cgo: adjust return value location to account for stack copies.

During a cgo call, the stack can be copied.  This copy invalidates
the pointer that cgo has into the return value area.  To fix this
problem, pass the address of the location containing the stack
top value (which is in the G struct).  For cgo functions which
return values, read the stktop before and after the cgo call to
compute the adjustment necessary to write the return value.

Fixes issue 8771

LGTM=iant, rsc
R=iant, rsc, khr
CC=golang-codereviews
https://codereview.appspot.com/144130043

9 files changed, 161 insertions, 50 deletions

diff --git a/misc/cgo/test/callback.go b/misc/cgo/test/callback.go
index a7f1a3ecd..44167e6e9 100644
--- a/misc/cgo/test/callback.go
+++ b/misc/cgo/test/callback.go
@@ -10,6 +10,9 @@ void callGoFoo(void);
 void callGoStackCheck(void);
 void callPanic(void);
 void callCgoAllocate(void);
+int callGoReturnVal(void);
+int returnAfterGrow(void);
+int returnAfterGrowFromGo(void);
 */
 import "C"
 
@@ -212,6 +215,48 @@ func testAllocateFromC(t *testing.T) {
 	C.callCgoAllocate() // crashes or exits on failure
 }
 
+// Test that C code can return a value if it calls a Go function that
+// causes a stack copy.
+func testReturnAfterGrow(t *testing.T) {
+	// Use a new goroutine so that we get a small stack.
+	c := make(chan int)
+	go func() {
+		c <- int(C.returnAfterGrow())
+	}()
+	if got, want := <-c, 123456; got != want {
+		t.Errorf("got %d want %d", got, want)
+	}
+}
+
+// Test that we can return a value from Go->C->Go if the Go code
+// causes a stack copy.
+func testReturnAfterGrowFromGo(t *testing.T) {
+	// Use a new goroutine so that we get a small stack.
+	c := make(chan int)
+	go func() {
+		c <- int(C.returnAfterGrowFromGo())
+	}()
+	if got, want := <-c, 129*128/2; got != want {
+		t.Errorf("got %d want %d", got, want)
+	}
+}
+
+//export goReturnVal
+func goReturnVal() (r C.int) {
+	// Force a stack copy.
+	var f func(int) int
+	f = func(i int) int {
+		var buf [256]byte
+		use(buf[:])
+		if i == 0 {
+			return 0
+		}
+		return i + f(i-1)
+	}
+	r = C.int(f(128))
+	return
+}
+
 func testCallbackStack(t *testing.T) {
 	// Make cgo call and callback with different amount of stack stack available.
 	// We do not do any explicit checks, just ensure that it does not crash.
diff --git a/misc/cgo/test/callback_c.c b/misc/cgo/test/callback_c.c
index dcd4ddd4e..5bb642534 100644
--- a/misc/cgo/test/callback_c.c
+++ b/misc/cgo/test/callback_c.c
@@ -64,3 +64,19 @@ callGoStackCheck(void)
 	extern void goStackCheck(void);
 	goStackCheck();
 }
+
+int
+returnAfterGrow(void)
+{
+	extern int goReturnVal(void);
+	goReturnVal();
+	return 123456;
+}
+
+int
+returnAfterGrowFromGo(void)
+{
+	extern int goReturnVal(void);
+	return goReturnVal();
+}
+
diff --git a/misc/cgo/test/cgo_test.go b/misc/cgo/test/cgo_test.go
index 1d1abf729..fcfad8304 100644
--- a/misc/cgo/test/cgo_test.go
+++ b/misc/cgo/test/cgo_test.go
@@ -10,53 +10,55 @@ import "testing"
 // so that they can use cgo (import "C").
 // These wrappers are here for gotest to find.
 
-func TestAlign(t *testing.T)               { testAlign(t) }
-func TestConst(t *testing.T)               { testConst(t) }
-func TestEnum(t *testing.T)                { testEnum(t) }
-func TestAtol(t *testing.T)                { testAtol(t) }
-func TestErrno(t *testing.T)               { testErrno(t) }
-func TestMultipleAssign(t *testing.T)      { testMultipleAssign(t) }
-func TestUnsignedInt(t *testing.T)         { testUnsignedInt(t) }
-func TestCallback(t *testing.T)            { testCallback(t) }
-func TestCallbackGC(t *testing.T)          { testCallbackGC(t) }
-func TestCallbackPanic(t *testing.T)       { testCallbackPanic(t) }
-func TestCallbackPanicLoop(t *testing.T)   { testCallbackPanicLoop(t) }
-func TestCallbackPanicLocked(t *testing.T) { testCallbackPanicLocked(t) }
-func TestPanicFromC(t *testing.T)          { testPanicFromC(t) }
-func TestAllocateFromC(t *testing.T)       { testAllocateFromC(t) }
-func TestZeroArgCallback(t *testing.T)     { testZeroArgCallback(t) }
-func TestBlocking(t *testing.T)            { testBlocking(t) }
-func Test1328(t *testing.T)                { test1328(t) }
-func TestParallelSleep(t *testing.T)       { testParallelSleep(t) }
-func TestSetEnv(t *testing.T)              { testSetEnv(t) }
-func TestHelpers(t *testing.T)             { testHelpers(t) }
-func TestLibgcc(t *testing.T)              { testLibgcc(t) }
-func Test1635(t *testing.T)                { test1635(t) }
-func TestPrintf(t *testing.T)              { testPrintf(t) }
-func Test4029(t *testing.T)                { test4029(t) }
-func TestBoolAlign(t *testing.T)           { testBoolAlign(t) }
-func Test3729(t *testing.T)                { test3729(t) }
-func Test3775(t *testing.T)                { test3775(t) }
-func TestCthread(t *testing.T)             { testCthread(t) }
-func TestCallbackCallers(t *testing.T)     { testCallbackCallers(t) }
-func Test5227(t *testing.T)                { test5227(t) }
-func TestCflags(t *testing.T)              { testCflags(t) }
-func Test5337(t *testing.T)                { test5337(t) }
-func Test5548(t *testing.T)                { test5548(t) }
-func Test5603(t *testing.T)                { test5603(t) }
-func Test6833(t *testing.T)                { test6833(t) }
-func Test3250(t *testing.T)                { test3250(t) }
-func TestCallbackStack(t *testing.T)       { testCallbackStack(t) }
-func TestFpVar(t *testing.T)               { testFpVar(t) }
-func Test4339(t *testing.T)                { test4339(t) }
-func Test6390(t *testing.T)                { test6390(t) }
-func Test5986(t *testing.T)                { test5986(t) }
-func Test7665(t *testing.T)                { test7665(t) }
-func TestNaming(t *testing.T)              { testNaming(t) }
-func Test7560(t *testing.T)                { test7560(t) }
-func Test5242(t *testing.T)                { test5242(t) }
-func Test8092(t *testing.T)                { test8092(t) }
-func Test7978(t *testing.T)                { test7978(t) }
-func Test8694(t *testing.T)                { test8694(t) }
+func TestAlign(t *testing.T)                 { testAlign(t) }
+func TestConst(t *testing.T)                 { testConst(t) }
+func TestEnum(t *testing.T)                  { testEnum(t) }
+func TestAtol(t *testing.T)                  { testAtol(t) }
+func TestErrno(t *testing.T)                 { testErrno(t) }
+func TestMultipleAssign(t *testing.T)        { testMultipleAssign(t) }
+func TestUnsignedInt(t *testing.T)           { testUnsignedInt(t) }
+func TestCallback(t *testing.T)              { testCallback(t) }
+func TestCallbackGC(t *testing.T)            { testCallbackGC(t) }
+func TestCallbackPanic(t *testing.T)         { testCallbackPanic(t) }
+func TestCallbackPanicLoop(t *testing.T)     { testCallbackPanicLoop(t) }
+func TestCallbackPanicLocked(t *testing.T)   { testCallbackPanicLocked(t) }
+func TestPanicFromC(t *testing.T)            { testPanicFromC(t) }
+func TestAllocateFromC(t *testing.T)         { testAllocateFromC(t) }
+func TestZeroArgCallback(t *testing.T)       { testZeroArgCallback(t) }
+func TestBlocking(t *testing.T)              { testBlocking(t) }
+func Test1328(t *testing.T)                  { test1328(t) }
+func TestParallelSleep(t *testing.T)         { testParallelSleep(t) }
+func TestSetEnv(t *testing.T)                { testSetEnv(t) }
+func TestHelpers(t *testing.T)               { testHelpers(t) }
+func TestLibgcc(t *testing.T)                { testLibgcc(t) }
+func Test1635(t *testing.T)                  { test1635(t) }
+func TestPrintf(t *testing.T)                { testPrintf(t) }
+func Test4029(t *testing.T)                  { test4029(t) }
+func TestBoolAlign(t *testing.T)             { testBoolAlign(t) }
+func Test3729(t *testing.T)                  { test3729(t) }
+func Test3775(t *testing.T)                  { test3775(t) }
+func TestCthread(t *testing.T)               { testCthread(t) }
+func TestCallbackCallers(t *testing.T)       { testCallbackCallers(t) }
+func Test5227(t *testing.T)                  { test5227(t) }
+func TestCflags(t *testing.T)                { testCflags(t) }
+func Test5337(t *testing.T)                  { test5337(t) }
+func Test5548(t *testing.T)                  { test5548(t) }
+func Test5603(t *testing.T)                  { test5603(t) }
+func Test6833(t *testing.T)                  { test6833(t) }
+func Test3250(t *testing.T)                  { test3250(t) }
+func TestCallbackStack(t *testing.T)         { testCallbackStack(t) }
+func TestFpVar(t *testing.T)                 { testFpVar(t) }
+func Test4339(t *testing.T)                  { test4339(t) }
+func Test6390(t *testing.T)                  { test6390(t) }
+func Test5986(t *testing.T)                  { test5986(t) }
+func Test7665(t *testing.T)                  { test7665(t) }
+func TestNaming(t *testing.T)                { testNaming(t) }
+func Test7560(t *testing.T)                  { test7560(t) }
+func Test5242(t *testing.T)                  { test5242(t) }
+func Test8092(t *testing.T)                  { test8092(t) }
+func Test7978(t *testing.T)                  { test7978(t) }
+func Test8694(t *testing.T)                  { test8694(t) }
+func TestReturnAfterGrow(t *testing.T)       { testReturnAfterGrow(t) }
+func TestReturnAfterGrowFromGo(t *testing.T) { testReturnAfterGrowFromGo(t) }
 
 func BenchmarkCgoCall(b *testing.B) { benchCgoCall(b) }
diff --git a/src/cmd/cgo/out.go b/src/cmd/cgo/out.go
index 2d14f766f..4e5b3a245 100644
--- a/src/cmd/cgo/out.go
+++ b/src/cmd/cgo/out.go
@@ -44,6 +44,7 @@ func (p *Package) writeDefs() {
 	fmt.Fprintf(fm, "int main() { return 0; }\n")
 	if *importRuntimeCgo {
 		fmt.Fprintf(fm, "void crosscall2(void(*fn)(void*, int), void *a, int c) { }\n")
+		fmt.Fprintf(fm, "char* cgo_topofstack(void) { return (char*)0; }\n")
 	} else {
 		// If we're not importing runtime/cgo, we *are* runtime/cgo,
 		// which provides crosscall2.  We just need a prototype.
@@ -519,9 +520,13 @@ func (p *Package) writeOutputFunc(fgcc *os.File, n *Name) {
 	// Use packed attribute to force no padding in this struct in case
 	// gcc has different packing requirements.
 	fmt.Fprintf(fgcc, "\t%s %v *a = v;\n", ctype, p.packedAttribute())
+	if n.FuncType.Result != nil {
+		// Save the stack top for use below.
+		fmt.Fprintf(fgcc, "\tchar *stktop = cgo_topofstack();\n")
+	}
 	fmt.Fprintf(fgcc, "\t")
 	if t := n.FuncType.Result; t != nil {
-		fmt.Fprintf(fgcc, "a->r = ")
+		fmt.Fprintf(fgcc, "__typeof__(a->r) r = ")
 		if c := t.C.String(); c[len(c)-1] == '*' {
 			fmt.Fprint(fgcc, "(__typeof__(a->r)) ")
 		}
@@ -544,6 +549,13 @@ func (p *Package) writeOutputFunc(fgcc *os.File, n *Name) {
 		fmt.Fprintf(fgcc, "a->p%d", i)
 	}
 	fmt.Fprintf(fgcc, ");\n")
+	if n.FuncType.Result != nil {
+		// The cgo call may have caused a stack copy (via a callback).
+		// Adjust the return value pointer appropriately.
+		fmt.Fprintf(fgcc, "\ta = (void*)((char*)a + (cgo_topofstack() - stktop));\n")
+		// Save the return value.
+		fmt.Fprintf(fgcc, "\ta->r = r;\n")
+	}
 	if n.AddError {
 		fmt.Fprintf(fgcc, "\treturn errno;\n")
 	}
@@ -1131,6 +1143,8 @@ __cgo_size_assert(__cgo_long_long, 8)
 __cgo_size_assert(float, 4)
 __cgo_size_assert(double, 8)
 
+extern char* cgo_topofstack(void);
+
 #include <errno.h>
 #include <string.h>
 `
diff --git a/src/runtime/asm_386.s b/src/runtime/asm_386.s
index 846a214d5..f1b3346e8 100644
--- a/src/runtime/asm_386.s
+++ b/src/runtime/asm_386.s
@@ -2275,3 +2275,13 @@ TEXT runtime·fastrand1(SB), NOSPLIT, $0-4
 TEXT runtime·return0(SB), NOSPLIT, $0
 	MOVL	$0, AX
 	RET
+
+// Called from cgo wrappers, this function returns g->m->curg.stack.hi.
+// Must obey the gcc calling convention.
+TEXT cgo_topofstack(SB),NOSPLIT,$0
+	get_tls(CX)
+	MOVL	g(CX), AX
+	MOVL	g_m(AX), AX
+	MOVL	m_curg(AX), AX
+	MOVL	(g_stack+stack_hi)(AX), AX
+	RET
diff --git a/src/runtime/asm_amd64.s b/src/runtime/asm_amd64.s
index 7304d79a2..b4c6c6bdc 100644
--- a/src/runtime/asm_amd64.s
+++ b/src/runtime/asm_amd64.s
@@ -2220,3 +2220,14 @@ TEXT runtime·fastrand1(SB), NOSPLIT, $0-4
 TEXT runtime·return0(SB), NOSPLIT, $0
 	MOVL	$0, AX
 	RET
+
+
+// Called from cgo wrappers, this function returns g->m->curg.stack.hi.
+// Must obey the gcc calling convention.
+TEXT cgo_topofstack(SB),NOSPLIT,$0
+	get_tls(CX)
+	MOVQ	g(CX), AX
+	MOVQ	g_m(AX), AX
+	MOVQ	m_curg(AX), AX
+	MOVQ	(g_stack+stack_hi)(AX), AX
+	RET
diff --git a/src/runtime/asm_arm.s b/src/runtime/asm_arm.s
index 38d97b78f..2c5de8afb 100644
--- a/src/runtime/asm_arm.s
+++ b/src/runtime/asm_arm.s
@@ -1300,3 +1300,11 @@ yieldloop:
 	RET
 	SUB	$1, R1
 	B yieldloop
+
+// Called from cgo wrappers, this function returns g->m->curg.stack.hi.
+// Must obey the gcc calling convention.
+TEXT cgo_topofstack(SB),NOSPLIT,$0
+	MOVW	g_m(g), R0
+	MOVW	m_curg(R0), R0
+	MOVW	(g_stack+stack_hi)(R0), R0
+	RET
diff --git a/src/runtime/cgo/callbacks.c b/src/runtime/cgo/callbacks.c
index 16614d03d..cea9b1667 100644
--- a/src/runtime/cgo/callbacks.c
+++ b/src/runtime/cgo/callbacks.c
@@ -78,3 +78,6 @@ void (*_cgo_free)(void*) = x_cgo_free;
 #pragma cgo_import_static x_cgo_thread_start
 extern void x_cgo_thread_start(void*);
 void (*_cgo_thread_start)(void*) = x_cgo_thread_start;
+
+#pragma cgo_export_static cgo_topofstack
+#pragma cgo_export_dynamic cgo_topofstack
diff --git a/src/runtime/stack.c b/src/runtime/stack.c
index 0d8814731..2d23c717b 100644
--- a/src/runtime/stack.c
+++ b/src/runtime/stack.c
@@ -827,7 +827,9 @@ runtime·shrinkstack(G *gp)
 	if(used >= oldsize / 4)
 		return; // still using at least 1/4 of the segment.
 
-	if(gp->syscallsp != 0) // TODO: can we handle this case?
+	// We can't copy the stack if we're in a syscall.
+	// The syscall might have pointers into the stack.
+	if(gp->syscallsp != 0)
 		return;
 
 #ifdef GOOS_windows