[PATCH] elf: Support DT_RELR relative relocation format [BZ #27924]

[Fangrui Song]

On 2021-10-08, H.J. Lu wrote:
>On Thu, Oct 7, 2021 at 11:58 PM Fangrui Song via Binutils
><binutils@sourceware.org> wrote:
>>
>> PIC objects (especially PIE and symbolic shared objects) usually have many
>> relative relocations. In 2017/2018, SHT_RELR/DT_RELR was proposed on
>> https://groups.google.com/g/generic-abi/c/bX460iggiKg/m/GxjM0L-PBAAJ
>> ("Proposal for a new section type SHT_RELR") and welcomed by many parties
>> (including Solaris). This packed format can typically save 95% dynamic
>> relocation section size for PIE. The vaddr size of a PIE can be 10% smaller.
>>
>> * Chrome OS folks have carried a local patch for a while (latest version:
>>   https://chromium.googlesource.com/chromiumos/overlays/chromiumos-overlay/+/refs/heads/main/sys-libs/glibc/files/local/glibc-2.32).
>>   I.e. this feature has been battle tested.
>> * Android bionic supports DT_RELR.
>> * The Linux kernel has supported CONFIG_RELR since 2019-08
>>   (https://git.kernel.org/linus/5cf896fb6be3effd9aea455b22213e27be8bdb1d).
>> * A musl patch (by me) exists but is not applied:
>>   https://www.openwall.com/lists/musl/2019/03/06/3
>>
>> I believe upstream glibc should support DT_RELR to benefit all Linux
>> distributions.
>>
>> As of linker support (to the best of my knowledge):
>>
>> * LLD support DT_RELR.
>> * https://chromium.googlesource.com/chromiumos/overlays/chromiumos-overlay/+/refs/heads/main/sys-devel/binutils/files/
>>   has a gold patch.
>> * GNU ld feature request https://sourceware.org/bugzilla/show_bug.cgi?id=27923
>>
>> I wish that GNU ld and gold maintainers can implement the feature as well :)
>
>This feature should be implemented in GNU linker first before getting
>into glibc.
>
>
>H.J.

Why? I think the situation is quite different from other features where
we generally want producers before consumers.  The feature is validated
by multiple parties and has multiple independent implementations and has
an implementation in a glibc supported linker (LLD).

Landing into glibc first can do GNU linker folks a favor: they don't
need to apply the glibc patch themselves to validate correctness of the
linker change.