Re: The trouble with __weak and objtool got worse

[Segher Boessenkool <segher@kernel.crashing.org>]

On Fri, Apr 15, 2022 at 02:17:40PM -0700, Fangrui Song wrote:
> On 2022-04-15, Nick Desaulniers wrote:
> >>> Uh, the email says -- prefix, but reality shows a single prefix? What
> >>> happened there? Maybe that link is to an earlier version than what
> >>> landed?
> >>
> >>Neither has landed.  You get the -g option, which can take options of
> >>itself, parsed by the target or file format code.  "as -gobbledygook"
> >>works fine as well :-)
> >
> >https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=d1bcae833b32f1408485ce69f844dcd7ded093a8
> >Looks like it's been in binutils since 2.36 (IIUC the ChangeLog correctly).
> 
> (Came to this thread from https://reviews.llvm.org/D123874)
> 
> H.J.'s patch discards unused STT_SECTION symbols.

Yup, it doesn't add any command line option, it is an internals patch.

> The ability to add back such symbols 
> (--generate-unused-section-symbols={yes|no}) is not in binutils.
> Neither 2.36 nor master branch has 
> --generate-unused-section-symbols={yes|no}.

Which is a good thing really, it is a horrible option name.

> as -generate-unused-section-symbols=yes is just a GNU as quirk that it
> interprets nearly all -g* options as -g, which generates .debug_arange
> and .debug_info compile units.

As -gdwarf-2 in fact, for most ELF targets (specific targets can
override it still, although hopefully they don't :-) )

> The feature is very different from
> restoring STT_SECTION symbols.
> 
> I know little about the kernel requirement but I think it shouldn't be
> too difficult to teach a relocatable object file consumer to not assume
> existence of STT_SECTION. The converse (add the feature to GNU as and
> LLVM integrated assembler) would be much more unideal.

Yeah, there are sections already (those *are* required in ELF), so
objtool should just use those, instead of relying on coincidences that
were true in old tools versions, but were never guaranteed :-)


Segher