[llvm-dev] How to force address of symbol

[Vicente Bergas via llvm-dev]

On Wed, Dec 22, 2021 at 9:53 PM Jessica Clarke <jrtc27 at jrtc27.com> wrote:
>
> On 20 Dec 2021, at 16:03, Vicente Bergas via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> >
> > Hello,
> > i would like to force the address of the symbol "start" which is a function.
> > (there is also the use case where the symbol of interest is a data array)
> > The symbol is declared in file test.c:
> > __asm__ (
> >  "  .section .test         \n"
> >  "  .global start          \n"
> >  "  .syntax unified        \n"
> >  "  .thumb                 \n"
> >  "  .thumb_func            \n"
> >  "  .type start, %function \n"
> >  "start:                   \n"
> >  "    ldr r0, stack_addr   \n"
> >  "    mov sp, r0           \n"
> >  "    b   reset            \n"
> >  "  .align 2               \n"
> >  "stack_addr:              \n"
> >  "  .word stack            \n"
> > );
> > void reset(void) { while (1); }
> >
> > and compiled this way:
> > clang --target=armv6-m -mfloat-abi=soft -c test.c -o test.o
> > or
> > ${CROSS_COMPILE}gcc -c test.c -o test.o
> >
> > When linking i am using the script file link.ld:
> > MEMORY { RAM (rwx) : ORIGIN = 0x1000, LENGTH = 0x1000 }
> > ENTRY(start)
> > SECTIONS {
> >  .text : { *(.test) } > RAM
> >  stack = ORIGIN(RAM) + LENGTH(RAM);
> > }
> >
> > and linked this way:
> > ld.lld -T link.ld test.o --gc-sections -o test.elf
> > or
> > ${CROSS_COMPILE}ld -T link.ld test.o --gc-sections -o test.elf
> >
> > In the llvm case, there is a warning reported:
> > ld.lld: warning: test.c:(.test+0x4): has non-ABS relocation
> > R_ARM_THM_JUMP11 against symbol 'reset'
> > but it succeeds.
> >
> > Finally, i check that the "start" symbol is placed at the correct address with:
> > llvm-objdump --syms test.elf
> > or
> > ${CROSS_COMPILE}objdump --syms test.elf
> >
> > The result is wrong for the llvm case (address is zero) and correct
> > for the gnu case (address is 0x1000).
> >
> > Am i doing something wrong?
> > Is this difference between gnu and llvm expected or is it a bug?
> >
> > LLVM/CLANG version: 13.0.0 (https://archlinux.org/)
> > GCC version: 11.2.1 (http://musl.cc/)
> > LD version: 2.37 (http://musl.cc/)
>
> You need to set the flags for .test in your .section directive; currently
> it ends up with no flags when using LLVM’s integrated assembler, and so
> isn’t an SHF_ALLOC section. You want “ax”, and should probably also be
> providing %progbits for good measure, i.e.:
>
>   .section .test, "ax", %progbits
>
> Jess

Thank you Jess, your solution solved the problem!

Regards,
  Vicenç.