runtime: mark cgo callback results as written for msan

This is a fix for the -msan option when using cgo callbacks.  A cgo
callback works by writing out C code that puts a struct on the stack and
passes the address of that struct into Go.  The result parameters are
fields of the struct.  The Go code will write to the result parameters,
but the Go code thinks it is just writing into the Go stack, and
therefore won't call msanwrite.  This CL adds a call to msanwrite in the
cgo callback code so that the C knows that results were written.

Change-Id: I80438dbd4561502bdee97fad3f02893a06880ee1
Reviewed-on: https://go-review.googlesource.com/16611Reviewed-by: David Crawshaw <crawshaw@golang.org>

misc/cgo/testsanitizers/msan3.go

+// Copyright 2015 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.
+
+package main
+
+/*
+extern int *GoFn(void);
+
+// Yes, you can have definitions if you use //export, as long as they are weak.
+int f(void) __attribute__ ((weak));
+
+int f() {
+  int *p = GoFn();
+  if (*p != 12345)
+    return 0;
+  return 1;
+}
+*/
+import "C"
+
+//export GoFn
+func GoFn() *C.int {
+	i := C.int(12345)
+	return &i
+}
+
+func main() {
+	if r := C.f(); r != 1 {
+		panic(r)
+	}
+}

misc/cgo/testsanitizers/test.bash

@@ -17,7 +17,7 @@ export CC
 
 TMPDIR=${TMPDIR:-/tmp}
 echo > ${TMPDIR}/testsanitizers$$.c
-if $CC -fsanitize=memory -c ${TMPDIR}/testsanitizers$$.c 2>&1 | grep "unrecognized" >& /dev/null; then
+if $CC -fsanitize=memory -c ${TMPDIR}/testsanitizers$$.c -o ${TMPDIR}/testsanitizers$$.o 2>&1 | grep "unrecognized" >& /dev/null; then
   echo "skipping msan test: -fsanitize=memory not supported"
   rm -f ${TMPDIR}/testsanitizers$$.*
   exit 0
@@ -52,6 +52,11 @@ if ! go run -msan msan2.go; then
   status=1
 fi
 
+if ! go run -msan msan3.go; then
+  echo "FAIL: msan3"
+  status=1
+fi
+
 if go run -msan msan_fail.go 2>/dev/null; then
   echo "FAIL: msan_fail"
   status=1

src/runtime/cgocall.go

@@ -266,6 +266,13 @@ func cgocallbackg1() {
 	if raceenabled {
 		racereleasemerge(unsafe.Pointer(&racecgosync))
 	}
+	if msanenabled {
+		// Tell msan that we wrote to the entire argument block.
+		// This tells msan that we set the results.
+		// Since we have already called the function it doesn't
+		// matter that we are writing to the non-result parameters.
+		msanwrite(cb.arg, cb.argsize)
+	}
 
 	// Do not unwind m->g0->sched.sp.
 	// Our caller, cgocallback, will do that.