ELF oddities and dynamic linking in ARM linux

Question

There are things in ARM elf binaries I'd like to understand better.

I need to figure this out get my homebrew assembler output ELF executables for gp2x f200. So I started by compiling this program with open2x cross-compiling toolchain:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
int main(){
    chdir("/usr/gp2x");
    execl("/usr/gp2x/gp2xmenu", "/usr/gp2x/gp2xmenu", NULL);
    return 0;
}

It otherwise looks like just okay, nothing special compared to x86. In the ELF headers the Flags -field is gotten used! I found some ARM elf specs but it is not mentioned why these are needed.

ELF Header:
  Magic:   7f 45 4c 46 01 01 01 61 00 00 00 00 00 00 00 00 
  Class:                             ELF32
  Data:                              2's complement, little endian
  Version:                           1 (current)
  OS/ABI:                            ARM
  ABI Version:                       0
  Type:                              EXEC (Executable file)
  Machine:                           ARM
  Version:                           0x1
  Entry point address:               0x82f8
  Start of program headers:          52 (bytes into file)
  Start of section headers:          3032 (bytes into file)
  Flags:                             0x202, has entry point, GNU EABI, software FP
  Size of this header:               52 (bytes)
  Size of program headers:           32 (bytes)
  Number of program headers:         6
  Size of section headers:           40 (bytes)
  Number of section headers:         31
  Section header string table index: 28

Now the other structures do not appear very different from what they are on x86. It actually looks very much familiar! Even the type of relocation is familiar (R_386_JUMP_SLOT vs. R_ARM_JUMP_SLOT). Though here it starts going weird.

Relocation section '.rel.plt' at offset 0x280 contains 4 entries:
 Offset     Info    Type            Sym.Value  Sym. Name
00010638  00000116 R_ARM_JUMP_SLOT   000082c8   abort
0001063c  00000416 R_ARM_JUMP_SLOT   000082d4   __libc_start_main
00010640  00000516 R_ARM_JUMP_SLOT   000082e0   execl
00010644  00000716 R_ARM_JUMP_SLOT   000082ec   chdir

Disassembly of section .plt:

000082b4 <.plt>:
    82b4:   e52de004    str lr, [sp, #-4]!
    82b8:   e59fe004    ldr lr, [pc, #4]    ; 82c4 <.plt+0x10>
    82bc:   e08fe00e    add lr, pc, lr
    82c0:   e5bef008    ldr pc, [lr, #8]!
    82c4:   00008368    andeq   r8, r0, r8, ror #6
    82c8:   e28fc600    add ip, pc, #0  ; 0x0
    82cc:   e28cca08    add ip, ip, #32768  ; 0x8000
    82d0:   e5bcf368    ldr pc, [ip, #872]!
    82d4:   e28fc600    add ip, pc, #0  ; 0x0
    82d8:   e28cca08    add ip, ip, #32768  ; 0x8000
    82dc:   e5bcf360    ldr pc, [ip, #864]!
    82e0:   e28fc600    add ip, pc, #0  ; 0x0
    82e4:   e28cca08    add ip, ip, #32768  ; 0x8000
    82e8:   e5bcf358    ldr pc, [ip, #856]!
    82ec:   e28fc600    add ip, pc, #0  ; 0x0
    82f0:   e28cca08    add ip, ip, #32768  ; 0x8000
    82f4:   e5bcf350    ldr pc, [ip, #848]!

If you look at Sym.Value, it points to this PLT. The problem is I don't understand how this is supposed to work. What is R_ARM_JUMP_SLOT causing here? Why does my PLT contain these weird instructions and how does my dynamic linker take on them?

    82ec:   e28fc600    add ip, pc, #0  ; 0x0
    82f0:   e28cca08    add ip, ip, #32768  ; 0x8000
    82f4:   e5bcf350    ldr pc, [ip, #848]!

The program is getting loaded into 0x8000 so I sort of understand that part. Except that I can't understand how this can be same if I compile this with -fPIC -shared.

So.. How does dynamic linking work under ARM?

Answer 1

I solved it myself today evening. There are lots of possible relocation mechanisms on ARM linux. I looked onto specs and found a relatively sane kind of relocation: R_ARM_ABS32. Only thing I had to do was to use it.

Compared to X86 ELF backend, I didn't need to change much anything else except the bytes in elf header to match ones I found in gcc-produced binaries. For safety I aligned some structures in binary though.

For future I have to provide a way on intervening branching code into my PLT or take some other approach I can produce larger programs that are using shared libraries. This is an assembler design issue more than a trouble in understanding ELF format.

Ran my first app in gp2x f200 just few minutes ago! It used 'system' -libc function to write a greeting into file and returned to machine's main menu. \o/