[PATCH] D59120: [ELF] Sort notes by alignment in decreasing order

[Rui Ueyama via llvm-commits]

On Mon, Mar 11, 2019 at 3:09 AM Peter Smith via Phabricator <
reviews at reviews.llvm.org> wrote:

> peter.smith added a comment.
>
> I personally favour a separate PT_NOTE section per alignment, that gives a
> note parser simple rule to iterate through the section (the p_align field).
> I think ordering by decreasing alignment will work in practice for the note
> sections that we have now, but it may not be in the future. I also think
> that this is worth doing even if clang is changed to generate
> .note.gnu.property sections that are 4-byte aligned; clang will often use
> libraries compiled by gcc (or some other compiler) that does use 8-byte
> aligned sections.
>

I'm not sure if a separate PT_NOTE section per alignment would work for all
cases. I don't think the size of a note section with 8-byte alignment is
guaranteed to be a multiple of 8 byte. If that's permitted, there is a gap
after a note section whose size is not a multiple of 8. So I guess the
"most" correct approach would be to create a note segment for each note
section. But perhaps I'm overthinking now...

There is a long glibc thread that covers a similar discussion in the gnu
> community https://sourceware.org/ml/libc-alpha/2018-08/msg00340.html , it
> is quite long so I've pulled out a few bits of it here:
>
> - https://sourceware.org/ml/libc-alpha/2018-08/msg00344.html and
> follow-up https://sourceware.org/ml/libc-alpha/2018-08/msg00347.html on
> why two separate PT_NOTE sections were needed.
> - https://sourceware.org/ml/libc-alpha/2018-08/msg00468.html rejection of
> changing ld bfd to that of gold and just setting the alignment of
> .gnu.property to 4-bytes. Ostensibly due to presence of large number of
> binaries with 8-byte aligned sections.
>
> From what I can tell:
>
> - ld.bfd outputs a separate PT_NOTE per p_align.
> - ld.gold ignores the 8-byte alignment of the .note.gnu.property in the
> input object and changes it to 4 so that only one PT_NOTE is required.
>

If ignoring the alignment requirement would work in practice, it is a
tempting choice.


> - glibc works for both, given that .note.gnu.property doesn't strictly
> need 8 byte alignment.
>
>
> Repository:
>   rLLD LLVM Linker
>
> CHANGES SINCE LAST ACTION
>   https://reviews.llvm.org/D59120/new/
>
> https://reviews.llvm.org/D59120
>
>
>
>
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