avoid spurious POLLIN returns in signalfd

The new code in kernel/signal.c does not allow fetching private signals
from another task.  This patch avoid spurious POLLIN returns from a
signalfd poll(2) operation.
Signed-off-by: NDavide Libenzi <davidel@xmailserver.org>
Cc: Oleg Nesterov <oleg@tv-sign.ru>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

fs/signalfd.c

@@ -133,7 +133,8 @@ static unsigned int signalfd_poll(struct file *file, poll_table *wait)
 	 * the peer disconnects.
 	 */
 	if (signalfd_lock(ctx, &lk)) {
-		if (next_signal(&lk.tsk->pending, &ctx->sigmask) > 0 ||
+		if ((lk.tsk == current &&
+		     next_signal(&lk.tsk->pending, &ctx->sigmask) > 0) ||
 		    next_signal(&lk.tsk->signal->shared_pending,
 				&ctx->sigmask) > 0)
 			events |= POLLIN;