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Index > Linux > creating shared libraries - how to do make PIC with GOT

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LocoDelAssembly
Your code has a bug


Joined: 06 May 2005
Posts: 4633
Location: Argentina
LocoDelAssembly
[edit] SOLVED, check below [/edit]
No luck Sad

This is what I tryed:

The sources first...
Code:
; shared-lib.asm
format ELF

section '.text' executable

public getTheMostValuableMessage

getTheMostValuableMessage:
        push    ebx

        call    @f
    @@: pop     ebx
        sub     ebx,rva @b ; @b-rva @b is the GOT address

        lea     eax,[ebx+rva message]
        mov     edx, size-message

        pop     ebx
        ret

message db 'Which is the sense of the life? 42', 10 ;added "10" to get better output (and to make the output visible in some distros)
size:    

Code:
; shared-lib-test
format ELF

section '.text' executable

 public _start
 extrn getTheMostValuableMessage

 _start:
        call    getTheMostValuableMessage

        mov     ecx,eax
        mov     eax,4
        mov     ebx,1
        int     0x80

;       ret produces segmentation fault so I'll use int $80 to exit instead
        mov     eax, 1
        xor     ebx, ebx
        int     $80

    


Now the terminal:
Quote:
root@athlon64:/home/oem/Desktop/test# fasm shared-lib.asm
flat assembler version 1.67.10 (16384 kilobytes memory)
3 passes, 429 bytes.
root@athlon64:/home/oem/Desktop/test# fasm shared-lib-test.asm
flat assembler version 1.67.10 (16384 kilobytes memory)
1 passes, 404 bytes.
root@athlon64:/home/oem/Desktop/test# ld -m elf_i386 -shared -soname shared-lib.so -o shared-lib.so shared-lib.o
root@athlon64:/home/oem/Desktop/test# ld -m elf_i386 -o shared-lib-test shared-lib-test.o shared-lib.so
root@athlon64:/home/oem/Desktop/test# ./shared-lib-test
bash: ./shared-lib-test: No such file or directory
root@athlon64:/home/oem/Desktop/test# ls shared* -l
-rw-r--r-- 1 root root 369 2006-09-25 17:11 shared-lib.asm
-rw-r--r-- 1 root root 429 2006-09-25 17:13 shared-lib.o
-rwxr-xr-x 1 root root 1450 2006-09-25 17:13 shared-lib.so
-rwxr-xr-x 1 root root 1653 2006-09-25 17:15 shared-lib-test <- Here it is!!
-rw-r--r-- 1 root root 182 2006-09-25 17:11 shared-lib-test.asm
-rw-r--r-- 1 root root 404 2006-09-25 17:13 shared-lib-test.o


I don't know what happens here, I also tried copying shared-lib.so to /lib directory and /lib32 directory but same result Sad

Yeah, I know, I must be using a command wrong but what is the correct way then?

Regards

[edit] There is some modifications on the sources [/edit]


Last edited by LocoDelAssembly on 26 Sep 2006, 16:08; edited 1 time in total
Post 25 Sep 2006, 20:19
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LocoDelAssembly
Your code has a bug


Joined: 06 May 2005
Posts: 4633
Location: Argentina
LocoDelAssembly
Seems that I'll never get it work, 4 hours trying, 4 hours!!!

Even using "-rpath /home/oem/Desktop/test/" doesn't work, throws "bash: ./shared-lib-test: No such file or directory ".

My last attempt was this:
Quote:
root@athlon64:/home/oem/Desktop/test# ld -m elf_i386 -shared -o shared-lib.so shared-lib.o
root@athlon64:/home/oem/Desktop/test# ld -m elf_i386 -rpath /home/oem/Desktop/test/ -o shared-lib-test shared-lib.so shared-lib-test.o


But I'm still geting the "bash: ./shared-lib-test: No such file or directory" message (but the file exists of course).

Here the readelf -a shared-lib-test
[edit] <Removed to improve screen space>[/edit]


PS: Note that if I link statically I can execute shared-lib-test without problems (linking against shared-lib.o instead of shared-lib.so), but it's not shared in that case...

[edit]

Here the definitive sequence to get it work on Ubuntu 64-bit Dapper Drake (6.06.1 LTS)

Quote:
fasm shared-lib.asm
fasm shared-lib-test.asm

ld -m elf_i386 -shared -o shared-lib.so shared-lib.o
ld -m elf_i386 -rpath . -dynamic-linker /lib/ld-linux.so.2 -o shared-lib-test shared-lib-test.o shared-lib.so



Note that this is for testing, you have to follow a diferent procedure in order to install the shared library (check the link that Tomasz posted below).

Regards
[/edit]


Last edited by LocoDelAssembly on 26 Sep 2006, 16:04; edited 1 time in total
Post 26 Sep 2006, 02:44
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Tomasz Grysztar
Assembly Artist


Joined: 16 Jun 2003
Posts: 7430
Location: Kraków, Poland
Tomasz Grysztar
The way you linked it, to execute the program correctly you have to run it like:
Code:
/lib/ld-linux.so.2 --library-path . shared-lib-test    


Or better, link the test program like this:
Code:
ld -dynamic-linker /lib/ld-linux.so.2 -o shared-lib-test shared-lib-test.o shared-lib.so    

and then execute with command:
Code:
LD_LIBRARY_PATH=.:$LD_LIBRARY_PATH ./shared-lib-test    


PS. See http://tldp.org/HOWTO/Program-Library-HOWTO/shared-libraries.html for some info.
Post 26 Sep 2006, 07:33
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LocoDelAssembly
Your code has a bug


Joined: 06 May 2005
Posts: 4633
Location: Argentina
LocoDelAssembly


I read it yesterday but seems that I got it wrong...

Using "ld -m elf_i386 -dynamic-linker /lib/ld-linux.so.2 -o shared-lib-test shared-lib-test.o shared-lib.so" shared-lib-test works using /lib/shared-lib.so or /lib32/shared-lib.so and using "ld -m elf_i386 -rpath . -dynamic-linker /lib/ld-linux.so.2 -o shared-lib-test shared-lib-test.o shared-lib.so" works with ./shared-lib.so . No need to use LD_LIBRARY_PATH in both cases Very Happy

Thank you a lot!!

PS: I edited my previous posts

[edit] Tested in the 32-bit version, works too [/edit]
Post 26 Sep 2006, 16:15
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Tomasz Grysztar
Assembly Artist


Joined: 16 Jun 2003
Posts: 7430
Location: Kraków, Poland
Tomasz Grysztar
Strange news: I don't know how it was actually working, but I discovered that there was a bug that caused the generated code to be not really PIC. [Maybe ld can make shared library even from non-PIC code, by choosing some random fixed base address for it? That needs investigating.]

I fixed the bug and added the PLT operator at the same time. You use it just like:
Code:
call PLT printf    

to call the external procedure via PLT. You can also do something like:
Code:
extrn 'printf' as _printf
printf = PLT _printf    

and be able the to simply call the "printf" label, while assembler will make this call work through the Procedure Linkage Table.

Well, it appears I have to rewrite the first post with more information how to get this all working. A new thread perhaps?
Post 26 Sep 2006, 18:49
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LocoDelAssembly
Your code has a bug


Joined: 06 May 2005
Posts: 4633
Location: Argentina
LocoDelAssembly
aaahh, I forgot to tell something more, using "objdump -D shared-lib-test" I saw that it resolves using the same technique that MASM uses when you call an API (calling a jmp). However the jmp is not a "jmp dword ptr" like MASM but a "jmp rel32".

Do you mean that or I'm still getting it wrong?Razz

[edit] I just found this http://www.iecc.com/linker/linker10.html . Nice explanation [/edit]
Post 26 Sep 2006, 19:29
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Tomasz Grysztar
Assembly Artist


Joined: 16 Jun 2003
Posts: 7430
Location: Kraków, Poland
Tomasz Grysztar
No, it was about relocations in shared-lib.o, not shared-lib-test (and the latter is created by ld and not fasm, anyway).
Post 26 Sep 2006, 20:38
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FlashBurn



Joined: 06 Jan 2005
Posts: 87
FlashBurn
Thanks, I now will have a deeper look into the elf specifications and will see if I will get shared library support for my os working!
Post 27 Sep 2006, 06:21
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pelaillo
Missing in inaction


Joined: 19 Jun 2003
Posts: 875
Location: Colombia
pelaillo
Quote:

a bug that caused the generated code to be not really PIC.

Now I understand why a PIC version of my example (posted here) wasn't working. I arrived nowhere and then move the focus away. Maybe is time to fix it. Wink
Post 29 Sep 2006, 22:50
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LocoDelAssembly
Your code has a bug


Joined: 06 May 2005
Posts: 4633
Location: Argentina
LocoDelAssembly
Tomasz Grysztar wrote:
Well, it appears I have to rewrite the first post with more information how to get this all working. A new thread perhaps?


Could you? Because:
Code:
format ELF

section '.text' executable

public main
extrn  printf

main:
  push msg
  call PLT printf
  add  esp, 4
  ret

msg db "Hello World", 0xA,0

;----

;flat assembler  version 1.67.28  (16384 kilobytes memory)
;libcdemo.asm [10]:
;  call PLT printf
;error: invalid use of symbol.
    


PS: Note the incorrect version number 1.67.28
Post 07 Aug 2008, 00:26
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Tomasz Grysztar
Assembly Artist


Joined: 16 Jun 2003
Posts: 7430
Location: Kraków, Poland
Tomasz Grysztar
It's going to be fixed finally in 1.67.29.
Post 11 Nov 2008, 13:50
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