[PATCH 2/2] CRED: Fix __task_cred()'s lockdep check and banner comment
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From: David Howells on 29 Jul 2010 07:50 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(a)redhat.com> Acked-by: Jiri Olsa <jolsa(a)redhat.com> Cc: Paul E. McKenney <paulmck(a)linux.vnet.ibm.com> --- Documentation/credentials.txt | 3 +++ include/linux/cred.h | 15 ++++++++++----- include/linux/sched.h | 1 + 3 files changed, 14 insertions(+), 5 deletions(-) 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 @@ 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 ce40cbc..4d2c395 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 747fcae..0478888 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) \ -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/ |