libc: honor POSIX default-ignore disposition for SIGCHLD/SIGURG/SIGWINCH

libc/signal.cc

@@ -413,6 +413,15 @@ int kill(pid_t pid, int sig)
     }
     unsigned sigidx = sig - 1;
     if (is_sig_dfl(signal_actions[sigidx])) {
+        // Per POSIX, the default disposition of SIGCHLD, SIGURG and SIGWINCH is
+        // to IGNORE (not to terminate the process).  OSv treats an unhandled
+        // SIG_DFL signal as fatal and powers the VM off, which is wrong for
+        // these three: e.g. delivering SIGCHLD to a process with no handler
+        // installed must be silently ignored, not fatal.  Honor the ignore
+        // default for them instead of powering off.
+        if (sig == SIGCHLD || sig == SIGURG || sig == SIGWINCH) {
+            return 0;
+        }
         // Our default is to power off.
         debugf("Uncaught signal %d (\"%s\"). Powering off.\n",
                 sig, strsignal(sig));

modules/tests/Makefile

@@ -130,6 +130,7 @@ tests := tst-pthread.so misc-ramdisk.so tst-vblk.so tst-mq-smoke.so tst-bsd-evh.
 	tst-queue-mpsc.so tst-af-local.so tst-pipe.so tst-yield.so \
 	misc-ctxsw.so tst-read.so tst-symlink.so tst-openat.so \
 	tst-eventfd.so tst-remove.so misc-wake.so tst-epoll.so misc-lfring.so \
+	tst-sig-dfl-ignore.so \
 	misc-fsx.so tst-sleep.so tst-resolve.so tst-except.so \
 	misc-tcp-sendonly.so tst-tcp-nbwrite.so misc-tcp-hash-srv.so \
 	misc-loadbalance.so misc-scheduler.so tst-console.so tst-app.so \

tests/tst-sig-dfl-ignore.cc

@@ -0,0 +1,43 @@
+/*
+ * Copyright (C) 2026 Greg Burd
+ *
+ * This work is open source software, licensed under the terms of the
+ * BSD license as described in the LICENSE file in the top-level directory.
+ *
+ * SIGCHLD, SIGURG and SIGWINCH have a POSIX default disposition of "ignore".
+ * Delivering one of them to a process that has installed no handler must be a
+ * no-op, NOT fatal.  Before the fix, OSv's kill() treated any SIG_DFL signal as
+ * uncaught-and-fatal and powered the VM off, so raising SIGCHLD with no handler
+ * would kill the guest.  This test raises each of the three with the default
+ * disposition and asserts the process simply survives.
+ */
+
+#include <signal.h>
+#include <unistd.h>
+#include <cstdio>
+#include <cassert>
+#include <initializer_list>
+
+int main()
+{
+    printf("=== tst-sig-dfl-ignore ===\n");
+
+    // No handlers installed => default disposition.  Per POSIX these three
+    // default to ignore, so raise() must return 0 and we must keep running.
+    for (int sig : {SIGCHLD, SIGURG, SIGWINCH}) {
+        int r = raise(sig);
+        printf("raise(%d) -> %d\n", sig, r);
+        assert(r == 0);
+    }
+
+    // kill(getpid(), ...) of the same signals must also be a no-op survivor.
+    for (int sig : {SIGCHLD, SIGURG, SIGWINCH}) {
+        int r = kill(getpid(), sig);
+        printf("kill(self, %d) -> %d\n", sig, r);
+        assert(r == 0);
+    }
+
+    // If we reached here, the process was not terminated by any of them.
+    printf("=== tst-sig-dfl-ignore: PASS (survived SIGCHLD/SIGURG/SIGWINCH) ===\n");
+    return 0;
+}