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

3 files changed, 111 insertions, 48 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) }