未定义行为之 NULL dereference

下面这段代码中 is_valid() 解引用了空指针 str，我们的直觉是编译运行后将迎来 SIGSEGV，然而事情并非所期望的那样。

/*
 * ub_null.c - 未定义行为演示 之 NULL dereference
 */
#include <stdio.h>
#include <string.h>

int is_valid(const char *str)
{
  if(*str == 0x80) return 1;
  if(str == NULL) return 0;
  return strcmp(str, "expected string") == 0;
}

int main(void)
{
  const char *str = NULL;
  printf("%d\n", is_valid(str));
  return 0;
}

lyazj@HelloWorld:~$ gcc --version
gcc (Ubuntu 11.3.0-1ubuntu1~22.04.1) 11.3.0
Copyright (C) 2021 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

lyazj@HelloWorld:~$ gcc -Wall -Wshadow -Wextra ub_null.c -o ub_null -O0
ub_null.c: In function ‘is_valid’:
ub_null.c:6:11: warning: comparison is always false due to limited range of data type [-Wtype-limits]
    6 |   if(*str == 0x80) return 0;
      |           ^~
lyazj@HelloWorld:~$ ./ub_null
0

结合 GCC 发出的警告，不难推断出条件表达式 *str == 0x80 在编译期被求值且相应的 if 语句被优化掉了，而且这是在 O0 的优化等级下。以下的反汇编结果验证了这一点。

lyazj@HelloWorld:~$ objdump --disassemble=is_valid -j.text ub_null

ub_null:     file format elf64-x86-64


Disassembly of section .text:

0000000000001169 <is_valid>:
    1169:	f3 0f 1e fa          	endbr64 
    116d:	55                   	push   %rbp
    116e:	48 89 e5             	mov    %rsp,%rbp
    1171:	48 83 ec 10          	sub    $0x10,%rsp
    1175:	48 89 7d f8          	mov    %rdi,-0x8(%rbp)
    1179:	48 83 7d f8 00       	cmpq   $0x0,-0x8(%rbp)
    117e:	75 07                	jne    1187 <is_valid+0x1e>
    1180:	b8 00 00 00 00       	mov    $0x0,%eax
    1185:	eb 1e                	jmp    11a5 <is_valid+0x3c>
    1187:	48 8b 45 f8          	mov    -0x8(%rbp),%rax
    118b:	48 8d 15 72 0e 00 00 	lea    0xe72(%rip),%rdx        # 2004 <_IO_stdin_used+0x4>
    1192:	48 89 d6             	mov    %rdx,%rsi
    1195:	48 89 c7             	mov    %rax,%rdi
    1198:	e8 d3 fe ff ff       	call   1070 <strcmp@plt>
    119d:	85 c0                	test   %eax,%eax
    119f:	0f 94 c0             	sete   %al
    11a2:	0f b6 c0             	movzbl %al,%eax
    11a5:	c9                   	leave  
    11a6:	c3                   	ret    

我们在同一环境对 O3 优化等级做相同的实验，得到了相同的结果：

lyazj@HelloWorld:~$ gcc -Wall -Wshadow -Wextra ub_null.c -o ub_null -O3
ub_null.c: In function ‘is_valid’:
ub_null.c:6:11: warning: comparison is always false due to limited range of data type [-Wtype-limits]
    6 |   if(*str == 0x80) return 0;
      |           ^~
lyazj@HelloWorld:~$ ./ub_null
0
lyazj@HelloWorld:~$ objdump --disassemble=is_valid -j.text ub_null

ub_null:     file format elf64-x86-64


Disassembly of section .text:

00000000000011a0 <is_valid>:
    11a0:	f3 0f 1e fa          	endbr64 
    11a4:	48 85 ff             	test   %rdi,%rdi
    11a7:	74 27                	je     11d0 <is_valid+0x30>
    11a9:	48 83 ec 08          	sub    $0x8,%rsp
    11ad:	48 8d 35 50 0e 00 00 	lea    0xe50(%rip),%rsi        # 2004 <_IO_stdin_used+0x4>
    11b4:	e8 a7 fe ff ff       	call   1060 <strcmp@plt>
    11b9:	85 c0                	test   %eax,%eax
    11bb:	0f 94 c0             	sete   %al
    11be:	48 83 c4 08          	add    $0x8,%rsp
    11c2:	0f b6 c0             	movzbl %al,%eax
    11c5:	c3                   	ret    
    11c6:	66 2e 0f 1f 84 00 00 	cs nopw 0x0(%rax,%rax,1)
    11cd:	00 00 00 
    11d0:	31 c0                	xor    %eax,%eax
    11d2:	c3                   	ret    

接下来我们用下面的两行代码替换被优化掉的 if 语句，看看会发生什么：

char head = *str;
if(head == 0x80) return 0;

lyazj@HelloWorld:~$ gcc -Wall -Wshadow -Wextra ub_null.c -o ub_null -O0
ub_null.c: In function ‘is_valid’:
ub_null.c:10:11: warning: comparison is always false due to limited range of data type [-Wtype-limits]
   10 |   if(head == 0x80) return 0;
      |           ^~
lyazj@HelloWorld:~$ ./ub_null 
Segmentation fault
lyazj@HelloWorld:~$ objdump --disassemble=is_valid -j.text ub_null

ub_null:     file form