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>
diff --git a/Documentation/credentials.txt b/Documentation/credentials.txt
index a2db352..995baf3 100644
--- a/Documentation/credentials.txt
+++ b/Documentation/credentials.txt
@@ -417,6 +417,9 @@
This does all the RCU magic inside of it. The caller must call put_cred() on
the credentials so obtained when they're finished with.
+ [*] Note: 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:
diff --git a/include/linux/cred.h b/include/linux/cred.h
index ce40cbc..4d2c395 100644
--- a/include/linux/cred.h
+++ b/include/linux/cred.h
@@ -274,13 +274,18 @@
* @task: The task to query
*
* Access the objective credentials of a task. The caller must hold the RCU
- * readlock.
+ * readlock or the task must be dead and unable to change its own credentials.
*
- * The caller must make sure task doesn't go away, either by holding a ref on
- * task or by holding tasklist_lock to prevent it from being unlinked.
+ * The result of this function should not be passed directly to get_cred();
+ * rather get_task_cred() should be used instead.
*/
-#define __task_cred(task) \
- ((const struct cred *)(rcu_dereference_check((task)->real_cred, rcu_read_lock_held() || lockdep_tasklist_lock_is_held())))
+#define __task_cred(task) \
+ ({ \
+ const struct task_struct *__t = (task); \
+ rcu_dereference_check(__t->real_cred, \
+ rcu_read_lock_held() || \
+ task_is_dead(__t)); \
+ })
/**
* get_current_cred - Get the current task's subjective credentials
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 747fcae..0478888 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -214,6 +214,7 @@
#define task_is_traced(task) ((task->state & __TASK_TRACED) != 0)
#define task_is_stopped(task) ((task->state & __TASK_STOPPED) != 0)
+#define task_is_dead(task) ((task)->exit_state != 0)
#define task_is_stopped_or_traced(task) \
((task->state & (__TASK_STOPPED | __TASK_TRACED)) != 0)
#define task_contributes_to_load(task) \