From 89e54f5594a917c0d064b426e1ee77a9241c7d85 Mon Sep 17 00:00:00 2001
From: Sam Clegg <sbc@chromium.org>
Date: Thu, 23 Sep 2021 18:27:34 -0700
Subject: [PATCH] Handle NULL arguments in sigaction

Fixes: #15062
---
 system/lib/libc/sigaction.c |  5 ++++-
 tests/test_signals.c        | 18 ++++++++++++++++++
 2 files changed, 22 insertions(+), 1 deletion(-)

diff --git a/system/lib/libc/sigaction.c b/system/lib/libc/sigaction.c
index e76456559a848..080ad45c5a815 100644
--- a/system/lib/libc/sigaction.c
+++ b/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;
 }
 
diff --git a/tests/test_signals.c b/tests/test_signals.c
index 711493215ecb7..10f7839319c5e 100644
--- a/tests/test_signals.c
+++ b/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();