[PATCH v4 0/5] Support DT_RELR relative relocation format

[H.J. Lu]

The empty version map is used for GLIBC_ABI_DT_RELR.  But lld doesn't
support empty version map.  Should we require linker with empty version
map support for glibc build?

H.J.
---
Changes in v4:

1. Always enable GLIBC_ABI_DT_RELR check.
2. Use $(OBJDUMP) instead of $(NM) for GLIBC_ABI_DT_RELR check. 

Changes in v3:

1. Don't define SUPPORT_DT_RELR.
2. Enable DT_RELR in glibc shared libraries and position independent
executables (PIE) automatically if linker supports -z pack-relative-relocs.

Changes in v2:

1. Enable DT_RELR for all targets.
2. Issue an error if there is a DT_RELR entry without GLIBC_ABI_DT_RELR
dependency nor GLIBC_PRIVATE definition.

---
PIE and 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 is a pre-standard. RELR
usually takes 3% or smaller space than R_*_RELATIVE relocations. The
virtual memory size of a mostly statically linked PIE is typically 5~10%
smaller.

Binutils 2.38 supports DT_RELR on x86 with the -z report-relative-reloc
option.  When DT_RELR is enabled, ld adds a GLIBC_ABI_DT_RELR symbol
version dependency on libc.so to outputs.  Issue an error if there is a
DT_RELR entry without GLIBC_ABI_DT_RELR dependency nor GLIBC_PRIVATE
definition.

DT_RELR is enabled in glibc shared libraries and position independent
executables (PIE) automatically if linker supports -z pack-relative-relocs.

The DT_RELR usage in glibc can be disabled with --disable-default-dt-relr.

Tested with binutils 2.38 on i686, x86-64 and x32.

Fangrui Song (1):
  elf: Support DT_RELR relative relocation format [BZ #27924]

H.J. Lu (4):
  elf: Properly handle zero DT_RELA/DT_REL values
  Add GLIBC_ABI_DT_RELR for DT_RELR support
  Add --disable-default-dt-relr
  NEWS: Mention DT_RELR support

 INSTALL                  |  6 ++++
 Makeconfig               | 19 ++++++++++++
 Makerules                |  2 ++
 NEWS                     |  2 +-
 configure                | 60 +++++++++++++++++++++++++++++++++++++
 configure.ac             | 23 +++++++++++++++
 elf/Makefile             | 34 +++++++++++++++++++--
 elf/Versions             |  5 ++++
 elf/dl-version.c         | 33 +++++++++++++++++++--
 elf/dynamic-link.h       | 40 ++++++++++++++++++++++++-
 elf/elf.h                | 13 ++++++--
 elf/get-dynamic-info.h   | 19 ++++++++++--
 elf/libc-abi-version.exp |  1 +
 elf/tst-relr-pie.c       |  1 +
 elf/tst-relr.c           | 64 ++++++++++++++++++++++++++++++++++++++++
 include/link.h           |  6 ++++
 manual/install.texi      |  5 ++++
 scripts/abilist.awk      |  2 ++
 scripts/versions.awk     |  7 ++++-
 19 files changed, 329 insertions(+), 13 deletions(-)
 create mode 100644 elf/libc-abi-version.exp
 create mode 100644 elf/tst-relr-pie.c
 create mode 100644 elf/tst-relr.c

-- 
2.35.1