1// +build linux darwin
2// run
3
4// Copyright 2015 The Go Authors. All rights reserved.
5// Use of this source code is governed by a BSD-style
6// license that can be found in the LICENSE file.
7
8// Test that if a slice access causes a fault, a deferred func
9// sees the most recent value of the variables it accesses.
10// This is true today; the role of the test is to ensure it stays true.
11//
12// In the test, memcopy is the function that will fault, during dst[i] = src[i].
13// The deferred func recovers from the error and returns, making memcopy
14// return the current value of n. If n is not being flushed to memory
15// after each modification, the result will be a stale value of n.
16//
17// The test is set up by mmapping a 64 kB block of memory and then
18// unmapping a 16 kB hole in the middle of it. Running memcopy
19// on the resulting slice will fault when it reaches the hole.
20
21package main
22
23import (
24	"log"
25	"runtime/debug"
26	"syscall"
27)
28
29func memcopy(dst, src []byte) (n int, err error) {
30	defer func() {
31		if r, ok := recover().(error); ok {
32			err = r
33		}
34	}()
35
36	for i := 0; i < len(dst) && i < len(src); i++ {
37		dst[i] = src[i]
38		n++
39	}
40	return
41}
42
43func main() {
44	// Turn the eventual fault into a panic, not a program crash,
45	// so that memcopy can recover.
46	debug.SetPanicOnFault(true)
47
48	size := syscall.Getpagesize()
49
50	// Map 16 pages of data with a 4-page hole in the middle.
51	data, err := syscall.Mmap(-1, 0, 16*size, syscall.PROT_READ|syscall.PROT_WRITE, syscall.MAP_ANON|syscall.MAP_PRIVATE)
52	if err != nil {
53		log.Fatalf("mmap: %v", err)
54	}
55
56	// Create a hole in the mapping that's PROT_NONE.
57	// Note that we can't use munmap here because the Go runtime
58	// could create a mapping that ends up in this hole otherwise,
59	// invalidating the test.
60	hole := data[len(data)/2 : 3*(len(data)/4)]
61	if err := syscall.Mprotect(hole, syscall.PROT_NONE); err != nil {
62		log.Fatalf("mprotect: %v", err)
63	}
64
65	// Check that memcopy returns the actual amount copied
66	// before the fault.
67	const offset = 5
68	n, err := memcopy(data[offset:], make([]byte, len(data)))
69	if err == nil {
70		log.Fatal("no error from memcopy across memory hole")
71	}
72	if expect := len(data)/2 - offset; n != expect {
73		log.Fatalf("memcopy returned %d, want %d", n, expect)
74	}
75}
76