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Linux x86. call error when linking gcc

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AntonPotapov:
Linux x86.
When I link with ld, everything works.
I can't understand why when linking with gcc I can't call my function from the declared my_section?
Here's my code. If executed correctly, it should return error 1
main.asm

--- Code: ---section .text
global main

main:
mov eax, 1
call sum
int 0x80

section my_section
sum:
mov ebx, 1
ret

--- End code ---

Code compilation and linking.
Release:

--- Code: ---#/bin/bash
nasm -f elf main.asm
gcc -m32 -o main main.o

--- End code ---

Debug:

--- Code: ---#/bin/bash
nasm -f elf -Lw -g main.asm
gcc -m32 -o main main.o

--- End code ---

When I run the code I get:

--- Code: ---Segmentation fault (core dumped)

--- End code ---

This is what gdb says when running the Debug version:

--- Code: ---Single stepping until exit from function sum,
which has no line number information.

Program received signal SIGSEGV, Segmentation fault.

--- End code ---

Frank Kotler:


--- Code: ---section my_section ; read only!
mov ebx, 1 ; seg fault

--- End code ---

I think that's your problem.

Why the "home made" section?

If you must... call it "rw"...

gdb will like it better with "-F dwarf" on Nasm's command line...

untested!

Best,
Frank

Frank Kotler:
Ignore me.

But lose the home made section.

Best,
Frank

AntonPotapov:
I don't understand why exactly "mov ebx, 1" is causing the error?  why there are no errors when linking with ld?

fredericopissarra:
The problem is simple: If you compile and take a look at the object file:

--- Code: ---$ nasm -felf32 -o main.o main.asm
$ objdump -h main.o
Sections:
Idx Name          Siz.      VMA       LMA       File off  Algn
  0 .text         0000000b  00000000  00000000  00000160  2**4
                  CONTENTS, ALLOC, LOAD, RELOC, READONLY, CODE
  1 my_section    00000006  00000000  00000000  00000170  2**0
                  CONTENTS, ALLOC, LOAD, READONLY, DATA
--- End code ---
Where's the "CODE" attribute!? This section is not "exectable". If you add this attribute to the section:

--- Code: ---...
  section my_section exec
...

--- End code ---
And compile again, you'll get:

--- Code: ---$ nasm -felf32 -o main.o main.asm
$ objdump -h main.o
Sections:
Idx Name          Siz.      VMA       LMA       File off  Algn
  0 .text         0000000b  00000000  00000000  00000160  2**4
                  CONTENTS, ALLOC, LOAD, RELOC, READONLY, CODE
  1 my_section    00000006  00000000  00000000  00000170  2**0
                  CONTENTS, ALLOC, LOAD, READONLY, CODE
$ gcc -s -m32 -o main main.o
$ ./main
$ echo $?
1
--- End code ---

PS: Ahhhh.. I've changed the code a little:

--- Code: ---  bits 32

  section .text

  global main

main:
  mov eax, 1
  call sum

  ; Since you are using the main() function, called from crt0.o, the
  ; end of your program is a simple return.
  ret

  section my_section exec

sum:
  ;mov ebx, 1    ; This should be avoided, since EBX should be preserved!
  ret
--- End code ---
In general, if you are building a standalone program in assembly, avoid using external libraries like libc... But if you still want to use it, you MUST obey the ABI.

The difference:

--- Code: ---$ nasm -felf32 -o main.o main.asm     # exit with ret
$ gcc -s -m32 main.o -o main
$ nasm -felf32 -o main2.o main2.asm      # exit with sys_exit.
$ ld -s -melf_i386 -e main main2.o -o main2

$ ls -goh main*
-rwxr-xr-x 1  14K apr 25 11:40 main
-rwxr-xr-x 1 4,5K apr 25 11:42 main2
...
$ ldd main main2
main:
        linux-gate.so.1 (0xf7f7c000)
        libc.so.6 => /lib32/libc.so.6 (0xf7d78000)
        /lib/ld-linux.so.2 (0xf7f7e000)
main2:
        Not a dynamic executable

--- End code ---

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