Source file test/recover4.go

     1  // run
     2  
     3  //go:build linux || darwin
     4  
     5  // Copyright 2015 The Go Authors. All rights reserved.
     6  // Use of this source code is governed by a BSD-style
     7  // license that can be found in the LICENSE file.
     8  
     9  // Test that if a slice access causes a fault, a deferred func
    10  // sees the most recent value of the variables it accesses.
    11  // This is true today; the role of the test is to ensure it stays true.
    12  //
    13  // In the test, memcopy is the function that will fault, during dst[i] = src[i].
    14  // The deferred func recovers from the error and returns, making memcopy
    15  // return the current value of n. If n is not being flushed to memory
    16  // after each modification, the result will be a stale value of n.
    17  //
    18  // The test is set up by mmapping a 64 kB block of memory and then
    19  // unmapping a 16 kB hole in the middle of it. Running memcopy
    20  // on the resulting slice will fault when it reaches the hole.
    21  
    22  package main
    23  
    24  import (
    25  	"log"
    26  	"runtime/debug"
    27  	"syscall"
    28  )
    29  
    30  func memcopy(dst, src []byte) (n int, err error) {
    31  	defer func() {
    32  		if r, ok := recover().(error); ok {
    33  			err = r
    34  		}
    35  	}()
    36  
    37  	for i := 0; i < len(dst) && i < len(src); i++ {
    38  		dst[i] = src[i]
    39  		n++
    40  	}
    41  	return
    42  }
    43  
    44  func main() {
    45  	// Turn the eventual fault into a panic, not a program crash,
    46  	// so that memcopy can recover.
    47  	debug.SetPanicOnFault(true)
    48  
    49  	size := syscall.Getpagesize()
    50  
    51  	// Map 16 pages of data with a 4-page hole in the middle.
    52  	data, err := syscall.Mmap(-1, 0, 16*size, syscall.PROT_READ|syscall.PROT_WRITE, syscall.MAP_ANON|syscall.MAP_PRIVATE)
    53  	if err != nil {
    54  		log.Fatalf("mmap: %v", err)
    55  	}
    56  
    57  	// Create a hole in the mapping that's PROT_NONE.
    58  	// Note that we can't use munmap here because the Go runtime
    59  	// could create a mapping that ends up in this hole otherwise,
    60  	// invalidating the test.
    61  	hole := data[len(data)/2 : 3*(len(data)/4)]
    62  	if err := syscall.Mprotect(hole, syscall.PROT_NONE); err != nil {
    63  		log.Fatalf("mprotect: %v", err)
    64  	}
    65  
    66  	// Check that memcopy returns the actual amount copied
    67  	// before the fault.
    68  	const offset = 5
    69  	n, err := memcopy(data[offset:], make([]byte, len(data)))
    70  	if err == nil {
    71  		log.Fatal("no error from memcopy across memory hole")
    72  	}
    73  	if expect := len(data)/2 - offset; n != expect {
    74  		log.Fatalf("memcopy returned %d, want %d", n, expect)
    75  	}
    76  }
    77  

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