Debugging Issue on Assemble Code by using GDB - debugging

Question

I tried to use gdb to debug the Assemble code but got error message,it said:
(gdb) run Starting program: /root/assembler_program/bsawp.o
/bin/bash: /root/assembler_program/bsawp.o: cannot execute binary file
The code:
.section .text
.globl _start
_start:
nop
movl 0x12345678 , %ebx
bswap %ebx
movl $1 , %eax
int $0x80
Then I use gdb :
(gdb) break *_start+1
Breakpoint 1 at 0x400079
(gdb) run
Starting program: /root/assembler_program/bsawp
Breakpoint 1, 0x0000000000400079 in _start ()
(gdb) step
Single stepping until exit from function _start,
which has no line number information.
Program received signal SIGSEGV, Segmentation fault.
0x0000000000400079 in _start ()
Could you please provide any hint and suggestion?
Thanks!
Best regards

Answer 1

Here:
Starting program: /root/assembler_program/bsawp.o
you are trying to run a relocatable object file. Don't do that.
You must link your object into an executable first. Something like this:
gcc -nostdlib -nostartfile test.s
Here:
Starting program: /root/assembler_program/bsawp
you apparently did link the bsap.o into an executable. The crash is happening here:
(gdb) x/i $pc
=> 0x400079 <_start+1>: mov 0x12345678,%ebx
This instruction is trying to load value from address 0x12345678, into register. But that address does not point to a valid memory location.
You most likely meant for it to load a constant 0x12345678, in which case the instruction you want is:
movl $0x12345678, %ebx
With that fix, I get expected:
(gdb) run
Starting program: /tmp/a.out
[Inferior 1 (process 238270) exited with code 022]
(gdb) p/x 022
$1 = 0x12