diff options
authorDavid Howells <dhowells@redhat.com>2010-07-29 12:45:55 +0100
committerLinus Torvalds <torvalds@linux-foundation.org>2010-07-29 15:16:18 -0700
commit8f92054e7ca1d3a3ae50fb42d2253ac8730d9b2a (patch)
parentde09a9771a5346029f4d11e4ac886be7f9bfdd75 (diff)
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>
3 files changed, 14 insertions, 5 deletions
diff --git a/Documentation/credentials.txt b/Documentation/credentials.txt
index a2db35287003..995baf379c07 100644
--- a/Documentation/credentials.txt
+++ b/Documentation/credentials.txt
@@ -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:
diff --git a/include/linux/cred.h b/include/linux/cred.h
index ce40cbc791e2..4d2c39573f36 100644
--- a/include/linux/cred.h
+++ b/include/linux/cred.h
@@ -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
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 747fcaedddb7..0478888c6899 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -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) \