c - 通过 EBP 从调用函数获取本地变量

Question

我正在尝试通过 EBP 从被调用方获取调用方中定义的局部变量。这应该是可能的，但如果不是，请解释原因。

这是我现在的代码:

#include <stdio.h>

void TEST(int a, int b, int c){
  int answer;
  printf("a: %d | b: %d | c: %d\n", a, b, c);

  __asm__ __volatile__(
    ".intel_syntax;"
    "mov %0, dword ptr ds:[ebp + 20];"        <-- EBP+20 == d == 42
    ".att_syntax;"
    : "=r" (answer)
    :
    :
  );

  printf("Answer: %d\n", answer);
}

int main(void){
  int a = 13;
  int b = 14;
  int c = 15;
  int d = 42;
  TEST(a, b, c);
}

当 TEST 被调用时，我希望堆栈看起来像下面这样:

|+20|  | 13     <-- d
|+16|  | 14     <-- c
|+12|  | 15     <-- b
|+ 8|  | 42     <-- a
|+ 4|  | return address
|  0|  | EBP
|- 4|  | local var answer
|- 8|  | ...

如果我尝试编译以下代码，我会得到:

test.c: Assembler messages:
test.c:7: Error: segment register name expected

哪里出错了？

Answer 1

move mem2,mem1

是不允许的。但这将由 gcc 处理，即使在 intel 语法上也是如此。我的假设是不正确的。 Gcc 将为 intel 和 at&t 语法生成正确的代码。

 "movl  mem1,%0"
        : "=r" (mem2) 

Gcc 会为我们生成有效的代码:

mov    mem1,%eax
 mov    %eax,mem2

错误:需要段寄存器名称:

gcc 无法获取段寄存器名称。但是有段ds 寄存器。而且即使不提及段寄存器也应该没问题。实际问题只是前缀问题

应该做的:要解决这个问题，我们必须使用 nonprefix或者加上前缀 %

同样的例子:

它在 x64 机器上。你应该改变 rbp->ebp。并且0x10也可以改变

#include <stdio.h>
#include <stdlib.h> 
 #include <stdint.h>
void TEST(int a, int b, int c) {
    int answer;

    int x = 16;


  __asm__ __volatile__(  
            ".intel_syntax noprefix ;"
            "mov %0, dword ptr   ds:[rbp + %1+0];"
  ".att_syntax;"
            : "=r" (answer) 
            : "r"((uintptr_t)x) /* x is input operand */
             );  
      printf("Answer: %d\n", answer);
    __asm__ __volatile__(  
   " movl    0(%%rbp,%1,1),%0"
            : "=r" (answer) 
            : "r"((uintptr_t)x) /* x is input operand */
             );  
      printf("Answer: %d\n", answer);
    __asm__ __volatile__(
            "movl  0x10(%%rbp),%0"
            : "=r" (answer)
            );
    printf("Answer: %d\n", answer);
    __asm__ __volatile__(
            ".intel_syntax;"
            "mov %0, dword ptr  [%%rbp + 0x10];"
            ".att_syntax;"
            : "=r" (answer)
            );
    printf("Answer: %d\n", answer);

    //the same as you wrote, just I added noprefix
    __asm__ __volatile__(
            ".intel_syntax noprefix ;"
            "mov %0, dword ptr   ds:[rbp + 0x10];"
            ".att_syntax;"
            : "=r" (answer)
            );
    printf("Answer: %d\n", answer);
    //this lines just to test if we refer the same address as address of d
    void *addressd;
    __asm__ __volatile__(
            "lea  0x10(%%rbp),%0"
            : "=r" (addressd)
            );
    printf("TEST: address of d %p\n", addressd);
}

int main(void) {
    int a = 13;
    int b = 14;
    int c = 15;
    int d = 42;
    TEST(a, b, c);
    printf("address of d %p", &d);
}