gdb Cannot access memory at address of $ebp

Question

Entire code is:

#include <stdio.h>
#include <string.h>
#include <unistd.h>

char *secret = "1234";

void go_shell()
{
    char *shell = "/bin/sh";
    char *cmd[] = { "/bin/sh", 0};
    printf("WOuld  you like to play a game?\n");
    setreuid(0, 0); 
    execve(shell, cmd, 0);
}

int authorize()
{
    char password[64];
    printf("Enter password: ");
    gets(password);
    if (!strcmp(password, secret))
        return 1;
    else
        return 0;
}

int main()
{
    if (authorize()) {
        printf("Login successfully\n");
        go_shell();
    } else {
        printf("Incorrect password\n");
    }

    return 0;
}

I want to see the $ebp when authorize() is just called.

Compiled with

gcc -Wall -ggdb <source_code> -o <exe>

Then I started gdb

gdb <exe>

(gdb) disass authorize
Dump of assembler code for function authorize:
   0x000000000040076d <+0>:     push   %rbp
   0x000000000040076e <+1>:     mov    %rsp,%rbp
   0x0000000000400771 <+4>:     sub    $0x50,%rsp
   0x0000000000400775 <+8>:     mov    %fs:0x28,%rax
   0x000000000040077e <+17>:    mov    %rax,-0x8(%rbp)
   0x0000000000400782 <+21>:    xor    %eax,%eax
   0x0000000000400784 <+23>:    mov    $0x4008d8,%edi
   0x0000000000400789 <+28>:    mov    $0x0,%eax
   0x000000000040078e <+33>:    callq  0x400590 <printf@plt>
   0x0000000000400793 <+38>:    lea    -0x50(%rbp),%rax
   0x0000000000400797 <+42>:    mov    %rax,%rdi
   0x000000000040079a <+45>:    mov    $0x0,%eax
   0x000000000040079f <+50>:    callq  0x4005d0 <gets@plt>
   0x00000000004007a4 <+55>:    mov    0x2008bd(%rip),%rdx        # 0x601068 <secret>
   0x00000000004007ab <+62>:    lea    -0x50(%rbp),%rax
   0x00000000004007af <+66>:    mov    %rdx,%rsi
   0x00000000004007b2 <+69>:    mov    %rax,%rdi
   0x00000000004007b5 <+72>:    callq  0x4005c0 <strcmp@plt>
   0x00000000004007ba <+77>:    test   %eax,%eax
   0x00000000004007bc <+79>:    jne    0x4007c5 <authorize+88>
   0x00000000004007be <+81>:    mov    $0x1,%eax
   0x00000000004007c3 <+86>:    jmp    0x4007ca <authorize+93>
   0x00000000004007c5 <+88>:    mov    $0x0,%eax
   0x00000000004007ca <+93>:    mov    -0x8(%rbp),%rcx
   0x00000000004007ce <+97>:    xor    %fs:0x28,%rcx
   0x00000000004007d7 <+106>:   je     0x4007de <authorize+113>
   0x00000000004007d9 <+108>:   callq  0x400580 <__stack_chk_fail@plt>
   0x00000000004007de <+113>:   leaveq
   0x00000000004007df <+114>:   retq
End of assembler dump.
(gdb) br *authorize+33
Breakpoint 1 at 0x40078e: file simple_login.c, line 19.
(gdb) run
Starting program: ./a.out

Breakpoint 1, 0x000000000040078e in authorize () at simple_login.c:19
19          printf("Enter password: ");
(gdb) where
#0  0x000000000040078e in authorize () at simple_login.c:19
#1  0x00000000004007ee in main () at simple_login.c:29
(gdb) x/2x $ebp
0xffffffffffffe4a0:     Cannot access memory at address 0xffffffffffffe4a0

---

On ubuntu64 bit, Linux version 4.8.0-44-generic (buildd@xxxx) (gcc version 5.4.0 20160609 (Ubuntu 5.4.0-6ubuntu1~16.04.4) ) #47~16.04.1-Ubuntu SMP

Thanks in advance.

Answer 1

Dump of assembler code for function authorize: 0x000000000040076d <+0>: push %rbp

This is a 64-bit build. You should use x/2x $rbp instead.