flat assembler - 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: 4624
Location: Argentina

LocoDelAssembly 25 Sep 2006, 20:19

[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

25 Sep 2006, 20:19

LocoDelAssembly
Your code has a bug

Joined: 06 May 2005
Posts: 4624
Location: Argentina

LocoDelAssembly 26 Sep 2006, 02:44

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

26 Sep 2006, 02:44

Tomasz Grysztar

Joined: 16 Jun 2003
Posts: 8356
Location: Kraków, Poland

Tomasz Grysztar 26 Sep 2006, 07:33

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.

26 Sep 2006, 07:33

LocoDelAssembly
Your code has a bug

Joined: 06 May 2005
Posts: 4624
Location: Argentina

LocoDelAssembly 26 Sep 2006, 16:15

Quote:

PS. See http://tldp.org/HOWTO/Program-Library-HOWTO/shared-libraries.html for some info.

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]

26 Sep 2006, 16:15

Tomasz Grysztar

Joined: 16 Jun 2003
Posts: 8356
Location: Kraków, Poland

Tomasz Grysztar 26 Sep 2006, 18:49

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?

26 Sep 2006, 18:49

LocoDelAssembly
Your code has a bug

Joined: 06 May 2005
Posts: 4624
Location: Argentina

LocoDelAssembly 26 Sep 2006, 19:29

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]

26 Sep 2006, 19:29

Tomasz Grysztar

Joined: 16 Jun 2003
Posts: 8356
Location: Kraków, Poland

Tomasz Grysztar 26 Sep 2006, 20:38

No, it was about relocations in shared-lib.o, not shared-lib-test (and the latter is created by ld and not fasm, anyway).

26 Sep 2006, 20:38

FlashBurn

Joined: 06 Jan 2005
Posts: 87

FlashBurn 27 Sep 2006, 06:21

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!

27 Sep 2006, 06:21

pelaillo
Missing in inaction

Joined: 19 Jun 2003
Posts: 878
Location: Colombia

pelaillo 29 Sep 2006, 22:50

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

29 Sep 2006, 22:50

LocoDelAssembly
Your code has a bug

Joined: 06 May 2005
Posts: 4624
Location: Argentina

LocoDelAssembly 07 Aug 2008, 00:26

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

07 Aug 2008, 00:26

Tomasz Grysztar

Joined: 16 Jun 2003
Posts: 8356
Location: Kraków, Poland

Tomasz Grysztar 11 Nov 2008, 13:50

It's going to be fixed finally in 1.67.29.

11 Nov 2008, 13:50

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