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[release-branch.go1.7] runtime: fix getArgInfo for deferred reflectio…

…n calls

Fixes #18333 (backport)

getArgInfo for reflect.makeFuncStub and reflect.methodValueCall is
necessarily special. These have dynamically determined argument maps
that are stored in their context (that is, their *funcval). These
functions are written to store this context at 0(SP) when called, and
getArgInfo retrieves it from there.

This technique works if getArgInfo is passed an active call frame for
one of these functions. However, getArgInfo is also used in
tracebackdefers, where the "call" is not a true call with an active
stack frame, but a deferred call. In this situation, getArgInfo
currently crashes because tracebackdefers passes a frame with sp set
to 0. However, the entire approach used by getArgInfo is flawed in
this situation because the wrapper has not actually executed, and
hence hasn't saved this metadata to any stack frame.

In the defer case, we know the *funcval from the _defer itself, so we
can fix this by teaching getArgInfo to use the *funcval context
directly when its available, and otherwise get it from the active call

While we're here, this commit simplifies getArgInfo a bit by making it
play more nicely with the type system. Rather than decoding the
*reflect.methodValue that is the wrapper's context as a *[2]uintptr,
just write out a copy of the reflect.methodValue type in the runtime.

Fixes #16331. Fixes #17471.

Change-Id: I81db4d985179b4a81c68c490cceeccbfc675456a
Run-TryBot: Austin Clements <>
TryBot-Result: Gobot Gobot <>
Reviewed-by: Keith Randall <>
Run-TryBot: Brad Fitzpatrick <>
Reviewed-by: Austin Clements <>
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aclements authored and bradfitz committed Oct 16, 2016
1 parent c4552c1 commit efdb1813a0f65b04fdfe9982b68f4f6d31eda277
Showing with 86 additions and 7 deletions.
  1. +2 −0 src/reflect/makefunc.go
  2. +36 −7 src/runtime/traceback.go
  3. +48 −0 test/fixedbugs/issue16331.go
@@ -70,6 +70,8 @@ func MakeFunc(typ Type, fn func(args []Value) (results []Value)) Value {
// word in the passed-in argument frame.
func makeFuncStub()

// This type is partially duplicated as runtime.reflectMethodValue.
// Any changes should be reflected in both.
type methodValue struct {
fn uintptr
stack *bitVector // stack bitmap for args - offset known to runtime
@@ -107,7 +107,7 @@ func tracebackdefers(gp *g, callback func(*stkframe, unsafe.Pointer) bool, v uns
frame.fn = f
frame.argp = uintptr(deferArgs(d))
frame.arglen, frame.argmap = getArgInfo(&frame, f, true)
frame.arglen, frame.argmap = getArgInfo(&frame, f, true, fn)
frame.continpc = frame.pc
if !callback((*stkframe)(noescape(unsafe.Pointer(&frame))), v) {
@@ -339,7 +339,7 @@ func gentraceback(pc0, sp0, lr0 uintptr, gp *g, skip int, pcbuf *uintptr, max in
// metadata recorded by f's caller.
if callback != nil || printing {
frame.argp = frame.fp + sys.MinFrameSize
frame.arglen, frame.argmap = getArgInfo(&frame, f, callback != nil)
frame.arglen, frame.argmap = getArgInfo(&frame, f, callback != nil, nil)

// Determine frame's 'continuation PC', where it can continue.
@@ -546,19 +546,48 @@ func gentraceback(pc0, sp0, lr0 uintptr, gp *g, skip int, pcbuf *uintptr, max in
return n

func getArgInfo(frame *stkframe, f *_func, needArgMap bool) (arglen uintptr, argmap *bitvector) {
// reflectMethodValue is a partial duplicate of reflect.methodValue.
type reflectMethodValue struct {
fn uintptr
stack *bitvector // args bitmap

// getArgInfo returns the argument frame information for a call to f
// with call frame frame.
// This is used for both actual calls with active stack frames and for
// deferred calls that are not yet executing. If this is an actual
// call, ctxt must be nil (getArgInfo will retrieve what it needs from
// the active stack frame). If this is a deferred call, ctxt must be
// the function object that was deferred.
func getArgInfo(frame *stkframe, f *_func, needArgMap bool, ctxt *funcval) (arglen uintptr, argmap *bitvector) {
arglen = uintptr(f.args)
if needArgMap && f.args == _ArgsSizeUnknown {
// Extract argument bitmaps for reflect stubs from the calls they made to reflect.
switch funcname(f) {
case "reflect.makeFuncStub", "reflect.methodValueCall":
arg0 := frame.sp + sys.MinFrameSize
fn := *(**[2]uintptr)(unsafe.Pointer(arg0))
if fn[0] != f.entry {
// These take a *reflect.methodValue as their
// context register.
var mv *reflectMethodValue
if ctxt != nil {
// This is not an actual call, but a
// deferred call. The function value
// is itself the *reflect.methodValue.
mv = (*reflectMethodValue)(unsafe.Pointer(ctxt))
} else {
// This is a real call that took the
// *reflect.methodValue as its context
// register and immediately saved it
// to 0(SP). Get the methodValue from
// 0(SP).
arg0 := frame.sp + sys.MinFrameSize
mv = *(**reflectMethodValue)(unsafe.Pointer(arg0))
if mv.fn != f.entry {
print("runtime: confused by ", funcname(f), "\n")
throw("reflect mismatch")
bv := (*bitvector)(unsafe.Pointer(fn[1]))
bv := mv.stack
arglen = uintptr(bv.n * sys.PtrSize)
argmap = bv
@@ -0,0 +1,48 @@
// run

// 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.

// Perform tracebackdefers with a deferred reflection method.

package main

import "reflect"

type T struct{}

func (T) M() {

func F(args []reflect.Value) (results []reflect.Value) {
return nil

func main() {
done := make(chan bool)
go func() {
// Test reflect.makeFuncStub.
t := reflect.TypeOf((func())(nil))
f := reflect.MakeFunc(t, F).Interface().(func())
defer f()
done <- true
go func() {
// Test reflect.methodValueCall.
f := reflect.ValueOf(T{}).Method(0).Interface().(func())
defer f()
done <- true

func growstack(x int) {
if x == 0 {
growstack(x - 1)

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