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0c31992eb3
This came up in private mail. It works today and I want to make sure it stays working. Change-Id: I13ebdc2dfadb3c72d7f179be89883137320c05d0 Reviewed-on: https://go-review.googlesource.com/7390 Reviewed-by: Austin Clements <austin@google.com> Reviewed-by: Rob Pike <r@golang.org>
72 lines
2.1 KiB
Go
72 lines
2.1 KiB
Go
// +build linux darwin
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// run
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// Copyright 2015 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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// Test that if a slice access causes a fault, a deferred func
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// sees the most recent value of the variables it accesses.
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// This is true today; the role of the test is to ensure it stays true.
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//
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// In the test, memcopy is the function that will fault, during dst[i] = src[i].
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// The deferred func recovers from the error and returns, making memcopy
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// return the current value of n. If n is not being flushed to memory
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// after each modification, the result will be a stale value of n.
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//
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// The test is set up by mmapping a 64 kB block of memory and then
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// unmapping a 16 kB hole in the middle of it. Running memcopy
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// on the resulting slice will fault when it reaches the hole.
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package main
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import (
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"log"
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"runtime/debug"
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"syscall"
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"unsafe"
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)
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func memcopy(dst, src []byte) (n int, err error) {
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defer func() {
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err = recover().(error)
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}()
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for i := 0; i < len(dst) && i < len(src); i++ {
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dst[i] = src[i]
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n++
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}
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return
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}
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func main() {
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// Turn the eventual fault into a panic, not a program crash,
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// so that memcopy can recover.
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debug.SetPanicOnFault(true)
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// Map 64 kB block of data with 16 kB hole in middle.
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data, err := syscall.Mmap(-1, 0, 64*1024, syscall.PROT_READ|syscall.PROT_WRITE, syscall.MAP_ANON|syscall.MAP_PRIVATE)
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if err != nil {
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log.Fatalf("mmap: %v", err)
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}
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// Note: Cannot call syscall.Munmap, because Munmap checks
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// that you are unmapping a whole region returned by Mmap.
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// We are trying to unmap just a hole in the middle.
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if _, _, err := syscall.Syscall(syscall.SYS_MUNMAP, uintptr(unsafe.Pointer(&data[32*1024])), 16*1024, 0); err != 0 {
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log.Fatalf("munmap: %v", err)
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}
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other := make([]byte, 64*1024)
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// Check that memcopy returns the actual amount copied
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// before the fault (32kB - 5, the offset we skip in the argument).
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n, err := memcopy(data[5:], other)
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if err == nil {
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log.Fatal("no error from memcopy across memory hole")
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}
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if n != 32*1024-5 {
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log.Fatal("memcopy returned %d, want %d", n, 32*1024-5)
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}
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}
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