Fix signalfd interaction with thread-private signals

Don't let signalfd dequeue private signals off other threads (in the
case of things like SIGILL or SIGSEGV, trying to do so would result
in undefined behaviour on who actually gets the signal, since they
are force unblocked).

Signed-off-by: Benjamin Herrenschmidt <[e-mail]>
Acked-by: Davide Libenzi <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

kernel/signal.c

···
  */
 int dequeue_signal(struct task_struct *tsk, sigset_t *mask, siginfo_t *info)
 {
-	int signr = __dequeue_signal(&tsk->pending, mask, info);
+	int signr = 0;
+
+	/* We only dequeue private signals from ourselves, we don't let
+	 * signalfd steal them
+	 */
+	if (tsk == current)
+		signr = __dequeue_signal(&tsk->pending, mask, info);
 	if (!signr) {
 		signr = __dequeue_signal(&tsk->signal->shared_pending,
 					 mask, info);