runtime: crash on SI_USER SigPanic signal

Clean up the code a little bit to make it clearer:

Don't check throwsplit for a SI_USER signal.

If throwsplit is set for a SigPanic signal, always throw;
discard any other flags.

Fixes #36420

Change-Id: Ic9dcd1108603d241f71c040504dfdc6e528f9767
Reviewed-on: https://go-review.googlesource.com/c/go/+/228900
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

1 files changed, 7 insertions, 3 deletions

diff --git a/src/runtime/signal_unix.go b/src/runtime/signal_unix.go
index c33f88b046..f5d79e561c 100644
--- a/src/runtime/signal_unix.go
+++ b/src/runtime/signal_unix.go
@@ -546,10 +546,10 @@ func sighandler(sig uint32, info *siginfo, ctxt unsafe.Pointer, gp *g) {
 	if sig < uint32(len(sigtable)) {
 		flags = sigtable[sig].flags
 	}
-	if flags&_SigPanic != 0 && gp.throwsplit {
+	if c.sigcode() != _SI_USER && flags&_SigPanic != 0 && gp.throwsplit {
 		// We can't safely sigpanic because it may grow the
 		// stack. Abort in the signal handler instead.
-		flags = (flags &^ _SigPanic) | _SigThrow
+		flags = _SigThrow
 	}
 	if isAbortPC(c.sigpc()) {
 		// On many architectures, the abort function just
@@ -588,7 +588,11 @@ func sighandler(sig uint32, info *siginfo, ctxt unsafe.Pointer, gp *g) {
 		dieFromSignal(sig)
 	}
 
-	if flags&_SigThrow == 0 {
+	// _SigThrow means that we should exit now.
+	// If we get here with _SigPanic, it means that the signal
+	// was sent to us by a program (c.sigcode() == _SI_USER);
+	// in that case, if we didn't handle it in sigsend, we exit now.
+	if flags&(_SigThrow|_SigPanic) == 0 {
 		return
 	}