CRED: Fix __task_cred()'s lockdep check and banner comment
Fix __task_cred()'s lockdep check by removing the following validation condition: lockdep_tasklist_lock_is_held() as commit_creds() does not take the tasklist_lock, and nor do most of the functions that call it, so this check is pointless and it can prevent detection of the RCU lock not being held if the tasklist_lock is held. Instead, add the following validation condition: task->exit_state >= 0 to permit the access if the target task is dead and therefore unable to change its own credentials. Fix __task_cred()'s comment to: (1) discard the bit that says that the caller must prevent the target task from being deleted. That shouldn't need saying. (2) Add a comment indicating the result of __task_cred() should not be passed directly to get_cred(), but rather than get_task_cred() should be used instead. Also put a note into the documentation to enforce this point there too. Signed-off-by: David Howells <dhowells@redhat.com> Acked-by: Jiri Olsa <jolsa@redhat.com> Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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@ -417,6 +417,9 @@ reference on them using:
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This does all the RCU magic inside of it. The caller must call put_cred() on
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This does all the RCU magic inside of it. The caller must call put_cred() on
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the credentials so obtained when they're finished with.
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the credentials so obtained when they're finished with.
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[*] Note: The result of __task_cred() should not be passed directly to
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get_cred() as this may race with commit_cred().
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There are a couple of convenience functions to access bits of another task's
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There are a couple of convenience functions to access bits of another task's
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credentials, hiding the RCU magic from the caller:
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credentials, hiding the RCU magic from the caller:
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@ -274,13 +274,18 @@ static inline void put_cred(const struct cred *_cred)
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* @task: The task to query
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* @task: The task to query
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*
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*
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* Access the objective credentials of a task. The caller must hold the RCU
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* Access the objective credentials of a task. The caller must hold the RCU
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* readlock.
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* readlock or the task must be dead and unable to change its own credentials.
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*
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*
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* The caller must make sure task doesn't go away, either by holding a ref on
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* The result of this function should not be passed directly to get_cred();
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* task or by holding tasklist_lock to prevent it from being unlinked.
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* rather get_task_cred() should be used instead.
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*/
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*/
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#define __task_cred(task) \
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#define __task_cred(task) \
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((const struct cred *)(rcu_dereference_check((task)->real_cred, rcu_read_lock_held() || lockdep_tasklist_lock_is_held())))
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({ \
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const struct task_struct *__t = (task); \
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rcu_dereference_check(__t->real_cred, \
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rcu_read_lock_held() || \
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task_is_dead(__t)); \
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})
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/**
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/**
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* get_current_cred - Get the current task's subjective credentials
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* get_current_cred - Get the current task's subjective credentials
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@ -214,6 +214,7 @@ extern char ___assert_task_state[1 - 2*!!(
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#define task_is_traced(task) ((task->state & __TASK_TRACED) != 0)
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#define task_is_traced(task) ((task->state & __TASK_TRACED) != 0)
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#define task_is_stopped(task) ((task->state & __TASK_STOPPED) != 0)
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#define task_is_stopped(task) ((task->state & __TASK_STOPPED) != 0)
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#define task_is_dead(task) ((task)->exit_state != 0)
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#define task_is_stopped_or_traced(task) \
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#define task_is_stopped_or_traced(task) \
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((task->state & (__TASK_STOPPED | __TASK_TRACED)) != 0)
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((task->state & (__TASK_STOPPED | __TASK_TRACED)) != 0)
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#define task_contributes_to_load(task) \
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#define task_contributes_to_load(task) \
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