runtime: check for nil g and m in msanread

fixes #18707.

Change-Id: Ibc4efef01197799f66d10bfead22faf8ac00473c
Reviewed-on: https://go-review.googlesource.com/35452
Run-TryBot: Bryan Mills <bcmills@google.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

misc/cgo/testsanitizers/msan_shared.go

+// Copyright 2017 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.
+
+// This program segfaulted during libpreinit when built with -msan:
+// http://golang.org/issue/18707
+
+package main
+
+import "C"
+
+func main() {}

misc/cgo/testsanitizers/test.bash

@@ -68,6 +68,25 @@ fi
 
 status=0
 
+testmsanshared() {
+  goos=$(go env GOOS)
+  suffix="-installsuffix testsanitizers"
+  libext="so"
+  if [ "$goos" == "darwin" ]; then
+	  libext="dylib"
+  fi
+  go build -msan -buildmode=c-shared $suffix -o ${TMPDIR}/libmsanshared.$libext msan_shared.go
+
+	echo 'int main() { return 0; }' > ${TMPDIR}/testmsanshared.c
+  $CC $(go env GOGCCFLAGS) -fsanitize=memory -o ${TMPDIR}/testmsanshared ${TMPDIR}/testmsanshared.c ${TMPDIR}/libmsanshared.$libext
+
+  if ! LD_LIBRARY_PATH=. ${TMPDIR}/testmsanshared; then
+    echo "FAIL: msan_shared"
+    status=1
+  fi
+  rm -f ${TMPDIR}/{testmsanshared,testmsanshared.c,libmsanshared.$libext}
+}
+
 if test "$msan" = "yes"; then
     if ! go build -msan std; then
 	echo "FAIL: build -msan std"
@@ -108,6 +127,8 @@ if test "$msan" = "yes"; then
 	echo "FAIL: msan_fail"
 	status=1
     fi
+
+    testmsanshared
 fi
 
 if test "$tsan" = "yes"; then

src/runtime/msan.go

@@ -28,9 +28,11 @@ const msanenabled = true
 // the runtime, but operations like a slice copy can call msanread
 // anyhow for values on the stack. Just ignore msanread when running
 // on the system stack. The other msan functions are fine.
+//
+//go:nosplit
 func msanread(addr unsafe.Pointer, sz uintptr) {
 	g := getg()
-	if g == g.m.g0 || g == g.m.gsignal {
+	if g == nil || g.m == nil || g == g.m.g0 || g == g.m.gsignal {
 		return
 	}
 	domsanread(addr, sz)