> +        let ptr = unsafe { bindings::get_current() };
> +
> +        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() },
> +            _not_send: PhantomData,
> +        }
> +    }
> +

I don't think this API is sound, as you can do `&*Task::current()` and
get a `&'static Task`, which is very problematic.

A sound API would be

	pub fn with_current<R>(f: imp FnOnce(&Task) -> R) -> R { ... }

(which also is how thread local works in Rust)

You would have to write `Task::with_current(|cur| cur.pid())` though,
which unfortunately is a bit less ergonomic.

Best,
Gary