CRED: Fix __task_cred()'s lockdep check and banner comment
David Howells [Thu, 29 Jul 2010 11:45:55 +0000 (12:45 +0100)]
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>

Documentation/credentials.txt
include/linux/cred.h
include/linux/sched.h

index a2db352..995baf3 100644 (file)
@@ -417,6 +417,9 @@ reference on them using:
 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:
 
index ce40cbc..4d2c395 100644 (file)
@@ -274,13 +274,18 @@ static inline void put_cred(const struct cred *_cred)
  * @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
index 747fcae..0478888 100644 (file)
@@ -214,6 +214,7 @@ extern char ___assert_task_state[1 - 2*!!(
 
 #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) \