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

[H.J. Lu]

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.