[llvm-dev] How to force address of symbol

[Vicente Bergas via llvm-dev]

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/)

Regards,
Vicenç.
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[Jessica Clarke via llvm-dev]

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

[Vicente Bergas via llvm-dev]

Thank you Jess, your solution solved the problem!

Regards,
Vicenç.