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>

doc/go1.15.html

@@ -198,6 +198,17 @@ TODO
       <code>uint</code>, <code>uint8</code>, <code>uint16</code>, <code>uint32</code>, <code>uint64</code>, <code>uintptr</code>,
       then the value will be printed, instead of just its address.
     </p>
+
+    <p><!-- CL -->
+      On a Unix system, if the <code>kill</code> command
+      or <code>kill</code> system call is used to send
+      a <code>SIGSEGV</code>, <code>SIGBUS</code>,
+      or <code>SIGFPE</code> signal to a Go program, and if the signal
+      is not being handled via
+      <a href="/pkg/os/signal/#Notify"><code>os/signal.Notify</code></a>,
+      the Go program will now reliably crash with a stack trace.
+      In earlier releases the behavior was unpredictable.
+    </p>
   </dd>
 </dl>
 

src/runtime/crash_cgo_test.go

@@ -555,3 +555,21 @@ func findTrace(text, top string) []string {
 	}
 	return nil
 }
+
+func TestSegv(t *testing.T) {
+	switch runtime.GOOS {
+	case "plan9", "windows":
+		t.Skipf("no signals on %s", runtime.GOOS)
+	}
+
+	for _, test := range []string{"Segv", "SegvInCgo"} {
+		t.Run(test, func(t *testing.T) {
+			t.Parallel()
+			got := runTestProg(t, "testprogcgo", test)
+			t.Log(got)
+			if !strings.Contains(got, "SIGSEGV") {
+				t.Errorf("expected crash from signal")
+			}
+		})
+	}
+}

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
 	}
 

src/runtime/testdata/testprogcgo/segv.go

@@ -0,0 +1,60 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build !plan9,!windows
+
+package main
+
+// static void nop() {}
+import "C"
+
+import (
+	"sync"
+	"syscall"
+)
+
+func init() {
+	register("Segv", Segv)
+	register("SegvInCgo", SegvInCgo)
+}
+
+var Sum int
+
+func Segv() {
+	c := make(chan bool)
+	var wg sync.WaitGroup
+	wg.Add(1)
+	go func() {
+		defer wg.Done()
+		close(c)
+		for i := 0; i < 10000; i++ {
+			Sum += i
+		}
+	}()
+
+	<-c
+
+	syscall.Kill(syscall.Getpid(), syscall.SIGSEGV)
+
+	wg.Wait()
+}
+
+func SegvInCgo() {
+	c := make(chan bool)
+	var wg sync.WaitGroup
+	wg.Add(1)
+	go func() {
+		defer wg.Done()
+		close(c)
+		for i := 0; i < 10000; i++ {
+			C.nop()
+		}
+	}()
+
+	<-c
+
+	syscall.Kill(syscall.Getpid(), syscall.SIGSEGV)
+
+	wg.Wait()
+}