issue8606b.go

     1  // run
     2  
     3  //go:build linux || darwin
     4  
     5  // Copyright 2020 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  // This is an optimization check. We want to make sure that we compare
    10  // string lengths, and other scalar fields, before checking string
    11  // contents.  There's no way to verify this in the language, and
    12  // codegen tests in test/codegen can't really detect ordering
    13  // optimizations like this. Instead, we generate invalid strings with
    14  // bad backing store pointers but nonzero length, so we can check that
    15  // the backing store never gets compared.
    16  //
    17  // We use two different bad strings so that pointer comparisons of
    18  // backing store pointers fail.
    19  
    20  package main
    21  
    22  import (
    23  	"fmt"
    24  	"reflect"
    25  	"syscall"
    26  	"unsafe"
    27  )
    28  
    29  type SI struct {
    30  	s string
    31  	i int
    32  }
    33  
    34  type SS struct {
    35  	s string
    36  	t string
    37  }
    38  
    39  func main() {
    40  	bad1 := "foo"
    41  	bad2 := "foo"
    42  
    43  	p := syscall.Getpagesize()
    44  	b, err := syscall.Mmap(-1, 0, p, syscall.PROT_READ|syscall.PROT_WRITE, syscall.MAP_ANON|syscall.MAP_PRIVATE)
    45  	if err != nil {
    46  		panic(err)
    47  	}
    48  	err = syscall.Mprotect(b, syscall.PROT_NONE)
    49  	if err != nil {
    50  		panic(err)
    51  	}
    52  	// write inaccessible pointers as the data fields of bad1 and bad2.
    53  	(*reflect.StringHeader)(unsafe.Pointer(&bad1)).Data = uintptr(unsafe.Pointer(&b[0]))
    54  	(*reflect.StringHeader)(unsafe.Pointer(&bad2)).Data = uintptr(unsafe.Pointer(&b[1]))
    55  
    56  	for _, test := range []struct {
    57  		a, b interface{}
    58  	}{
    59  		{SI{s: bad1, i: 1}, SI{s: bad2, i: 2}},
    60  		{SS{s: bad1, t: "a"}, SS{s: bad2, t: "aa"}},
    61  		{SS{s: "a", t: bad1}, SS{s: "b", t: bad2}},
    62  		// This one would panic because the length of both strings match, and we check
    63  		// the body of the bad strings before the body of the good strings.
    64  		//{SS{s: bad1, t: "a"}, SS{s: bad2, t: "b"}},
    65  	} {
    66  		if test.a == test.b {
    67  			panic(fmt.Sprintf("values %#v and %#v should not be equal", test.a, test.b))
    68  		}
    69  	}
    70  
    71  }
    72
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