Restrict clearing TIF_SIGPENDING

This patch should get a few birds.  It prevents sigaction calls from
clearing TIF_SIGPENDING in other threads, which could leak -ERESTART*.
And It fixes ptrace_stop not to clear it, which done at the syscall exit
stop could leak -ERESTART*.  It probably removes the harm from signalfd,
at least assuming it never calls dequeue_signal on kernel threads that
might have used block_all_signals.
Signed-off-by: Roland McGrath <roland@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Satoru Takeuchi <takeuchi_satoru@jp.fujitsu.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/signal.c

@@ -105,7 +105,11 @@ static int recalc_sigpending_tsk(struct task_struct *t)
 		set_tsk_thread_flag(t, TIF_SIGPENDING);
 		return 1;
 	}
-	clear_tsk_thread_flag(t, TIF_SIGPENDING);
+	/*
+	 * We must never clear the flag in another thread, or in current
+	 * when it's possible the current syscall is returning -ERESTART*.
+	 * So we don't clear it here, and only callers who know they should do.
+	 */
 	return 0;
 }
 
@@ -121,7 +125,9 @@ void recalc_sigpending_and_wake(struct task_struct *t)
 
 void recalc_sigpending(void)
 {
-	recalc_sigpending_tsk(current);
+	if (!recalc_sigpending_tsk(current))
+		clear_thread_flag(TIF_SIGPENDING);
+
 }
 
 /* Given the mask, find the first available signal that should be serviced. */
@@ -385,7 +391,8 @@ int dequeue_signal(struct task_struct *tsk, sigset_t *mask, siginfo_t *info)
 			}
 		}
 	}
-	recalc_sigpending_tsk(tsk);
+	if (likely(tsk == current))
+		recalc_sigpending();
 	if (signr && unlikely(sig_kernel_stop(signr))) {
 		/*
 		 * Set a marker that we have dequeued a stop signal.  Our
@@ -1580,8 +1587,9 @@ static void ptrace_stop(int exit_code, int nostop_code, siginfo_t *info)
 	/*
 	 * Queued signals ignored us while we were stopped for tracing.
 	 * So check for any that we should take before resuming user mode.
+	 * This sets TIF_SIGPENDING, but never clears it.
 	 */
-	recalc_sigpending();
+	recalc_sigpending_tsk(current);
 }
 
 void ptrace_notify(int exit_code)