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

[Fangrui Song <i@maskray.me>]

On Tue, Apr 30, 2024 at 4:30 AM mengqinggang <mengqinggang@loongson.cn> wrote:
>
>
> 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.

The SHN_ABS STB_GLOBAL STV_HIDDEN symbol can be undefined.
The linker can define it like other special symbols
(_GLOBAL_OFFSET_TABLE, .TOC., _TLS_MODULE_OFFSET_, _SDA_BASE_,
__global_pointer$).

> 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.

Then we can remove the special case for R_LARCH_ALIGN when copying
relocations for -r / --emit-relocs.

>
> >
> >>> 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).
>