[llvm-dev] How to force address of symbol

[Jessica Clarke via llvm-dev]

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