cred: delete task_euid()

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 [...]").

Since usage in Binder has now been removed, get rid of the resulting
dead code.

Changes to the zh_CN translation was carried out with the help of
Gemini and Google Translate, and since adjusted as per Alex Shi's
feedback.

Suggested-by: Jann Horn <jannh@google.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>

Documentation/security/credentials.rst

@@ -393,16 +393,14 @@ the credentials so obtained when they're finished with.
    The result of ``__task_cred()`` should not be passed directly to
    ``get_cred()`` as this may race with ``commit_cred()``.
 
-There are a couple of convenience functions to access bits of another task's
-credentials, hiding the RCU magic from the caller::
+There is a convenience function to access bits of another task's credentials,
+hiding the RCU magic from the caller::
 
 	uid_t task_uid(task)		Task's real UID
-	uid_t task_euid(task)		Task's effective UID
 
 If the caller is holding the RCU read lock at the time anyway, then::
 
 	__task_cred(task)->uid
-	__task_cred(task)->euid
 
 should be used instead.  Similarly, if multiple aspects of a task's credentials
 need to be accessed, RCU read lock should be used, ``__task_cred()`` called,

Documentation/translations/zh_CN/security/credentials.rst

@@ -337,15 +337,13 @@ const指针上操作，因此不需要进行类型转换，但需要临时放弃
    ``__task_cred()`` 的结果不应直接传递给 ``get_cred()`` ，
    因为这可能与 ``commit_cred()`` 发生竞争条件。
 
-还有一些方便的函数可以访问另一个任务凭据的特定部分，将RCU操作对调用方隐藏起来::
+有一个方便的函数可用于访问另一个任务凭据的特定部分，从而对调用方隐藏RCU机制::
 
 	uid_t task_uid(task)		Task's real UID
-	uid_t task_euid(task)		Task's effective UID
 
 如果调用方在此时已经持有RCU读锁，则应使用::
 
 	__task_cred(task)->uid
-	__task_cred(task)->euid
 
 类似地，如果需要访问任务凭据的多个方面，应使用RCU读锁，调用 ``__task_cred()``
 函数，将结果存储在临时指针中，然后从临时指针中调用凭据的各个方面，最后释放锁。

include/linux/cred.h

@@ -371,7 +371,6 @@ DEFINE_FREE(put_cred, struct cred *, if (!IS_ERR_OR_NULL(_T)) put_cred(_T))
 })
 
 #define task_uid(task)		(task_cred_xxx((task), uid))
-#define task_euid(task)		(task_cred_xxx((task), euid))
 #define task_ucounts(task)	(task_cred_xxx((task), ucounts))
 
 #define current_cred_xxx(xxx)			\

rust/helpers/task.c

@@ -28,11 +28,6 @@ __rust_helper kuid_t rust_helper_task_uid(struct task_struct *task)
 	return task_uid(task);
 }
 
-__rust_helper kuid_t rust_helper_task_euid(struct task_struct *task)
-{
-	return task_euid(task);
-}
-
 #ifndef CONFIG_USER_NS
 __rust_helper uid_t rust_helper_from_kuid(struct user_namespace *to, kuid_t uid)
 {

rust/kernel/task.rs

@@ -217,16 +217,6 @@ pub fn uid(&self) -> Kuid {
         Kuid::from_raw(unsafe { bindings::task_uid(self.as_ptr()) })
     }
 
-    /// 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.
-        Kuid::from_raw(unsafe { bindings::task_euid(self.as_ptr()) })
-    }
-
     /// Determines whether the given task has pending signals.
     #[inline]
     pub fn signal_pending(&self) -> bool {