The latter part of https://sourceware.org/pipermail/binutils/2020-May/111086.html
Position independent executables and shared objects with -Bsymbolic/-Bsymbolic-functions may have many relative relocations (instead of absolute relocations (e.g. R_X86_64_64)).
Relative relocations can be packed in a compact format to save a lot of space. See the last few messages on https://groups.google.com/g/generic-abi/c/bX460iggiKg
DT_RELR has support in the Linux kernel (arch/arm64/Kconfig ARCH_HAS_RELR) since 2019 https://git.kernel.org/linus/5cf896fb6be3effd9aea455b22213e27be8bdb1d
Thanks for sharing and instructing
[hjl@gnu-cfl-2 tmp]$ gcc -pie -fPIE -O2 tst-relr.c
[hjl@gnu-cfl-2 tmp]$ ./a.out
Segmentation fault (core dumped)
Given that the current lld implementation generates broken
binaries for existing glibc without any warning at run-time,
we need a different linker command line option to implement
it properly so that the new binary will fail to run on glibc
without DT_RELR support at run-time.
We can do "-z relr=glibc" which generates DT_RELR and adds a
glibc version dependency against the first version of glibc
with DT_RELR support.
(In reply to H.J. Lu from comment #2)
> We can do "-z relr=glibc" which generates DT_RELR and adds a
> glibc version dependency against the first version of glibc
> with DT_RELR support.
Here is the proposal:
We need an updated proposal without mysterious crashes. One option
is that the linker should bump EI_ABIVERSION when generating DT_RELR.
Fixed for i386 and x86-64 by
Author: H.J. Lu <email@example.com>
Date: Wed Jan 5 19:21:45 2022 -0800
ld: Add glibc dependency for DT_RELR
When DT_RELR is enabled, to avoid random run-time crash with older glibc
binaries without DT_RELR support, add a GLIBC_ABI_DT_RELR symbol version,
which is provided by glibc with DT_RELR support, dependency on the shared
C library if it provides a GLIBC_2.XX symbol version.