Anton Carver | 17285b1 | 2015-10-16 10:26:06 +0100 | [diff] [blame] | 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 | |
| 21 | package main |
| 22 | |
| 23 | import ( |
| 24 | "log" |
| 25 | "runtime/debug" |
| 26 | "syscall" |
| 27 | "unsafe" |
| 28 | ) |
| 29 | |
| 30 | func memcopy(dst, src []byte) (n int, err error) { |
| 31 | defer func() { |
| 32 | err = recover().(error) |
| 33 | }() |
| 34 | |
| 35 | for i := 0; i < len(dst) && i < len(src); i++ { |
| 36 | dst[i] = src[i] |
| 37 | n++ |
| 38 | } |
| 39 | return |
| 40 | } |
| 41 | |
| 42 | func main() { |
| 43 | // Turn the eventual fault into a panic, not a program crash, |
| 44 | // so that memcopy can recover. |
| 45 | debug.SetPanicOnFault(true) |
| 46 | |
| 47 | size := syscall.Getpagesize() |
| 48 | |
| 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) |
| 51 | if err != nil { |
| 52 | log.Fatalf("mmap: %v", err) |
| 53 | } |
| 54 | |
| 55 | // Note: Cannot call syscall.Munmap, because Munmap checks |
| 56 | // that you are unmapping a whole region returned by Mmap. |
| 57 | // We are trying to unmap just a hole in the middle. |
| 58 | if _, _, err := syscall.Syscall(syscall.SYS_MUNMAP, uintptr(unsafe.Pointer(&data[8*size])), uintptr(4*size), 0); err != 0 { |
| 59 | log.Fatalf("munmap: %v", err) |
| 60 | } |
| 61 | |
| 62 | other := make([]byte, 16*size) |
| 63 | |
| 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) |
| 67 | if err == nil { |
| 68 | log.Fatal("no error from memcopy across memory hole") |
| 69 | } |
| 70 | if n != 8*size-5 { |
| 71 | log.Fatal("memcopy returned %d, want %d", n, 8*size-5) |
| 72 | } |
| 73 | } |