[PATCH v4 1/2] rust: task: clarify comments on task UID accessors

[Gary Guo]

On Fri May 29, 2026 at 10:33 AM BST, Alice Ryhl wrote:
> From: Jann Horn <jannh at google.com>
> 
> 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.
> 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.
> 
> Signed-off-by: Jann Horn <jannh at google.com>
> Signed-off-by: Alice Ryhl <aliceryhl at google.com>

Reviewed-by: Gary Guo <gary at garyguo.net>

> ---
> Originally sent as:
> https://lore.kernel.org/r/20260212-rust-uid-v1-1-deff4214c766@google.com
> ---
>  rust/kernel/task.rs | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)