1// run
2
3//go:build linux || darwin
4
5// Copyright 2015 The Go Authors. All rights reserved.
6// Use of this source code is governed by a BSD-style
7// license that can be found in the LICENSE file.
8
9// Test that if a slice access causes a fault, a deferred func
10// sees the most recent value of the variables it accesses.
11// This is true today; the role of the test is to ensure it stays true.
12//
13// In the test, memcopy is the function that will fault, during dst[i] = src[i].
14// The deferred func recovers from the error and returns, making memcopy
15// return the current value of n. If n is not being flushed to memory
16// after each modification, the result will be a stale value of n.
17//
18// The test is set up by mmapping a 64 kB block of memory and then
19// unmapping a 16 kB hole in the middle of it. Running memcopy
20// on the resulting slice will fault when it reaches the hole.
21
22package main
23
24import (
25	"log"
26	"runtime/debug"
27	"syscall"
28)
29
30func memcopy(dst, src []byte) (n int, err error) {
31	defer func() {
32		if r, ok := recover().(error); ok {
33			err = r
34		}
35	}()
36
37	for i := 0; i < len(dst) && i < len(src); i++ {
38		dst[i] = src[i]
39		n++
40	}
41	return
42}
43
44func main() {
45	// Turn the eventual fault into a panic, not a program crash,
46	// so that memcopy can recover.
47	debug.SetPanicOnFault(true)
48
49	size := syscall.Getpagesize()
50
51	// Map 16 pages of data with a 4-page hole in the middle.
52	data, err := syscall.Mmap(-1, 0, 16*size, syscall.PROT_READ|syscall.PROT_WRITE, syscall.MAP_ANON|syscall.MAP_PRIVATE)
53	if err != nil {
54		log.Fatalf("mmap: %v", err)
55	}
56
57	// Create a hole in the mapping that's PROT_NONE.
58	// Note that we can't use munmap here because the Go runtime
59	// could create a mapping that ends up in this hole otherwise,
60	// invalidating the test.
61	hole := data[len(data)/2 : 3*(len(data)/4)]
62	if err := syscall.Mprotect(hole, syscall.PROT_NONE); err != nil {
63		log.Fatalf("mprotect: %v", err)
64	}
65
66	// Check that memcopy returns the actual amount copied
67	// before the fault.
68	const offset = 5
69	n, err := memcopy(data[offset:], make([]byte, len(data)))
70	if err == nil {
71		log.Fatal("no error from memcopy across memory hole")
72	}
73	if expect := len(data)/2 - offset; n != expect {
74		log.Fatalf("memcopy returned %d, want %d", n, expect)
75	}
76}
77