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.
30 func memcopy(dst, src []byte) (n int, err error) {
32 err = recover().(error)
35 for i := 0; i < len(dst) && i < len(src); i++ {
43 // Turn the eventual fault into a panic, not a program crash,
44 // so that memcopy can recover.
45 debug.SetPanicOnFault(true)
47 size := syscall.Getpagesize()
49 // Map 16 pages of data with a 4-page hole in the middle.
50 data, err := syscall.Mmap(-1, 0, 16*size, syscall.PROT_READ|syscall.PROT_WRITE, syscall.MAP_ANON|syscall.MAP_PRIVATE)
52 log.Fatalf("mmap: %v", err)
55 other := make([]byte, 16*size)
57 // Note: Cannot call syscall.Munmap, because Munmap checks
58 // that you are unmapping a whole region returned by Mmap.
59 // We are trying to unmap just a hole in the middle.
60 if _, _, err := syscall.Syscall(syscall.SYS_MUNMAP, uintptr(unsafe.Pointer(&data[8*size])), uintptr(4*size), 0); err != 0 {
61 log.Fatalf("munmap: %v", err)
64 // Check that memcopy returns the actual amount copied
65 // before the fault (8*size - 5, the offset we skip in the argument).
66 n, err := memcopy(data[5:], other)
68 log.Fatal("no error from memcopy across memory hole")
71 log.Fatalf("memcopy returned %d, want %d", n, 8*size-5)