Trying to load position independent code on cortex-m3

Question

I have an embedded application which will have a bootloader which will decide to run 1 of two applications directly from internal flash. I am trying to make these apps posi

Answer 1

PIE (and PIC) code needs a relocation process after loading at some address (different from default) and before it will be runned. I suggest you consult the code of ld.so. Also, you should check the relocations table in your binary (e.g. using readelf -r).

Here is a good presentation on PIE (it is about OpenBSD, but the process is same). http://www.openbsd.org/papers/nycbsdcon08-pie/ or http://www.dcbsdcon.org/speakers/slides/miller_dcbsdcon2009.pdf

I guess you should not only to change a GOT, but to also to find all Relocations and to do them.

Basically, processing of PIE binary by ld.so is almost the same as processing a dynamic library with PIC, with relocating not a library, but the executable image itself.

The "Wrong addresses" you see is a place, where an actual value would be written by relocation solving. As for i386 http://books.google.com/books?id=Id9cYsIdjIwC&pg=PA174 there are relocations:

• R_386_GOTPC
• R_386_GOT32
• R_386_GOTOFF
• R_386_RELATIVE

Linker should resolve all of them before the code can access a global data.

Readelf -r sample:

Dynamically linked one

$ readelf -r fdyn

Relocation section '.rel.dyn' at offset 0x27c contains 1 entries:
 Offset     Info    Type            Sym.Value  Sym. Name
08049ff0  00000106 R_386_GLOB_DAT    00000000   __gmon_start__

Relocation section '.rel.plt' at offset 0x284 contains 2 entries:
 Offset     Info    Type            Sym.Value  Sym. Name
0804a000  00000107 R_386_JUMP_SLOT   00000000   __gmon_start__
0804a004  00000207 R_386_JUMP_SLOT   00000000   __libc_start_main

PIE:

$ readelf -r fPIE

Relocation section '.rel.dyn' at offset 0x388 contains 6 entries:
 Offset     Info    Type            Sym.Value  Sym. Name
00001fe8  00000008 R_386_RELATIVE
00001ff0  00000008 R_386_RELATIVE
00002010  00000008 R_386_RELATIVE
00001fe0  00000106 R_386_GLOB_DAT    00000000   __gmon_start__
00001fe4  00000206 R_386_GLOB_DAT    00000000   _Jv_RegisterClasses
00001fec  00000406 R_386_GLOB_DAT    00000000   __cxa_finalize

Relocation section '.rel.plt' at offset 0x3b8 contains 3 entries:
 Offset     Info    Type            Sym.Value  Sym. Name
00002000  00000107 R_386_JUMP_SLOT   00000000   __gmon_start__
00002004  00000307 R_386_JUMP_SLOT   00000000   __libc_start_main
00002008  00000407 R_386_JUMP_SLOT   00000000   __cxa_finalize