runtime: call msanwrite on object passed to runtime/cgo

Avoids an msan error when runtime/cgo is explicitly rebuilt with
-fsanitize=memory.

Fixes #13815.

Change-Id: I70308034011fb308b63585bcd40b0d1e62ec93ef
Reviewed-on: https://go-review.googlesource.com/18263Reviewed-by: Russ Cox <rsc@golang.org>

misc/cgo/testsanitizers/test.bash

@@ -61,7 +61,12 @@ if ! go run -msan msan.go; then
   status=1
 fi
 
-if ! go run -msan msan2.go; then
+if ! CGO_LDFLAGS="-fsanitize=memory" CGO_CPPFLAGS="-fsanitize=memory" go run -msan -a msan2.go; then
+  echo "FAIL: msan2 with -fsanitize=memory"
+  status=1
+fi
+
+if ! go run -msan -a msan2.go; then
   echo "FAIL: msan2"
   status=1
 fi

src/runtime/proc.go

@@ -1489,6 +1489,9 @@ func newm(fn func(), _p_ *p) {
 		ts.g.set(mp.g0)
 		ts.tls = (*uint64)(unsafe.Pointer(&mp.tls[0]))
 		ts.fn = unsafe.Pointer(funcPC(mstart))
+		if msanenabled {
+			msanwrite(unsafe.Pointer(&ts), unsafe.Sizeof(ts))
+		}
 		asmcgocall(_cgo_thread_start, unsafe.Pointer(&ts))
 		return
 	}