GDB Cannot insert breakpoint, Can't access memory at address XXX?

Question

I wrote a very simple program:

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

And I compiled it with -sfor split 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 figure out the main starting address of a function 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.

Since there is no symbol information in the file, I need to put the gap at the file entry point and parse it and find the starting address of the function main. Therefore, I used the command info fileto search for the entry pointaddress file :

(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 beginning of the section .text. Therefore, I set a breakpoint at 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 can't GDB insert this breakpoint?

Answer 1

, , ( ), gdb , . , gdb . . , , - 0, :

(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>

, PC 0x00007ffff7ddd190. , .

(: , ), :

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

Answer 2

, , . , file :

64- LSB ELF

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snoot.c

#include <stdio.h>

int square(int test) {
    return test*test;
}

int func() {
    n = 7;
    printf("The answer is %d\n", square(n)-5);
}

gcc -shared -fpic snoot.c -o libsnoot.so
strip libsnoot.so

. objdump -T libsnoot.so, :

libsnoot.so:     file format elf64-x86-64

DYNAMIC SYMBOL TABLE:
0000000000000580 l    d  .init  0000000000000000              .init
0000000000000000  w   D  *UND*  0000000000000000              _ITM_deregisterTMCloneTable
0000000000000000      DF *UND*  0000000000000000  GLIBC_2.2.5 printf
0000000000000000  w   D  *UND*  0000000000000000              __gmon_start__
0000000000000000  w   D  *UND*  0000000000000000              _Jv_RegisterClasses
0000000000000000  w   D  *UND*  0000000000000000              _ITM_registerTMCloneTable
0000000000000000  w   DF *UND*  0000000000000000  GLIBC_2.2.5 __cxa_finalize
0000000000201028 g    D  .got.plt   0000000000000000  Base        _edata
00000000000006e0 g    DF .text  0000000000000010  Base        square
0000000000201030 g    D  .bss   0000000000000000  Base        _end
0000000000201028 g    D  .bss   0000000000000000  Base        __bss_start
0000000000000580 g    DF .init  0000000000000000  Base        _init
0000000000000724 g    DF .fini  0000000000000000  Base        _fini
00000000000006f0 g    DF .text  0000000000000032  Base        func

.text , . , , ( , C), . , . , square :

testsnoot.c

extern void square(void);

int main() {
    square();
}

, so , :

gcc testsnoot.c -o testsnoot -L. -lsnoot

, :

LD_LIBRARY_PATH = "" gdb./testsnoot

, LD_LIBRARY_PATH, , , .

(gdb) b square
Breakpoint 1 at 0x400560
(gdb) r
Starting program: /home/edward/test/testsnoot 
Missing separate debuginfos, use: dnf debuginfo-install glibc-2.24-4.fc25.x86_64

Breakpoint 1, 0x00007ffff7bd56e4 in square () from ./libsnoot.so
(gdb) x/20i $pc
=> 0x7ffff7bd56e4 <square+4>:   mov    %edi,-0x4(%rbp)
   0x7ffff7bd56e7 <square+7>:   mov    -0x4(%rbp),%eax
   0x7ffff7bd56ea <square+10>:  imul   -0x4(%rbp),%eax
   0x7ffff7bd56ee <square+14>:  pop    %rbp
   0x7ffff7bd56ef <square+15>:  retq   
   0x7ffff7bd56f0 <func>:   push   %rbp
   0x7ffff7bd56f1 <func+1>: mov    %rsp,%rbp
   0x7ffff7bd56f4 <func+4>: sub    $0x10,%rsp
   0x7ffff7bd56f8 <func+8>: movl   $0x7,-0x4(%rbp)
   0x7ffff7bd56ff <func+15>:    mov    -0x4(%rbp),%eax
   0x7ffff7bd5702 <func+18>:    mov    %eax,%edi
   0x7ffff7bd5704 <func+20>:    callq  0x7ffff7bd55b0 <square@plt>
   0x7ffff7bd5709 <func+25>:    sub    $0x5,%eax
   0x7ffff7bd570c <func+28>:    mov    %eax,%esi
   0x7ffff7bd570e <func+30>:    lea    0x18(%rip),%rdi        # 0x7ffff7bd572d
   0x7ffff7bd5715 <func+37>:    mov    $0x0,%eax
   0x7ffff7bd571a <func+42>:    callq  0x7ffff7bd55c0 <printf@plt>
   0x7ffff7bd571f <func+47>:    nop
   0x7ffff7bd5720 <func+48>:    leaveq 
   0x7ffff7bd5721 <func+49>:    retq   

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