ld bug

ld bug

[DJ.CnX]

hello guys....

source:

section .text

global main

msg db "och bin",0x0
msg_len equ $-msg

main:
           pusha
           mov eax,4
           mov ebx,1
           mov ecx,msg
           mov edx,msg_len
           int 0x80

           popa
           xor eax,eax
           inc eax
           int 0x80


compilation command:
sh# nasm -f elf -o new.o new.asm
sh# ld -e main -o new new.o
sh# strace ./new
execve("./new", ["./new"], [/* 78 vars */]) = 0
--- SIGSEGV (Segmentation fault) @ 0 (0) ---
+++ killed by SIGSEGV +++


i compiled the whole shit with gcc and it worked... .-/

Re: ld bug

[Willy Tarreau]

Hello,

On Sun, Jun 12, 2005 at 05:10:50PM +0200, DJ.CnX@phreaker.net wrote:
> 
> hello guys....
> 
> source:
> 
> section .text
> 
> global main
> 
> msg db "och bin",0x0
> msg_len equ $-msg
> 
> main:
>            pusha
>            mov eax,4
>            mov ebx,1
>            mov ecx,msg
>            mov edx,msg_len
>            int 0x80
> 
>            popa
>            xor eax,eax
>            inc eax
>            int 0x80
> 
> 
> compilation command:
> sh# nasm -f elf -o new.o new.asm
> sh# ld -e main -o new new.o
> sh# strace ./new
> execve("./new", ["./new"], [/* 78 vars */]) = 0
> --- SIGSEGV (Segmentation fault) @ 0 (0) ---
> +++ killed by SIGSEGV +++
> i compiled the whole shit with gcc and it worked... .-/

Although this is not the right list for this, it seems that the start
pointer in your program is 0, and since this page is not mapped, you
get a segfault. How to fix this I don't know, but this definitely is
not a kernel bug.

Willy

Re: ld bug

[Jan Engelhardt]

>
>Although this is not the right list for this, it seems that the start
>pointer in your program is 0, and since this page is not mapped, you
>get a segfault. How to fix this I don't know, but this definitely is
>not a kernel bug.

"ld" usually wants a .start symbol (or something...) - and it looks like
nasm does not put some into the object file.

>> compilation command:
>> sh# nasm -f elf -o new.o new.asm
>> sh# ld -e main -o new new.o
>> sh# strace ./new
>> execve("./new", ["./new"], [/* 78 vars */]) = 0
>> --- SIGSEGV (Segmentation fault) @ 0 (0) ---
>> +++ killed by SIGSEGV +++
>> i compiled the whole shit with gcc and it worked... .-/

Well, I did the same with GCC (GNU AS):

.intel_syntax noprefix;
.section .rodata;
.L_LC0:
	.string "och bin";
.text;
	pusha;
	mov eax, 4;
	mov ebx, 1;
	mov ecx, offset flat:.L_LC0;
	mov edx, 8;
	int 0x80;

18:43 eax:../hxtools/examples # cc x.S -c
18:43 eax:../hxtools/examples # ld x.o -o x.aout
ld: warning: cannot find entry symbol _start; defaulting to 0000000008048074
18:43 eax:../hxtools/examples # ./x.aout 
och binSegmentation fault

- The segfault is due to the abrupt end after int 0x80,
  so let's not consider it.

- "och bin" is printed

- As you can see, there is said _start symbol which I presume NASM
  fails to generate.



Jan Engelhardt                                                               
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