cgo, runtime: diagnose callback on non-Go thread

Before:
$ go run x.go
signal 11 (core dumped)
$

After:
$ go run x.go
runtime: cgo callback on thread not created by Go.
signal 11 (core dumped)
$

For issue 3068.
Not a fix, but as much of a fix as we can do before Go 1.

R=golang-dev, rogpeppe, gri
CC=golang-dev
https://golang.org/cl/5781047

src/cmd/cgo/out.go

@@ -573,8 +573,9 @@ func (p *Package) writeExports(fgo2, fc, fm *os.File) {
 			goname = "_cgoexpwrap" + cPrefix + "_" + fn.Recv.List[0].Names[0].Name + "_" + goname
 		}
 		fmt.Fprintf(fc, "#pragma dynexport %s %s\n", goname, goname)
-		fmt.Fprintf(fc, "extern void ·%s();\n", goname)
-		fmt.Fprintf(fc, "\nvoid\n")
+		fmt.Fprintf(fc, "extern void ·%s();\n\n", goname)
+		fmt.Fprintf(fc, "#pragma textflag 7\n") // no split stack, so no use of m or g
+		fmt.Fprintf(fc, "void\n")
 		fmt.Fprintf(fc, "_cgoexp%s_%s(void *a, int32 n)\n", cPrefix, exp.ExpName)
 		fmt.Fprintf(fc, "{\n")
 		fmt.Fprintf(fc, "\truntime·cgocallback(·%s, a, n);\n", goname)

src/pkg/runtime/asm_386.s

@@ -425,6 +425,14 @@ TEXT runtime·cgocallback(SB),7,$12
 	// Save current m->g0->sched.sp on stack and then set it to SP.
 	get_tls(CX)
 	MOVL	m(CX), BP
+
+	// If m is nil, it is almost certainly because we have been called
+	// on a thread that Go did not create.  We're going to crash as
+	// soon as we try to use m; instead, try to print a nice error and exit.
+	CMPL	BP, $0
+	JNE 2(PC)
+	CALL	runtime·badcallback(SB)
+
 	MOVL	m_g0(BP), SI
 	PUSHL	(g_sched+gobuf_sp)(SI)
 	MOVL	SP, (g_sched+gobuf_sp)(SI)

src/pkg/runtime/asm_amd64.s

@@ -471,6 +471,14 @@ TEXT runtime·cgocallback(SB),7,$24
 	// Save current m->g0->sched.sp on stack and then set it to SP.
 	get_tls(CX)
 	MOVQ	m(CX), BP
+	
+	// If m is nil, it is almost certainly because we have been called
+	// on a thread that Go did not create.  We're going to crash as
+	// soon as we try to use m; instead, try to print a nice error and exit.
+	CMPQ	BP, $0
+	JNE 2(PC)
+	CALL	runtime·badcallback(SB)
+
 	MOVQ	m_g0(BP), SI
 	PUSHQ	(g_sched+gobuf_sp)(SI)
 	MOVQ	SP, (g_sched+gobuf_sp)(SI)

src/pkg/runtime/thread_darwin.c

@@ -477,3 +477,13 @@ runtime·setprof(bool on)
 	else
 		runtime·sigprocmask(SIG_BLOCK, &sigset_prof, nil);
 }
+
+static int8 badcallback[] = "runtime: cgo callback on thread not created by Go.\n";
+
+// This runs on a foreign stack, without an m or a g.  No stack split.
+#pragma textflag 7
+void
+runtime·badcallback(void)
+{
+	runtime·write(2, badcallback, sizeof badcallback - 1);
+}

src/pkg/runtime/thread_freebsd.c

@@ -195,3 +195,13 @@ runtime·setprof(bool on)
 {
 	USED(on);
 }
+
+static int8 badcallback[] = "runtime: cgo callback on thread not created by Go.\n";
+
+// This runs on a foreign stack, without an m or a g.  No stack split.
+#pragma textflag 7
+void
+runtime·badcallback(void)
+{
+	runtime·write(2, badcallback, sizeof badcallback - 1);
+}

src/pkg/runtime/thread_linux.c

@@ -255,3 +255,13 @@ runtime·setprof(bool on)
 {
 	USED(on);
 }
+
+static int8 badcallback[] = "runtime: cgo callback on thread not created by Go.\n";
+
+// This runs on a foreign stack, without an m or a g.  No stack split.
+#pragma textflag 7
+void
+runtime·badcallback(void)
+{
+	runtime·write(2, badcallback, sizeof badcallback - 1);
+}

src/pkg/runtime/thread_netbsd.c

@@ -213,3 +213,13 @@ runtime·setprof(bool on)
 {
 	USED(on);
 }
+
+static int8 badcallback[] = "runtime: cgo callback on thread not created by Go.\n";
+
+// This runs on a foreign stack, without an m or a g.  No stack split.
+#pragma textflag 7
+void
+runtime·badcallback(void)
+{
+	runtime·write(2, badcallback, sizeof badcallback - 1);
+}

src/pkg/runtime/thread_openbsd.c

@@ -213,3 +213,13 @@ runtime·setprof(bool on)
 {
 	USED(on);
 }
+
+static int8 badcallback[] = "runtime: cgo callback on thread not created by Go.\n";
+
+// This runs on a foreign stack, without an m or a g.  No stack split.
+#pragma textflag 7
+void
+runtime·badcallback(void)
+{
+	runtime·write(2, badcallback, sizeof badcallback - 1);
+}

src/pkg/runtime/thread_plan9.c

@@ -247,3 +247,13 @@ runtime·setprof(bool on)
 {
 	USED(on);
 }
+
+static int8 badcallback[] = "runtime: cgo callback on thread not created by Go.\n";
+
+// This runs on a foreign stack, without an m or a g.  No stack split.
+#pragma textflag 7
+void
+runtime·badcallback(void)
+{
+	runtime·pwrite(2, badcallback, sizeof badcallback - 1, -1LL);
+}

src/pkg/runtime/thread_windows.c

@@ -422,3 +422,22 @@ runtime·setprof(bool on)
 {
 	USED(on);
 }
+
+static int8 badcallback[] = "runtime: cgo callback on thread not created by Go.\n";
+
+// This runs on a foreign stack, without an m or a g.  No stack split.
+#pragma textflag 7
+void
+runtime·badcallback(void)
+{
+	uint32 written;
+
+	runtime·stdcall(
+		runtime·WriteFile, 5,
+		runtime·stdcall(runtime·GetStdHandle, 1, (uintptr)-12), // stderr
+		badcallback,
+		(uintptr)(sizeof badcallback - 1),
+		&written,
+		nil
+	);
+}