Handle NULL arguments in sigaction (#15114)

Fixes: #15062

Author: sbc100

system/lib/libc/sigaction.c

@@ -22,7 +22,10 @@ int __sigaction(int sig, const struct sigaction *restrict sa, struct sigaction *
     *old = __sig_actions[sig];
   }
 
-  __sig_actions[sig] = *sa;
+  if (sa) {
+    __sig_actions[sig] = *sa;
+  }
+
   return 0;
 }
 

tests/test_signals.c

@@ -127,7 +127,25 @@ void test_sigwaitinfo() {
   assert(!recieved1);
 }
 
+void test_sigaction() {
+  // Use sigaction to find the existing handlers
+  struct sigaction action;
+  sigaction(SIGUSR1, NULL, &action);
+  assert(action.sa_handler == SIG_DFL);
+  assert((void (*)(int))action.sa_sigaction == SIG_DFL);
+
+  // Now install a new handler
+  action.sa_handler = handler1;
+  sigaction(SIGUSR1, &action, NULL);
+
+  // Verify that the new handler is returned
+  struct sigaction action2;
+  sigaction(SIGUSR1, NULL, &action2);
+  assert(action2.sa_handler == handler1);
+}
+
 int main() {
+  test_sigaction();
   test_bad_signal();
   test_raise_sigusr1();
   test_sigpenging();