Re: [PATCH v2 2/2] rust: task: use atomic read for pid()

[Alice Ryhl <aliceryhl@google.com>]

On Thu, Feb 12, 2026 at 11:12:07PM +0100, Jann Horn wrote:
> (Note: This is not a bugfix, it just cleans up an incorrect assumption.)
> 
> Task::pid() wrongly assumes that task::pid remains constant until the task
> refcount drops to zero.
> 
> However, Linux has a special quirk where, when execve() is called by a
> thread other than the thread group leader (the main thread), the thread
> calling execve() swaps its identity with the thread group leader's,
> becoming the new thread group leader. This means task::pid can't be assumed
> to be immutable for non-current tasks.
> (The actual swapping of PIDs is implemented in exchange_tids().)
> 
> Signed-off-by: Jann Horn <jannh@google.com>

Thanks.

Reviewed-by: Alice Ryhl <aliceryhl@google.com>

> ---
>  rust/kernel/task.rs | 12 +++++++++---
>  1 file changed, 9 insertions(+), 3 deletions(-)
> 
> diff --git a/rust/kernel/task.rs b/rust/kernel/task.rs
> index 91ad88cdfd3b..de0d90b47862 100644
> --- a/rust/kernel/task.rs
> +++ b/rust/kernel/task.rs
> @@ -10,6 +10,8 @@
>      mm::MmWithUser,
>      pid_namespace::PidNamespace,
>      sync::aref::ARef,
> +    sync::atomic::ordering::Relaxed,
> +    sync::atomic::Atomic,
>      types::{NotThreadSafe, Opaque},
>  };
>  use core::{
> @@ -206,9 +208,13 @@ pub fn as_ptr(&self) -> *mut bindings::task_struct {
>  
>      /// Returns the PID of the given task.
>      pub fn pid(&self) -> Pid {
> -        // SAFETY: The pid of a task never changes after initialization, so reading this field is
> -        // not a data race.
> -        unsafe { *ptr::addr_of!((*self.as_ptr()).pid) }
> +        // SAFETY: The pid of a task almost never changes after initialization,
> +        // so reading this field is usually not a data race.
> +        // The exception is a race where the task is part of a process that
> +        // goes through execve(), see exchange_tids().
> +        // A temporary mutable pointer is created, but only actually used for
> +        // a load.
> +        unsafe { Atomic::from_ptr(&raw mut (*self.as_ptr()).pid).load(Relaxed) }
>      }
>  
>      /// Returns the UID of the given task.
> 
> -- 
> 2.53.0.273.g2a3d683680-goog
>