[PATCH v2] elf: Support DT_RELR relative relocation format [BZ #27924]

Mon Oct 18 14:42:09 GMT 2021

On Sat, Oct 16, 2021 at 5:50 PM Fangrui Song via Binutils
<binutils@sourceware.org> wrote:
>
> 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.
>
> ---
>
> Notes I will not include in the submitted commit:
>
> Available on https://sourceware.org/git/?p=glibc.git;a=shortlog;h=refs/heads/maskray/relr
>
> "pre-standard": even Solaris folks are happy with the refined generic-abi
> proposal. Cary Coutant will apply the change
> https://sourceware.org/pipermail/libc-alpha/2021-October/131781.html
>
> This patch is simpler than Chrome OS's glibc patch and makes ELF_DYNAMIC_DO_RELR
> available to all ports. I don't think the current glibc implementation
> supports ia64 in an ELFCLASS32 container. That said, the style I used is
> works with an ELFCLASS32 container for 64-bit machine if ElfW(Addr) is
> 64-bit.
>
> * Chrome OS folks have carried a local patch since 2018 (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 2018 and switched to DT_RELR==36 in 2020.
> * 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
> * rtld-elf from FreeBSD 14 will support DT_RELR.
>
> I believe upstream glibc should support DT_RELR to benefit all Linux
> distributions. I filed some feature requests to get their attention:
>
> * Gentoo: https://bugs.gentoo.org/818376
> * Arch Linux: https://bugs.archlinux.org/task/72433
> * Debian https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=996598
> * Fedora https://bugzilla.redhat.com/show_bug.cgi?id=2014699
>
> 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 :)
>
> Tested on aarch64 and x86_64.
>
> Changes from v1 (https://sourceware.org/pipermail/libc-alpha/2021-October/131768.html)
> * Fix style, simplify code
> * Improve test
> ---
>  configure              | 31 +++++++++++++++++++++++++++++++
>  configure.ac           |  4 ++++
>  elf/Makefile           |  4 ++++
>  elf/dynamic-link.h     | 28 ++++++++++++++++++++++++++++
>  elf/elf.h              | 13 +++++++++++--
>  elf/get-dynamic-info.h |  3 +++
>  elf/tst-relr.c         | 27 +++++++++++++++++++++++++++
>  7 files changed, 108 insertions(+), 2 deletions(-)
>  create mode 100644 elf/tst-relr.c
>
> diff --git a/configure b/configure
> index 3227e434d3..fdab6a97ef 100755
> --- a/configure
> +++ b/configure
> @@ -6067,6 +6067,37 @@ $as_echo "$libc_linker_feature" >&6; }
>  config_vars="$config_vars
>  have-depaudit = $libc_cv_depaudit"
>
> +{ $as_echo "$as_me:${as_lineno-$LINENO}: checking for linker that supports --pack-dyn-relocs=relr" >&5
> +$as_echo_n "checking for linker that supports --pack-dyn-relocs=relr... " >&6; }
> +libc_linker_feature=no
> +if test x"$gnu_ld" = x"yes"; then
> +  cat > conftest.c <<EOF
> +int _start (void) { return 42; }
> +EOF
> +  if { ac_try='${CC-cc} $CFLAGS $CPPFLAGS $LDFLAGS $no_ssp
> +                   -Wl,--pack-dyn-relocs=relr -nostdlib -nostartfiles
> +                   -fPIC -shared -o conftest.so conftest.c
> +                   1>&5'
> +  { { eval echo "\"\$as_me\":${as_lineno-$LINENO}: \"$ac_try\""; } >&5
> +  (eval $ac_try) 2>&5
> +  ac_status=$?
> +  $as_echo "$as_me:${as_lineno-$LINENO}: \$? = $ac_status" >&5
> +  test $ac_status = 0; }; }
> +  then
> +    libc_linker_feature=yes
> +  fi
> +  rm -f conftest*
> +fi
> +if test $libc_linker_feature = yes; then
> +  libc_cv_relr=yes
> +else
> +  libc_cv_relr=no
> +fi
> +{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $libc_linker_feature" >&5
> +$as_echo "$libc_linker_feature" >&6; }
> +config_vars="$config_vars
> +have-relr = $libc_cv_relr"
> +
>  { $as_echo "$as_me:${as_lineno-$LINENO}: checking for linker that supports --no-dynamic-linker" >&5
>  $as_echo_n "checking for linker that supports --no-dynamic-linker... " >&6; }
>  libc_linker_feature=no
> diff --git a/configure.ac b/configure.ac
> index 00f49f09f7..96110f9d7d 100644
> --- a/configure.ac
> +++ b/configure.ac
> @@ -1354,6 +1354,10 @@ LIBC_LINKER_FEATURE([--depaudit], [-Wl,--depaudit,x],
>                     [libc_cv_depaudit=yes], [libc_cv_depaudit=no])
>  LIBC_CONFIG_VAR([have-depaudit], [$libc_cv_depaudit])
>
> +LIBC_LINKER_FEATURE([--pack-dyn-relocs=relr], [-Wl,--pack-dyn-relocs=relr],
> +                   [libc_cv_relr=yes], [libc_cv_relr=no])
> +LIBC_CONFIG_VAR([have-relr], [$libc_cv_relr])
> +
>  LIBC_LINKER_FEATURE([--no-dynamic-linker],
>                     [-Wl,--no-dynamic-linker],
>                     [libc_cv_no_dynamic_linker=yes],
> diff --git a/elf/Makefile b/elf/Makefile
> index bf45d8ee24..2c4cdfac68 100644
> --- a/elf/Makefile
> +++ b/elf/Makefile
> @@ -245,6 +245,10 @@ tests-special += $(objpfx)tst-audit14-cmp.out $(objpfx)tst-audit15-cmp.out \
>                  $(objpfx)tst-audit16-cmp.out
>  endif
>  endif
> +ifeq ($(have-relr),yes)
> +tests += tst-relr
> +LDFLAGS-tst-relr += -Wl,--pack-dyn-relocs=relr
> +endif
>  endif

Is DT_RELR only generated for PIE? If yes, you need to add it
to tests-pie and compile it as PIE.

[hjl@gnu-cfl-2 tmp]$ gcc -pie -fPIE -O2 tst-relr.c
-Wl,--pack-dyn-relocs=relr -fuse-ld=lld
[hjl@gnu-cfl-2 tmp]$ ./a.out
Segmentation fault (core dumped)
[hjl@gnu-cfl-2 tmp]$

Given that the current lld implementation generates broken
binaries for existing glibc without any warning at run-time,
we should use 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.

-- 
H.J.