Re: [PATCH v2] BFD: Fix the bug of R_LARCH_AGLIN caused by discard section

[mengqinggang <mengqinggang@loongson.cn>]

在 2024/4/20 上午1:52, Fangrui Song 写道:
>>> On Thu, Apr 18, 2024 at 11:04 PM Fangrui Song <i@maskray.me> wrote:
>>> I should make it clear that I think this R_LARCH_ALIGN referencing
>>> STT_SECTION with addend align+256*align_limit representation is
>>> questionable.
>>> Why do you break the regular semantics of STT_SECTION relocatable linking?
>>>
>>> Can an absolute symbol be used instead?
>> Here just some my thoughts about ABS symbol. It cause more symbol cost
>> in the "*.o" files. For ABS symbols, if several "*.o" files are linked
>> together, there will be several extra symbols. Llvm works OK by creating
>> ABS symbol (I tried, but forgot the details), but GNU AS is not. Because
>> it applies its ABS value to addend (, Qinggang has investigated before.).
> Elf64_Sym is relatively smaller with just 24 bytes (unlike other
> control structures in ELF).
> To bypass a specific oddity in relocatable linking, consider using a
> SHN_ABS symbol.
>
> You can define a SHN_ABS STB_GLOBAL STV_HIDDEN symbol to avoid
> redundant copies within the link unit.
> Duplicate SHN_ABS symbols of the same st_value do not cause duplicate
> symbol definitions (except mold).
> Alternatively, use STB_WEAK to make the deduplication intention clearer.


A SHN_ABS STB_GLOBAL/STB_GLOBAL  STV_HIDDEN and non-zero symbol can be 
referenced by a relocation.
A zero or local SHN_ABS symbol will directly add it's value to the 
addend of the relocation.

rela_normal can prevent common code from handling MIPS relocations in a 
relocatable link.
R_LARCH_ALIGN does not need to handle in a relocatable link. I prefer to 
use section symbol
instead of adding an SHN_ABS symbol.


>
>>> On Fri, Mar 22, 2024 at 1:29 AM mengqinggang <mengqinggang@loongson.cn> wrote:
>>> I just saw this was pushed as commit daeda14191c1710ce967259a47ef4e0a3fb6eebf.
>>>
>>> The addition of the generic elf_backend_is_rela_normal flag seems like
>>> something a global maintainer should take a closer look at.
>>> In particular, I'm curious if Alan, the author of the "rela_normal"
>>> commit (b491616acb5462a3694160ffef6413c160fed10a), has any thoughts on
>>> this.
>>>
>>> The idea appears to be
>>> (https://github.com/loongson/la-abi-specs/blob/release/laelf.adoc#:~:text=R_LARCH_ALIGN)
>>>
>>> .text
>>> break 1
>>> .p2align 4, , 8  // R_LARCH_ALIGN .text+0x0804
>>> break 8
>>>
>>> In a relocatable link, the addend associated with the STT_SECTION
>>> symbol is kept unchanged.
>> Relocatable link merge input file text sections into one text section.
>> If a relocation reference a section symbol, the addend would add the
>> section offset in the final one text section.
> This is exactly my concern. Using a STT_SECTION symbol requires a special case.
>
> https://inbox.sourceware.org/binutils/20020506132720.GT3698@bubble.sa.bigpond.net.au/
> specifies
>
>> mips is the fly in the ointment here, and the reason for elf_backend_rela_normal.
> !rela_normal is weird, and we should not introduce more weird stuff
> (R_LARCH_ALIGN referencing STT_SECTION).