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

[Jann Horn <jannh@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@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>