CODEWITHSUNDEEP
clinuxgccmemorygdb
Joel alexis
Joel alexis

Reputation: 43

Cant access a specific memory address. on C program

I am working on the book "HACKING Art Of Exploitation " exercise Convert2.c page 61.

Here's my code. Below is my question.

#include <stdio.h>

void usage(char *program_name) {
    printf("Usage: %s <message> <# of times to repeat>\n", program_name);
    exit(1);
}

int main(int argc, char *argv[]) {
    int i, count;

   // if(argc < 3) //if fewer than 3 arguments is used 
     //   usage(argv[0]); // display usage message and exit

    count = atoi(argv[2]); //convert the second arg into an  interger
    printf("Repeating %d times\n", count);

    for(i=0; i < count; i++) 
        printf("%3d - %s\n", i, argv[1]); // print the first arg 

}

GDB OUTPUT...

    ➜ git:(master) ✗ 👽 gdb -q a.out
Reading symbols from a.out...done.
(gdb) run test
Starting program: /home/fruitdealer/clones/C_zombie/hacking/a.out test

Program received signal SIGSEGV, Segmentation fault.
__GI_____strtol_l_internal (nptr=0x0, endptr=endptr@entry=0x0, base=base@entry=10, group=group@entry=0,
    loc=0x7ffff7dd0560 <_nl_global_locale>) at ../stdlib/strtol_l.c:292
292     ../stdlib/strtol_l.c: No such file or directory.
(gdb) break main
Breakpoint 1 at 0x555555554707: file convert.c, line 14.
(gdb) where
#0  __GI_____strtol_l_internal (nptr=0x0, endptr=endptr@entry=0x0, base=base@entry=10, group=group@entry=0,
    loc=0x7ffff7dd0560 <_nl_global_locale>) at ../stdlib/strtol_l.c:292
#1  0x00007ffff7a29122 in __strtol (nptr=<optimized out>, endptr=endptr@entry=0x0, base=base@entry=10)
    at ../stdlib/strtol.c:106
#2  0x00007ffff7a24690 in atoi (nptr=<optimized out>) at atoi.c:27
#3  0x000055555555471f in main (argc=2, argv=0x7fffffffdeb8) at convert.c:14
(gdb) run test
The program being debugged has been started already.
Start it from the beginning? (y or n) y
Starting program: /home/fruitdealer/clones/C_zombie/hacking/a.out test

Breakpoint 1, main (argc=2, argv=0x7fffffffdeb8) at convert.c:14
14          count = atoi(argv[2]); //convert the second arg into an  interger
(gdb) cont
Continuing.

Program received signal SIGSEGV, Segmentation fault.
__GI_____strtol_l_internal (nptr=0x0, endptr=endptr@entry=0x0, base=base@entry=10, group=group@entry=0,
    loc=0x7ffff7dd0560 <_nl_global_locale>) at ../stdlib/strtol_l.c:292
292     ../stdlib/strtol_l.c: No such file or directory.
(gdb) x/3xw 0x7fffffffdeb8
0x7fffffffdeb8: 0xffffe220      0x00007fff      0xffffe250
(gdb) x/s 0xffffe220
0xffffe220:     <error: Cannot access memory at address 0xffffe220>
(gdb) x/s 0xffffe250
0xffffe250:     <error: Cannot access memory at address 0xffffe250>
(gdb) x/sw 0xffffe250
0xffffe250:     <error: Cannot access memory at address 0xffffe250>
(gdb)

I posted all of gdb output because i wasn't sure how much of it you would need. My problem lies at the bottom of my GDB output when i run "x/s" on gdb and get the <error: Cannot access memory at address 0xffffe250> error.

On the book Jon Erickson is able to access 0xffffe220 and 0x00007fff and then he has an error on 0xffffe250 this part of memory.

What am i missing?

Why can't i access any of the three addresses in 0x7fffffffdeb8?

Upvotes: 3

Views: 1106

Answers (1)

jahneff
jahneff

Reputation: 73

The first half of the address is cut off. If you notice, it takes 8 bytes to store the addresses because you are on a 64 bit machine, not 32. You are trying to access a truncated address.

Rather than three addresses at 0x7fffffffdeb8, you are looking at one and a half. Try accessing a byte that starts with 0x00007fff...

Upvotes: 4

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