blob: 67ed970ecb2f00f215ad2e76aff8c517c353ce8b [file] [log] [blame]
// +build linux darwin
// run
// Copyright 2015 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.
// Test that if a slice access causes a fault, a deferred func
// sees the most recent value of the variables it accesses.
// This is true today; the role of the test is to ensure it stays true.
// In the test, memcopy is the function that will fault, during dst[i] = src[i].
// The deferred func recovers from the error and returns, making memcopy
// return the current value of n. If n is not being flushed to memory
// after each modification, the result will be a stale value of n.
// The test is set up by mmapping a 64 kB block of memory and then
// unmapping a 16 kB hole in the middle of it. Running memcopy
// on the resulting slice will fault when it reaches the hole.
package main
import (
func memcopy(dst, src []byte) (n int, err error) {
defer func() {
err = recover().(error)
for i := 0; i < len(dst) && i < len(src); i++ {
dst[i] = src[i]
func main() {
// Turn the eventual fault into a panic, not a program crash,
// so that memcopy can recover.
size := syscall.Getpagesize()
// Map 16 pages of data with a 4-page hole in the middle.
data, err := syscall.Mmap(-1, 0, 16*size, syscall.PROT_READ|syscall.PROT_WRITE, syscall.MAP_ANON|syscall.MAP_PRIVATE)
if err != nil {
log.Fatalf("mmap: %v", err)
other := make([]byte, 16*size)
// Note: Cannot call syscall.Munmap, because Munmap checks
// that you are unmapping a whole region returned by Mmap.
// We are trying to unmap just a hole in the middle.
if _, _, err := syscall.Syscall(syscall.SYS_MUNMAP, uintptr(unsafe.Pointer(&data[8*size])), uintptr(4*size), 0); err != 0 {
log.Fatalf("munmap: %v", err)
// Check that memcopy returns the actual amount copied
// before the fault (8*size - 5, the offset we skip in the argument).
n, err := memcopy(data[5:], other)
if err == nil {
log.Fatal("no error from memcopy across memory hole")
if n != 8*size-5 {
log.Fatalf("memcopy returned %d, want %d", n, 8*size-5)