ChibiOS Free Embedded RTOS

hi.
i want to do relocation of a elf file with chibios on stm32f4. ive managed to get it working with
a statically linked app. but now i want dynamical loading of functions. like shared libraries.
(i want to have something similar to what vxworks does). however i dont think i understand
the dynsym section properly. can anyone tell me how the plt section is supposted to be filled by my loader?

om able to find functions inside the strings offset. so now the plt needs to be updated somehow with addresses.

in the assembly code of what i try to load i havw a blx instruction
from what i know of x86 this is suppost to call function@plt wich should lookup the sym in question and put the pointer in the plt... should note that what i want is not lazy binding but rather linking before execution.
but on target of course.
i could post the code if anyone is interested.
br
matias

To dynamically link an elf you would need to generate a lookup table and store it in flash somewhere which contains a name-value pair for every function-address you are linking against. You can use a tool like 'nm' to generate the information for you base image (nm -n yourBinary.elf) where yourBinary.elf is the image running on the device which will be doing the loading. Then you would create a file/header combo with this table and rebuild. Then your loader can search this table for the string and find the load address to be filled in.

skute, yes i know howto solve that part. but i dont understand how to/ where to put the address.
when i parse the elf and find let say as an example printf. where do i write the adress in the loaded elf?
as far as i know its suppost to be in the got section. at the offset given by dynsym. i tried that but
i just get a unhandled exception when i try to single step the location where i assume the branch is occuring
that location contains a blx instrucion. wich i assume is trying to call got+offset.is this correct?
/matias

Once you have the relocation type and relocation address, you perform the specific relocation on the relocation address itself. There is an example here: http://infocenter.arm.com/help/index.js ... HDBCB.html

In the example they are using a relocation type of R_ARM_ABS32 (described here http://infocenter.arm.com/help/topic/co ... _aaelf.pdf).

So, for this example, to perform the relocation of this type you basically do *(uint32_t)relocationAddr += st_value where st_value is part of the symbol table entry (see http://www.skyfree.org/linux/references/ELF_Format.pdf).

I've never fully implemented an elf loader but it is something that I had played around with, so I apologize if my posts aren't as helpful as you'd like, or worse, incorrect/misleading.

Hi,Well any help is appriciated Ive checked the example in the link, but it does not mention R_ARM_JUMP_SLOT...however here is a bit more details on what I dont quite grasp:Disassembly of section .rel.plt:0000023c <.rel.plt>: 23c:   00008348        andeq   r8, r0, r8, asr #6            <--------  This is the address I parse from the elf file, it's a jumpslot as seen by readelf command later, 240:   00000416        andeq   r0, r0, r6, lsl r4Disassembly of section .plt:00000244 <.plt>: 244:   e52de004        push    {lr}            ; (str lr, [sp, #-4]!) 248:   e59fe004        ldr     lr, [pc, #4]    ; 254 <main-0x1c> 24c:   e08fe00e        add     lr, pc, lr 250:   e5bef008        ldr     pc, [lr, #8]! 254:   000080e8        andeq   r8, r0, r8, ror #1 258:   e28fc600        add     ip, pc, #0                              <--------  (A) whats this??, dont quite understand whats going on here, I thought this was going to be a jump instruction to the address given at .rel.plt+0x23c 25c:   e28cca08        add     ip, ip, #32768  ; 0x8000 260:   e5bcf0e8        ldr     pc, [ip, #232]! ; 0xe8Disassembly of section .text:00000270 <main>: 270:   f04f 0301       mov.w   r3, #1 274:   4618            mov     r0, r3 276:   4770            bx      lr 278:   f3af 8000       nop.w 27c:   f3af 8000       nop.w00000280 <_runmain>: 280:   b500            push    {lr} 282:   b083            sub     sp, #12 284:   9001            str     r0, [sp, #4] 286:   9100            str     r1, [sp, #0] 288:   f7ff efe6       blx     258 <main-0x18>   <-------- This is a function call to listTasks(), jumps to (A) above I suppose 28c:   f04f 0300       mov.w   r3, #0 290:   4618            mov     r0, r3readelf givesRelocation section '.rel.plt' at offset 0x23c contains 1 entries: Offset     Info    Type            Sym.Value  Sym. Name00008348  00000416 R_ARM_JUMP_SLOT   00000000   listTasks ^---- am i suppost to write address of "listTasks" af this offset ?, thats what I tried anyway, but with no success...//Matias

for anyone interested the code is available at github.com/matiashr/elfloader

Hi,

Consider using the forum "code" tags, the code would be more readable.

Giovanni

is there anone who knows why gcc produces large binary when i skip -pie flag?
my binary becomes 4 times larger (?!). im using nostdlib nostartfiles aswell.
it seems program headers move to offset 8000hex. any reason for this?

br matias
will use code tag from now on.

i get exception invalid state on the blx instruction. from what i anderstand this is because of switching between thum and arm. is this possible on stm32f4.?
the manual says its thumb2 and does not mention arm mode?
anyways where is one suppost to set the use_thumb that is mentioned in the chibi makefile. doxygen docs suggest that many diffrent thumb modes are available. thuminterworking etc..
/matias

Nope, Cortex-M processors only execute Thumb code so the LSb in the program counter must alwais be one.

Interworking, of course, is not needed.

Giovanni