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 other := make([]byte, 16*size) 56 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) 62 } 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 } 74
