[PATCH] signal: cleanup the .si_signo checks in dequeue_synchronous_signal()

[Oleg Nesterov <oleg@redhat.com>]

- Use SI_FROMKERNEL() instead of open-coded check

- Use rt_sigmask() instead of sigmask() to avoid UB on 32-bit if
  .si_signo > 32

- Pre-calculate the unblocked synchronous signal "mask" to not check
  .si_signo twice.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
 kernel/signal.c | 18 +++++++-----------
 1 file changed, 7 insertions(+), 11 deletions(-)

diff --git a/kernel/signal.c b/kernel/signal.c
index 7386b9552dc3..64259219bbdc 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -676,21 +676,17 @@ static int dequeue_synchronous_signal(kernel_siginfo_t *info)
 	struct task_struct *tsk = current;
 	struct sigpending *pending = &tsk->pending;
 	struct sigqueue *q, *sync = NULL;
+	unsigned long mask;
 
-	/*
-	 * Might a synchronous signal be in the queue?
-	 */
-	if (!((pending->signal.sig[0] & ~tsk->blocked.sig[0]) & SYNCHRONOUS_MASK))
+	/* Might a synchronous signal be in the queue? */
+	mask = (pending->signal.sig[0] & ~tsk->blocked.sig[0]) & SYNCHRONOUS_MASK;
+	if (!mask)
 		return 0;
 
-	/*
-	 * Return the first synchronous signal in the queue.
-	 */
+	/* Return the first synchronous signal in the queue. */
 	list_for_each_entry(q, &pending->list, list) {
-		/* Synchronous signals have a positive si_code */
-		if ((q->info.si_code > SI_USER) &&
-		    (sigmask(q->info.si_signo) & SYNCHRONOUS_MASK) &&
-		    !sigismember(&tsk->blocked, q->info.si_signo)) {
+		if (SI_FROMKERNEL(&q->info) &&
+		    (rt_sigmask(q->info.si_signo) & mask)) {
 			sync = q;
 			goto next;
 		}
-- 
2.55.0