mbox series

[v2,0/7] Support DT_RELR relative relocation format

Message ID 20220205172446.652011-1-hjl.tools@gmail.com
Headers
Series Support DT_RELR relative relocation format |

Message

H.J. Lu Feb. 5, 2022, 5:24 p.m. UTC 
  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
nd the architecture defines SUPPORT_DT_RELR in config.h.  At the moment,
only x86 targets define SUPPORT_DT_RELR.

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 (6):
  elf: Properly handle zero DT_RELA/DT_REL values
  Add GLIBC_ABI_DT_RELR for DT_RELR support
  x86/configure.ac: Define PI_STATIC_AND_HIDDEN/SUPPORT_STATIC_PIE
  x86: Define SUPPORT_DT_RELR
  Add --disable-default-dt-relr
  NEWS: Mention DT_RELR support

 INSTALL                     |  6 +++
 Makeconfig                  | 19 +++++++++
 Makerules                   |  2 +
 NEWS                        |  2 +-
 config.h.in                 |  3 ++
 configure                   | 80 +++++++++++++++++++++++++++++++++++++
 configure.ac                | 33 +++++++++++++++
 elf/Makefile                | 36 +++++++++++++++--
 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 +++-
 sysdeps/i386/configure      |  6 ---
 sysdeps/i386/configure.ac   |  7 ----
 sysdeps/x86/configure       |  9 +++++
 sysdeps/x86/configure.ac    | 10 +++++
 sysdeps/x86_64/configure    |  6 ---
 sysdeps/x86_64/configure.ac |  7 ----
 26 files changed, 383 insertions(+), 39 deletions(-)
 create mode 100644 elf/libc-abi-version.exp
 create mode 100644 elf/tst-relr-pie.c
 create mode 100644 elf/tst-relr.c

Comments

Joseph Myers Feb. 7, 2022, 8:27 p.m. UTC | #1 
On Sat, 5 Feb 2022, H.J. Lu via Libc-alpha wrote:

> DT_RELR is enabled in glibc shared libraries and position independent
> executables (PIE) automatically if linker supports -z pack-relative-relocs
> nd the architecture defines SUPPORT_DT_RELR in config.h.  At the moment,
> only x86 targets define SUPPORT_DT_RELR.

I still don't think the SUPPORT_DT_RELR part of this is appropriate.

If the linker is reporting -z pack-relative-relocs support on 
architectures where it won't work, that should be fixed in the linker.  
(Ideally the DT_RELR implementation in the linker would need as little 
architecture-specific code as possible, but I realise the BFD ELF linker 
isn't really designed to minimise code duplication betweeen different 
architectures.)

So just checking for linker support should be sufficient, without a 
separate hardcoding in glibc of which architectures have such support.  
(If there are particular binutils releases that claim support but where 
it's actually broken on some architectures, a more specific configure test 
would be preferable to hardcoded architecture-specific handling in glibc.)