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.
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.
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.
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.
29 func memcopy(dst, src []byte) (n int, err error) {
31 if r, ok := recover().(error); ok {
36 for i := 0; i < len(dst) && i < len(src); i++ {
44 // Turn the eventual fault into a panic, not a program crash,
45 // so that memcopy can recover.
46 debug.SetPanicOnFault(true)
48 size := syscall.Getpagesize()
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)
53 log.Fatalf("mmap: %v", err)
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)
65 // Check that memcopy returns the actual amount copied
68 n, err := memcopy(data[offset:], make([]byte, len(data)))
70 log.Fatal("no error from memcopy across memory hole")
72 if expect := len(data)/2 - offset; n != expect {
73 log.Fatalf("memcopy returned %d, want %d", n, expect)