Bug#996598: [ld][glibc]: Adopt SHT_RELR/DT_RELR to decrease PIE and shared object size
Source: glibc
Severity: wishlist
Tags: upstream
X-Debbugs-Cc: sylvestre@debian.org
The SHT_RELR/DT_RELR format encodes relative relocations in a very efficient way (quite usually takes just 3% or smaller space).
The size optimization can greatly decrease the virtual memory size of PIE and shared objects with many R_*_RELATIVE relocations.
E.g. The clang executable's virtual memory size is 8.2% smaller with SHT_RELR/DT_RELR. The size varies across projects, but I anticipate at least 5% decrease for most projects.
% ~/projects/bloaty/Release/bloaty clang.pie.relr -- clang.pie
FILE SIZE VM SIZE
-------------- --------------
[NEW] +163Ki [NEW] +163Ki .relr.dyn
+4.9% +32 +5.4% +32 .dynamic
+2.5% +8 [ = ] 0 .shstrtab
-99.5% -13.8Mi -99.5% -13.8Mi .rela.dyn
-8.3% -13.6Mi -8.2% -13.6Mi TOTAL
The SHT_RELR/DT_RELR relocation format requires linker and loader support.
* On the linkder side, ld.lld has supported .relr.dyn/SHT_RELR/DT_RELR (`--pack-dyn-relocs=relr`) for a long time and the support has been stable since 2019-09 (after I fixed an address oscillating bug).
* On the loader (glibc ld.so) side, a patch exists https://sourceware.org/pipermail/libc-alpha/2021-October/131768.html
but lack of GNU ld support may impede its adoption among Linux distributions.
User support is also important to push the patch forward. (ia64 according to folks isn't an issue. glibc doesn't implement ELFCLASS32 for ia64 AFAICT.)
(
Worth noting that the Linux kernel's arm64 port supports SHT_RELR/DT_RELR since 2019.
ChromeOS has maintained a local glibc patch since around 2018.)
So I file this ticket seeking for support (comment on https://sourceware.org/bugzilla/show_bug.cgi?id=27924). I hope that with sufficient attention from users,
someone (e.g. a GNU ld maintainer) will eventually stand up and implement `--pack-dyn-relocs=relr` for GNU ld (https://sourceware.org/bugzilla/show_bug.cgi?id=27923).
Even in the absence of GNU ld support, I hope glibc can accept the DT_RELR patch, so that ld.lld users can use `--pack-dyn-relocs=relr`.
See also Gentoo: https://bugs.gentoo.org/818376 Fedora: https://bugzilla.redhat.com/show_bug.cgi?id=2014699 Arch Linux: https://bugs.archlinux.org/task/72433
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