fix unintended observability of SIGABRT disposition reset via sigaction

the existing abort locking logic in sigaction only accounted for
attempts to change the disposition, not attempts to observe the change
made by abort.

unfortunately the change is still observable in at least one other
place: inheritance of signal dispositions across exec and posix_spawn.
fixing these is a separate task and it's not even clear whether a
complete fix is possible.

src/signal/sigaction.c

@@ -21,6 +21,13 @@ int __libc_sigaction(int sig, const struct sigaction *restrict sa, struct sigact
 {
 	struct k_sigaction ksa, ksa_old;
 	unsigned long set[_NSIG/(8*sizeof(long))];
+	/* Doing anything with the disposition of SIGABRT requires a lock,
+	 * so that it cannot be changed while abort is terminating the
+	 * process and so any change made by abort can't be observed. */
+	if (sig == SIGABRT) {
+		__block_all_sigs(&set);
+		LOCK(__abort_lock);
+	}
 	if (sa) {
 		if ((uintptr_t)sa->sa_handler > 1UL) {
 			a_or_l(handler_set+(sig-1)/(8*sizeof(long)),
@@ -44,21 +51,13 @@ int __libc_sigaction(int sig, const struct sigaction *restrict sa, struct sigact
 				a_store(&__eintr_valid_flag, 1);
 			}
 		}
-		/* Changing the disposition of SIGABRT to anything but
-		 * SIG_DFL requires a lock, so that it cannot be changed
-		 * while abort is terminating the process after simply
-		 * calling raise(SIGABRT) failed to do so. */
-		if (sa->sa_handler != SIG_DFL && sig == SIGABRT) {
-			__block_all_sigs(&set);
-			LOCK(__abort_lock);
-		}
 		ksa.handler = sa->sa_handler;
 		ksa.flags = sa->sa_flags | SA_RESTORER;
 		ksa.restorer = (sa->sa_flags & SA_SIGINFO) ? __restore_rt : __restore;
 		memcpy(&ksa.mask, &sa->sa_mask, _NSIG/8);
 	}
 	int r = __syscall(SYS_rt_sigaction, sig, sa?&ksa:0, old?&ksa_old:0, _NSIG/8);
-	if (sig == SIGABRT && sa && sa->sa_handler != SIG_DFL) {
+	if (sig == SIGABRT) {
 		UNLOCK(__abort_lock);
 		__restore_sigs(&set);
 	}