我正在尝试在运行时打印函数的操作码。为此,我编写了一个 C 程序,它应该在该地址打印地址和十六进制数据。这里它尝试打印 mul 函数的内容。
#include <stdio.h>
int add(int a, int b)
{
printf("Adding..\n");
return a+b;
}
int sub(int a, int b)
{
printf("Subtracting...\n");
return a-b;
}
int mul(int a, int b)
{
printf("Multiplying...\n");
return add(a,b) * sub(a,b);
}
int main()
{
char *ptr;
int i;
char a;
int (*func)(int,int);
mul(4,3);
func = &mul;
ptr = (char *)func;
do
{
a = *ptr;
printf("%p %x\n",ptr,a);
ptr++;
}while (a != 0xffffffc3);
//op code for ret is c3, which specifies end of function
//however, i am not certain why it opcode is being padded by 0xffffff
}
它给出的输出是
Multiplying...
Adding..
Subtracting...
0x4005a4 55
0x4005a5 48
0x4005a6 ffffff89
0x4005a7 ffffffe5
0x4005a8 53
0x4005a9 48
0x4005aa ffffff83
0x4005ab ffffffec
0x4005ac 18
0x4005ad ffffff89
0x4005ae 7d
0x4005af ffffffec
0x4005b0 ffffff89
0x4005b1 75
0x4005b2 ffffffe8
0x4005b3 ffffffbf
0x4005b4 c
0x4005b5 7
0x4005b6 40
0x4005b7 0
0x4005b8 ffffffe8
0x4005b9 63
0x4005ba fffffffe
0x4005bb ffffffff
0x4005bc ffffffff
0x4005bd ffffff8b
0x4005be 55
0x4005bf ffffffe8
0x4005c0 ffffff8b
0x4005c1 45
0x4005c2 ffffffec
0x4005c3 ffffff89
0x4005c4 ffffffd6
0x4005c5 ffffff89
0x4005c6 ffffffc7
0x4005c7 ffffffe8
0x4005c8 ffffff90
0x4005c9 ffffffff
0x4005ca ffffffff
0x4005cb ffffffff
0x4005cc ffffff89
0x4005cd ffffffc3
输出几乎如我所愿,但一些操作码在左侧被 0xffffff 填充,并被读取为负值。为什么会这样?
ELF文件的objdump如下
00000000004005a4 <mul>:
4005a4: 55 push %rbp
4005a5: 48 89 e5 mov %rsp,%rbp
4005a8: 53 push %rbx
4005a9: 48 83 ec 18 sub $0x18,%rsp
4005ad: 89 7d ec mov %edi,-0x14(%rbp)
4005b0: 89 75 e8 mov %esi,-0x18(%rbp)
4005b3: bf 0c 07 40 00 mov $0x40070c,%edi
4005b8: e8 63 fe ff ff callq 400420 <puts@plt>
4005bd: 8b 55 e8 mov -0x18(%rbp),%edx
4005c0: 8b 45 ec mov -0x14(%rbp),%eax
4005c3: 89 d6 mov %edx,%esi
4005c5: 89 c7 mov %eax,%edi
4005c7: e8 90 ff ff ff callq 40055c <add>
4005cc: 89 c3 mov %eax,%ebx
4005ce: 8b 55 e8 mov -0x18(%rbp),%edx
4005d1: 8b 45 ec mov -0x14(%rbp),%eax
4005d4: 89 d6 mov %edx,%esi
4005d6: 89 c7 mov %eax,%edi
4005d8: e8 a1 ff ff ff callq 40057e <sub>
4005dd: 0f af c3 imul %ebx,%eax
4005e0: 48 83 c4 18 add $0x18,%rsp
4005e4: 5b pop %rbx
4005e5: 5d pop %rbp
4005e6: c3 retq
十六进制代码几乎相同,除了 0xffffff 的填充。我无法弄清楚为什么?