Re: [PATCH] rust: task: clarify comments on task UID accessors

["Gary Guo" <gary@garyguo.net>]

On Fri Feb 13, 2026 at 2:00 AM CST, Jann Horn wrote:
> Linux has separate subjective and objective task credentials, see the
> comment above `struct cred`. Clarify which accessor functions operate on
> which set of credentials.
>
> Also document that Task::euid() is a very weird operation. You can see how
> weird it is by grepping for task_euid() - binder is its only user.
> Task::euid() obtains the objective effective UID - it looks at the
> credentials of the task for purposes of acting on it as an object, but then
> accesses the effective UID (which the credentials.7 man page describes as
> "[...] used by the kernel to determine the permissions that the process
> will have when accessing shared resources [...]").
>
> For context:
> Arguably, binder's use of task_euid() is a theoretical security problem,
> which only has no impact on Android because Android has no setuid binaries
> executable by apps.

If there's no setuid binary, then the `task_euid` can also just be replaced with
`task_uid`?

> commit 29bc22ac5e5b ("binder: use euid from cred instead of using task")
> fixed that by removing that only user of task_euid(), but the fix got
> reverted in commit c21a80ca0684 ("binder: fix test regression due to
> sender_euid change") because some Android test started failing.

What exactly is relying on the current behaviour? It'll better to fix that and
remove `task_euid` completely as I find "objective effective UID" quite
confusing, even with disclaimer that it shouldn't be used.

Best,
Gary


>
> Signed-off-by: Jann Horn <jannh@google.com>
> ---
>  rust/kernel/task.rs | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
>
> diff --git a/rust/kernel/task.rs b/rust/kernel/task.rs
> index 49fad6de0674..33e6d44b9a15 100644
> --- a/rust/kernel/task.rs
> +++ b/rust/kernel/task.rs
> @@ -223,14 +223,17 @@ pub fn pid(&self) -> Pid {
>          unsafe { *ptr::addr_of!((*self.as_ptr()).pid) }
>      }
>  
> -    /// Returns the UID of the given task.
> +    /// Returns the objective real UID of the given task.
>      #[inline]
>      pub fn uid(&self) -> Kuid {
>          // SAFETY: It's always safe to call `task_uid` on a valid task.
>          Kuid::from_raw(unsafe { bindings::task_uid(self.as_ptr()) })
>      }
>  
> -    /// Returns the effective UID of the given task.
> +    /// Returns the objective effective UID of the given task.
> +    ///
> +    /// You should probably not be using this; the effective UID is normally
> +    /// only relevant in subjective credentials.
>      #[inline]
>      pub fn euid(&self) -> Kuid {
>          // SAFETY: It's always safe to call `task_euid` on a valid task.
> @@ -363,7 +366,7 @@ unsafe fn dec_ref(obj: ptr::NonNull<Self>) {
>  }
>  
>  impl Kuid {
> -    /// Get the current euid.
> +    /// Get the current subjective euid.
>      #[inline]
>      pub fn current_euid() -> Kuid {
>          // SAFETY: Just an FFI call.
>
> ---
> base-commit: 192c0159402e6bfbe13de6f8379546943297783d
> change-id: 20260212-rust-uid-f1b3a45c8084
>
> --  
> Jann Horn <jannh@google.com>