cmd/cgo: mark stub functions as no_sanitize_thread

When the generated stub functions write back the results to the stack,
they can in some cases be writing to the same memory on the g0 stack.
There is no race here (assuming there is no race in the Go code), but
the thread sanitizer does not know that.  Turn off the thread sanitizer
for the stub functions to prevent false positive warnings.

Current clang suggests the no_sanitize("thread") attribute, but that
does not work with clang 3.6 or GCC.  clang 3.6, GCC, and current clang
all support the no_sanitize_thread attribute, so use that
unconditionally.

The test case and first version of the patch are from Dmitriy Vyukov.

Change-Id: I80ce92824c6c8cf88ea0fe44f21cf50cf62474c9
Reviewed-on: https://go-review.googlesource.com/23252
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

misc/cgo/testsanitizers/test.bash

@@ -134,6 +134,16 @@ if test "$tsan" = "yes"; then
 	status=1
     fi
 
+    if ! go run tsan3.go 2>$err; then
+	cat $err
+	echo "FAIL: tsan3"
+	status=1
+    elif grep -i warning $err >/dev/null 2>&1; then
+	cat $err
+	echo "FAIL: tsan3"
+	status=1
+    fi
+
     rm -f $err
 fi
 

misc/cgo/testsanitizers/tsan3.go

@@ -0,0 +1,40 @@
+// Copyright 2016 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
+
+// The stubs for the C functions read and write the same slot on the
+// g0 stack when copying arguments in and out.
+
+/*
+#cgo CFLAGS: -fsanitize=thread
+#cgo LDFLAGS: -fsanitize=thread
+
+int Func1() {
+	return 0;
+}
+
+void Func2(int x) {
+	(void)x;
+}
+*/
+import "C"
+
+func main() {
+	const N = 10000
+	done := make(chan bool, N)
+	for i := 0; i < N; i++ {
+		go func() {
+			C.Func1()
+			done <- true
+		}()
+		go func() {
+			C.Func2(0)
+			done <- true
+		}()
+	}
+	for i := 0; i < 2*N; i++ {
+		<-done
+	}
+}

src/cmd/cgo/out.go

@@ -560,6 +560,7 @@ func (p *Package) writeOutputFunc(fgcc *os.File, n *Name) {
 
 	// Gcc wrapper unpacks the C argument struct
 	// and calls the actual C function.
+	fmt.Fprintf(fgcc, "CGO_NO_SANITIZE_THREAD\n")
 	if n.AddError {
 		fmt.Fprintf(fgcc, "int\n")
 	} else {
@@ -635,6 +636,7 @@ func (p *Package) writeOutputFunc(fgcc *os.File, n *Name) {
 // wrapper, we can't refer to the function, since the reference is in
 // a different file.
 func (p *Package) writeGccgoOutputFunc(fgcc *os.File, n *Name) {
+	fmt.Fprintf(fgcc, "CGO_NO_SANITIZE_THREAD\n")
 	if t := n.FuncType.Result; t != nil {
 		fmt.Fprintf(fgcc, "%s\n", t.C.String())
 	} else {
@@ -817,6 +819,7 @@ func (p *Package) writeExports(fgo2, fm, fgcc, fgcch io.Writer) {
 		fmt.Fprintf(fgcch, "\nextern %s;\n", s)
 
 		fmt.Fprintf(fgcc, "extern void _cgoexp%s_%s(void *, int, __SIZE_TYPE__);\n", cPrefix, exp.ExpName)
+		fmt.Fprintf(fgcc, "\nCGO_NO_SANITIZE_THREAD")
 		fmt.Fprintf(fgcc, "\n%s\n", s)
 		fmt.Fprintf(fgcc, "{\n")
 		fmt.Fprintf(fgcc, "\t__SIZE_TYPE__ _cgo_ctxt = _cgo_wait_runtime_init_done();\n")
@@ -1020,7 +1023,7 @@ func (p *Package) writeGccgoExports(fgo2, fm, fgcc, fgcch io.Writer) {
 		fmt.Fprintf(fgcc, `extern %s %s %s __asm__("%s.%s");`, cRet, goName, cParams, gccgoSymbolPrefix, goName)
 		fmt.Fprint(fgcc, "\n")
 
-		fmt.Fprint(fgcc, "\n")
+		fmt.Fprint(fgcc, "\nCGO_NO_SANITIZE_THREAD\n")
 		fmt.Fprintf(fgcc, "%s %s %s {\n", cRet, exp.ExpName, cParams)
 		if resultCount > 0 {
 			fmt.Fprintf(fgcc, "\t%s r;\n", cRet)
@@ -1304,11 +1307,14 @@ extern char* _cgo_topofstack(void);
 
 // Prologue defining TSAN functions in C.
 const noTsanProlog = `
+#define CGO_NO_SANITIZE_THREAD
 #define _cgo_tsan_acquire()
 #define _cgo_tsan_release()
 `
 
 const yesTsanProlog = `
+#define CGO_NO_SANITIZE_THREAD __attribute__ ((no_sanitize_thread))
+
 long long _cgo_sync __attribute__ ((common));
 
 extern void __tsan_acquire(void*);