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

1 files changed, 15 insertions, 1 deletions

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>
 `