3 //go:build linux || darwin
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.
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.
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.
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.
30 func memcopy(dst, src []byte) (n int, err error) {
32 if r, ok := recover().(error); ok {
37 for i := 0; i < len(dst) && i < len(src); i++ {
45 // Turn the eventual fault into a panic, not a program crash,
46 // so that memcopy can recover.
47 debug.SetPanicOnFault(true)
49 size := syscall.Getpagesize()
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)
54 log.Fatalf("mmap: %v", err)
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)
66 // Check that memcopy returns the actual amount copied
69 n, err := memcopy(data[offset:], make([]byte, len(data)))
71 log.Fatal("no error from memcopy across memory hole")
73 if expect := len(data)/2 - offset; n != expect {
74 log.Fatalf("memcopy returned %d, want %d", n, expect)