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SubjectRe: [PATCH 0/4] Was: deferring __fput()
On Thu, Jun 28, 2012 at 06:45:39PM +0200, Oleg Nesterov wrote:
> Forgot to mention...
>
> And I still think that task_work_add() should not succeed unconditionally,
> it synchronize with exit_task_work(). Otherwise keyctl_session_to_parent()
> is broken.

Hmm... Look: if nothing else, we have
/* the parent mustn't be init and mustn't be a kernel thread */
if (parent->pid <= 1 || !parent->mm)
goto unlock;
in the caller. OTOH, on the exit side we have exit_mm() done first. And
that will have ->mm set to NULL. So we are closing a very narrow race to start
with. So why not do the following and be done with that?
diff --git a/security/keys/keyctl.c b/security/keys/keyctl.c
index 0291b3f..f1b59ae 100644
--- a/security/keys/keyctl.c
+++ b/security/keys/keyctl.c
@@ -1486,6 +1486,7 @@ long keyctl_session_to_parent(void)
oldwork = NULL;
parent = me->real_parent;

+ task_lock(parent);
/* the parent mustn't be init and mustn't be a kernel thread */
if (parent->pid <= 1 || !parent->mm)
goto unlock;
@@ -1529,6 +1530,7 @@ long keyctl_session_to_parent(void)
if (!ret)
newwork = NULL;
unlock:
+ task_unlock(parent);
write_unlock_irq(&tasklist_lock);
rcu_read_unlock();
if (oldwork)

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