GDB Cannot insert breakpoint, Cannot access memory at address XXX?

Question

I wrote a really simple program:

ebrahim@ebrahim:~/test$ cat main.c
int main() {
    int i = 0;
    return i;
}

And the I compiled it with -s for stripped mode:

ebrahim@ebrahim:~/test$ gcc -s main.c -o f3
ebrahim@ebrahim:~/test$ file f3
f3: ELF 64-bit LSB shared object, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, for GNU/Linux 2.6.32, BuildID[sha1]=4dc6b893fbae8b418ca41ddeef948df1fcb26d3d, stripped

Now, I'm trying to find out the main function start address using GDB:

ebrahim@ebrahim:~/test$ gdb -nh f3
GNU gdb (Ubuntu 7.11.90.20161005-0ubuntu2) 7.11.90.20161005-git
Copyright (C) 2016 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
and "show warranty" for details.
This GDB was configured as "x86_64-linux-gnu".
Type "show configuration" for configuration details.
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>.
Find the GDB manual and other documentation resources online at:
<http://www.gnu.org/software/gdb/documentation/>.
For help, type "help".
Type "apropos word" to search for commands related to "word"...
Reading symbols from f3...(no debugging symbols found)...done.

As there is no Symbol info inside the file, I need to put a break at the file entry point and the disassemble it and find the start address of main function. So I used info file command to find the file entry point address:

(gdb) info file
Symbols from "/home/ebrahim/test/f3".
Local exec file:
    `/home/ebrahim/test/f3', file type elf64-x86-64.
    Entry point: 0x530     <<<<=============
    0x0000000000000238 - 0x0000000000000254 is .interp
    0x0000000000000254 - 0x0000000000000274 is .note.ABI-tag
    0x0000000000000274 - 0x0000000000000298 is .note.gnu.build-id
    0x0000000000000298 - 0x00000000000002b4 is .gnu.hash
    0x00000000000002b8 - 0x0000000000000360 is .dynsym
    0x0000000000000360 - 0x00000000000003f1 is .dynstr
    0x00000000000003f2 - 0x0000000000000400 is .gnu.version
    0x0000000000000400 - 0x0000000000000420 is .gnu.version_r
    0x0000000000000420 - 0x00000000000004f8 is .rela.dyn
    0x00000000000004f8 - 0x000000000000050f is .init
    0x0000000000000510 - 0x0000000000000520 is .plt
    0x0000000000000520 - 0x0000000000000528 is .plt.got
    0x0000000000000530 - 0x00000000000006e2 is .text
    0x00000000000006e4 - 0x00000000000006ed is .fini
    0x00000000000006f0 - 0x00000000000006f4 is .rodata
    0x00000000000006f4 - 0x0000000000000728 is .eh_frame_hdr
    0x0000000000000728 - 0x000000000000081c is .eh_frame
    0x0000000000200de0 - 0x0000000000200de8 is .init_array
    0x0000000000200de8 - 0x0000000000200df0 is .fini_array
    0x0000000000200df0 - 0x0000000000200df8 is .jcr
    0x0000000000200df8 - 0x0000000000200fb8 is .dynamic
    0x0000000000200fb8 - 0x0000000000201000 is .got
    0x0000000000201000 - 0x0000000000201010 is .data
    0x0000000000201010 - 0x0000000000201018 is .bss

As we expected the entry point is the start of .text section. So I put a breakpoint on this address:

(gdb) b *0x0000000000000530
Breakpoint 1 at 0x530
(gdb) r
Starting program: /home/ebrahim/test/f3 
Warning:
Cannot insert breakpoint 1.
Cannot access memory at address 0x530

(gdb)

The question is why GDB cannot insert this breakpoint?

Answer 1

Debugging stripped code is probably very much useless (except for reverse engineering), but you can cause gdb to stop at the very first instruction, and you are already doing this accidentally. If the address of a breakpoint cannot be mapped, gdb stops and tells you the error. As a side effect, your program is stopped at its first instruction. An address that's guaranteed to be unmappable is 0, so just do the following:

(gdb) b *0
Breakpoint 1 at 0x0
(gdb) r
Starting program: [...]
Warning:
Cannot insert breakpoint 1.
Cannot access memory at address 0x0

(gdb) disas
Dump of assembler code for function _start:
=> 0x00007ffff7ddd190 <+0>: mov    %rsp,%rdi
   0x00007ffff7ddd193 <+3>: callq  0x7ffff7de0750 <_dl_start>

Here you see the PC sits at 0x00007ffff7ddd190. So this is your entry point at runtime.

In order to be able to continue (or: single-step for example), you have to delete the offending breakpoint:

(gdb) delete
Delete all breakpoints? (y or n) y
(gdb) c
Continuing.

Credits for this answer go to this answer on reverse engineering