[PATCH] override RLIMIT_SIGPENDING for non-RT signals

I can read POSIX to say that the siginfo_t data must be available when
`kill' was used, as well.  This patch makes it allocate the siginfo_t, even
when that exceeds {RLIMIT_SIGPENDING}, for any non-RT signal (< SIGRTMIN)
not sent by sigqueue (actually, any signal that couldn't have been faked by
a sigqueue call).  Of course, in an extreme memory shortage situation, you
are SOL and violate POSIX a little before you die horribly from being out
of memory anyway.

The LEGACY_QUEUE logic already ensures that, for non-RT signals, at most
one is ever on the queue.  So there really is no risk at all of unbounded
resource consumption; the usage can reach {RLIMIT_SIGPENDING} + 31, is all.

It's already the case that the limit can be exceeded by (in theory) up to
{RLIMIT_NPROC}-1 in race conditions because the bump and the limit check
are not atomic.  (Obviously you can only get anywhere near that many with
assloads of preemption, but exceeding it by a few is not too unlikely.)
This patch also fixes that accounting so that it should not be possible to
exceed {RLIMIT_SIGPENDING} + SIGRTMIN-1 queue items per user in races.
Signed-off-by: Roland McGrath <roland@redhat.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

kernel/signal.c

@@ -261,19 +261,23 @@ next_signal(struct sigpending *pending, sigset_t *mask)
 	return sig;
 }
 
-static struct sigqueue *__sigqueue_alloc(struct task_struct *t, int flags)
+static struct sigqueue *__sigqueue_alloc(struct task_struct *t, int flags,
+					 int override_rlimit)
 {
 	struct sigqueue *q = NULL;
 
-	if (atomic_read(&t->user->sigpending) <
+	atomic_inc(&t->user->sigpending);
+	if (override_rlimit ||
+	    atomic_read(&t->user->sigpending) <=
 			t->signal->rlim[RLIMIT_SIGPENDING].rlim_cur)
 		q = kmem_cache_alloc(sigqueue_cachep, flags);
-	if (q) {
+	if (unlikely(q == NULL)) {
+		atomic_dec(&t->user->sigpending);
+	} else {
 		INIT_LIST_HEAD(&q->list);
 		q->flags = 0;
 		q->lock = NULL;
 		q->user = get_uid(t->user);
-		atomic_inc(&q->user->sigpending);
 	}
 	return(q);
 }
@@ -805,7 +809,9 @@ static int send_signal(int sig, struct siginfo *info, struct task_struct *t,
 	   make sure at least one signal gets delivered and don't
 	   pass on the info struct.  */
 
-	q = __sigqueue_alloc(t, GFP_ATOMIC);
+	q = __sigqueue_alloc(t, GFP_ATOMIC, (sig < SIGRTMIN &&
+					     ((unsigned long) info < 2 ||
+					      info->si_code >= 0)));
 	if (q) {
 		list_add_tail(&q->list, &signals->list);
 		switch ((unsigned long) info) {
@@ -1328,7 +1334,7 @@ struct sigqueue *sigqueue_alloc(void)
 {
 	struct sigqueue *q;
 
-	if ((q = __sigqueue_alloc(current, GFP_KERNEL)))
+	if ((q = __sigqueue_alloc(current, GFP_KERNEL, 0)))
 		q->flags |= SIGQUEUE_PREALLOC;
 	return(q);
 }