Position Independent Code for MMU-less MCUs with XIP

[kirkolw kirkolw <kirkolw at gmail dot com>]

Hello!


Currently I develop Dynamic loader for one Nuttx fork for ARM architecture.


What do I want?

Load ELF files at runtime.
Execute code from FLASH (XIP).
No relocations into text section. (for sharing .text section between apps)


What do I have?

1) .text section is placed into FLASH
2)Function calls are sent to plt:

(test_func tries to print "Hello World!")
0000028c <test_func>:
...
 290:   e59f300c        ldr     r3, [pc, #12]   ; 2a4 <test_func+0x18> #
Get GOT-index of "Hello World!" string
 294:   e79a0003        ldr     r0, [sl, r3] # "sl" - addr of GOT. Load
"Hello world!" pointer to r0 and pass it to puts()
 298:   ebfffff0        bl      260 <puts@plt>
...

All seems OK. Code is position independent.
GOT is placed into RAM and after load-time relocations has correct
addresses of all stuff.
Important thing:
 298:   ebfffff0(!!!)        bl      260 <puts@plt>
Branch to puts@plt is relative. So, PLT must be placed into FLASH too.

With PLT into FLASH I have a problem: compiler generates pc-relative code!
00000260 <puts@plt>:
 260:   e28fc600         add     ip, pc, #0, 12
 264:   e28cca00         add     ip, ip, #0, 20
 268:   e5bcf0f0         ldr     pc, [ip, #240]! ; 0xf0
With pc-relative code into PLT, GOT must be placed into particular address
into RAM, but MMU-less system can't provide such opportunity.


I have found example of code for PLT functions into presentation [
https://www.slideshare.net/linaroorg/sfo15406-arm-fdpic-toolset-kernel-libraries-for-cortexm-cortexr-mmuless-cores
]:
plt(foo):    ldr      ip, .L1                 # foo's descriptor offset
             add      ip, ip, r9              # from caller's FDT
             ldr      r9, [ip, #4]            # foo's data address
             ldr      pc, [ip]                # foo's code address
L1.          word     foo(GOTOFFFUNCDESC)

Such code is exactly what I want.
It seems, they developed a number of patches, but they were not merged into
any compiler's upstream

I've tried:
    - arm-none-eabi-gcc
    - linaro
    - clang

Used compiler keys:
    -fpic -mpic-register=r10 -msingle-pic-base
-mno-pic-data-is-text-relative

How can I obtain PLT code mentiont above?

P.S. I desired something like -mno-plt flag to disable PLT and I will have
probably not too fast, but at least suitable for XIP code. But such flag is
implemented only for MIPS, as I can see. Is there similar flag for ARM?


Many thanks,
Dmitrii Rodionov