Re: [PATCH v7 3/6] elf: Support DT_RELR relative relocation format [BZ #27924]

[Adhemerval Zanella <adhemerval.zanella@linaro.org>]

On 31/03/2022 13:38, H.J. Lu wrote:
> From: Fangrui Song <maskray@google.com>
> 
> 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
> 
> Changes from the original patch:
> 
> 1. Check the linker option, -z pack-relative-relocs, which add a
> GLIBC_ABI_DT_RELR symbol version dependency on the shared C library if
> it provides a GLIBC_2.XX symbol version.
> 2. Change make variale to have-dt-relr.
> 3. Rename tst-relr-no-pie to tst-relr-pie for --disable-default-pie.
> 4. Use TEST_VERIFY in tst-relr.c.
> 5. Add the check-tst-relr-pie.out test to check for linker generated
> libc.so version dependency on GLIBC_ABI_DT_RELR.
> 6. Move ELF_DYNAMIC_DO_RELR before ELF_DYNAMIC_DO_REL.

I think it is worth to add a note on commit that DT_RELR relocation are
always applies a bind-now (do -Wl,-z,lazy or LD_BIND_NOW has not effect).

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

> ---
>  configure              | 37 ++++++++++++++++++++++++
>  configure.ac           |  5 ++++
>  elf/Makefile           | 25 ++++++++++++++++
>  elf/dynamic-link.h     | 34 ++++++++++++++++++++++
>  elf/get-dynamic-info.h |  3 ++
>  elf/tst-relr-pie.c     |  1 +
>  elf/tst-relr.c         | 65 ++++++++++++++++++++++++++++++++++++++++++
>  7 files changed, 170 insertions(+)
>  create mode 100644 elf/tst-relr-pie.c
>  create mode 100644 elf/tst-relr.c
> 
> diff --git a/configure b/configure
> index 2a3cb49b0b..05b72ceebf 100755
> --- a/configure
> +++ b/configure
> @@ -6115,6 +6115,43 @@ $as_echo "$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 -z pack-relative-relocs" >&5
> +$as_echo_n "checking for linker that supports -z pack-relative-relocs... " >&6; }
> +libc_linker_feature=no
> +if test x"$gnu_ld" = x"yes"; then
> +  cat > conftest.c <<EOF
> +int _start (void) { return 42; }
> +EOF
> +  if { ac_try='${CC-cc} $CFLAGS $CPPFLAGS $LDFLAGS $no_ssp
> +		    -Wl,-z,pack-relative-relocs -nostdlib -nostartfiles
> +		    -fPIC -shared -o conftest.so conftest.c
> +		    1>&5'
> +  { { eval echo "\"\$as_me\":${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
> +    if ${CC-cc} $CFLAGS $CPPFLAGS $LDFLAGS $no_ssp -Wl,-z,pack-relative-relocs -nostdlib \
> +	-nostartfiles -fPIC -shared -o conftest.so conftest.c 2>&1 \
> +	| grep "warning: -z pack-relative-relocs ignored" > /dev/null 2>&1; then
> +      true
> +    else
> +      libc_linker_feature=yes
> +    fi
> +  fi
> +  rm -f conftest*
> +fi
> +if test $libc_linker_feature = yes; then
> +  libc_cv_dt_relr=yes
> +else
> +  libc_cv_dt_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-dt-relr = $libc_cv_dt_relr"
> +
>  { $as_echo "$as_me:${as_lineno-$LINENO}: checking for linker that supports --no-dynamic-linker" >&5
>  $as_echo_n "checking for linker that supports --no-dynamic-linker... " >&6; }
>  libc_linker_feature=no
> diff --git a/configure.ac b/configure.ac
> index fa7d3c025b..f4ac25aa90 100644
> --- a/configure.ac
> +++ b/configure.ac
> @@ -1367,6 +1367,11 @@ LIBC_LINKER_FEATURE([--depaudit], [-Wl,--depaudit,x],
>  		    [libc_cv_depaudit=yes], [libc_cv_depaudit=no])
>  LIBC_CONFIG_VAR([have-depaudit], [$libc_cv_depaudit])
>  
> +LIBC_LINKER_FEATURE([-z pack-relative-relocs],
> +		    [-Wl,-z,pack-relative-relocs],
> +		    [libc_cv_dt_relr=yes], [libc_cv_dt_relr=no])
> +LIBC_CONFIG_VAR([have-dt-relr], [$libc_cv_dt_relr])
> +
>  LIBC_LINKER_FEATURE([--no-dynamic-linker],
>  		    [-Wl,--no-dynamic-linker],
>  		    [libc_cv_no_dynamic_linker=yes],

Ok.

> diff --git a/elf/Makefile b/elf/Makefile
> index 09d3d88336..979dde046d 100644
> --- a/elf/Makefile
> +++ b/elf/Makefile
> @@ -541,6 +541,25 @@ tests-special += \
>    # tests-special
>  endif
>  endif
> +ifeq ($(have-dt-relr),yes)
> +tests += \
> +  tst-relr \
> +# tests
> +ifeq ($(have-fpie),yes)
> +tests += \
> +  tst-relr-pie \
> +# tests
> +tests-pie += \
> +  tst-relr-pie \
> +# tests-pie
> +tests-special += \
> +  $(objpfx)check-tst-relr-pie.out \
> +# tests-special
> +endif
> +CFLAGS-tst-relr-pie.c += $(pie-ccflag)
> +LDFLAGS-tst-relr += -Wl,-z,pack-relative-relocs
> +LDFLAGS-tst-relr-pie += -Wl,-z,pack-relative-relocs
> +endif
>  endif
>  
>  ifeq ($(run-built-tests),yes)

Ok.

> @@ -2735,3 +2754,9 @@ $(objpfx)check-abi-version-libc.out: $(common-objpfx)libc.so
>  		| sed -ne '/.gnu.version_d/, /.gnu.version_r/ p' \
>  		| grep GLIBC_ABI_DT_RELR > $@; \
>  	$(evaluate-test)
> +
> +$(objpfx)check-tst-relr-pie.out: $(objpfx)tst-relr-pie
> +	LC_ALL=C $(OBJDUMP) -p $< \
> +		| sed -ne '/required from libc.so/,$$ p' \
> +		| grep GLIBC_ABI_DT_RELR > $@; \
> +	$(evaluate-test)
> diff --git a/elf/dynamic-link.h b/elf/dynamic-link.h
> index 25dd7ca4f2..d04c457e55 100644
> --- a/elf/dynamic-link.h
> +++ b/elf/dynamic-link.h
> @@ -146,12 +146,46 @@ elf_machine_lazy_rel (struct link_map *map, struct r_scope_elem *scope[],
>  #  define ELF_DYNAMIC_DO_RELA(map, scope, lazy, skip_ifunc) /* Nothing to do.  */
>  # endif
>  
> +# define ELF_DYNAMIC_DO_RELR(map)					      \
> +  do {									      \
> +    ElfW(Addr) l_addr = (map)->l_addr, *where = 0;			      \
> +    const ElfW(Relr) *r, *end;						      \
> +    if (!(map)->l_info[DT_RELR])					      \
> +      break;								      \
> +    r = (const ElfW(Relr) *)D_PTR((map), l_info[DT_RELR]);		      \
> +    end = (const ElfW(Relr) *)((const char *)r +			      \
> +                               (map)->l_info[DT_RELRSZ]->d_un.d_val);	      \
> +    for (; r < end; r++)						      \
> +      {									      \
> +	ElfW(Relr) entry = *r;						      \
> +	if ((entry & 1) == 0)						      \
> +	  {								      \
> +	    where = (ElfW(Addr) *)(l_addr + entry);			      \
> +	    *where++ += l_addr;						      \
> +	  }								      \
> +	else 								      \
> +	  {								      \
> +	    for (long i = 0; (entry >>= 1) != 0; i++)			      \

s/long/long int/

> +	      if ((entry & 1) != 0)					      \
> +		where[i] += l_addr;					      \
> +	    where += CHAR_BIT * sizeof(ElfW(Relr)) - 1;			      \
> +	  }								      \
> +      }									      \
> +  } while (0);
> +
>  /* This can't just be an inline function because GCC is too dumb
>     to inline functions containing inlines themselves.  */
> +# ifdef RTLD_BOOTSTRAP
> +#  define DO_RTLD_BOOTSTRAP 1
> +# else
> +#  define DO_RTLD_BOOTSTRAP 0
> +# endif
>  # define ELF_DYNAMIC_RELOCATE(map, scope, lazy, consider_profile, skip_ifunc) \
>    do {									      \
>      int edr_lazy = elf_machine_runtime_setup ((map), (scope), (lazy),	      \
>  					      (consider_profile));	      \
> +    if (((map) != &GL(dl_rtld_map) || DO_RTLD_BOOTSTRAP))		      \
> +      ELF_DYNAMIC_DO_RELR (map);					      \
>      ELF_DYNAMIC_DO_REL ((map), (scope), edr_lazy, skip_ifunc);		      \
>      ELF_DYNAMIC_DO_RELA ((map), (scope), edr_lazy, skip_ifunc);		      \
>    } while (0)

Ok.

> diff --git a/elf/get-dynamic-info.h b/elf/get-dynamic-info.h
> index 6c2ccd6db4..6c2a3a12b1 100644
> --- a/elf/get-dynamic-info.h
> +++ b/elf/get-dynamic-info.h
> @@ -89,6 +89,7 @@ elf_get_dynamic_info (struct link_map *l, bool bootstrap,
>  # if ! ELF_MACHINE_NO_REL
>        ADJUST_DYN_INFO (DT_REL);
>  # endif
> +      ADJUST_DYN_INFO (DT_RELR);
>        ADJUST_DYN_INFO (DT_JMPREL);
>        ADJUST_DYN_INFO (VERSYMIDX (DT_VERSYM));
>        ADJUST_DYN_INFO (ADDRIDX (DT_GNU_HASH));
> @@ -113,6 +114,8 @@ elf_get_dynamic_info (struct link_map *l, bool bootstrap,
>    if (info[DT_REL] != NULL)
>      assert (info[DT_RELENT]->d_un.d_val == sizeof (ElfW(Rel)));
>  #endif
> +  if (info[DT_RELR] != NULL)
> +    assert (info[DT_RELRENT]->d_un.d_val == sizeof (ElfW(Relr)));
>    if (bootstrap || static_pie_bootstrap)
>      {
>        assert (info[DT_RUNPATH] == NULL);

Ok.

> diff --git a/elf/tst-relr-pie.c b/elf/tst-relr-pie.c
> new file mode 100644
> index 0000000000..7df0cdbfd6
> --- /dev/null
> +++ b/elf/tst-relr-pie.c
> @@ -0,0 +1 @@
> +#include "tst-relr.c"

Ok.

> diff --git a/elf/tst-relr.c b/elf/tst-relr.c
> new file mode 100644
> index 0000000000..c634ce0d21
> --- /dev/null
> +++ b/elf/tst-relr.c
> @@ -0,0 +1,65 @@
> +/* Basic tests for DT_RELR.
> +   Copyright (C) 2022 Free Software Foundation, Inc.
> +   This file is part of the GNU C Library.
> +
> +   The GNU C Library is free software; you can redistribute it and/or
> +   modify it under the terms of the GNU Lesser General Public
> +   License as published by the Free Software Foundation; either
> +   version 2.1 of the License, or (at your option) any later version.
> +
> +   The GNU C Library is distributed in the hope that it will be useful,
> +   but WITHOUT ANY WARRANTY; without even the implied warranty of
> +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
> +   Lesser General Public License for more details.
> +
> +   You should have received a copy of the GNU Lesser General Public
> +   License along with the GNU C Library; if not, see
> +   <https://www.gnu.org/licenses/>.  */
> +
> +#include <link.h>
> +#include <stdbool.h>
> +#include <array_length.h>
> +#include <support/check.h>
> +
> +static int o, x;
> +
> +#define ELEMS O O O O O O O O X X X X X X X O O X O O X X X E X E E O X O E
> +#define E 0,
> +
> +#define O &o,
> +#define X &x,
> +void *arr[] = { ELEMS };
> +#undef O
> +#undef X
> +
> +#define O 1,
> +#define X 2,
> +static char val[] = { ELEMS };
> +
> +static int
> +do_test (void)
> +{
> +  ElfW(Dyn) *d = _DYNAMIC;
> +  if (d)
> +    {
> +      bool has_relr = false;
> +      for (; d->d_tag != DT_NULL; d++)
> +	if (d->d_tag == DT_RELR)
> +	  has_relr = true;
> +
> +#if defined __PIE__ || defined __pie__ || defined PIE || defined pie
> +      TEST_VERIFY (has_relr);
> +#else
> +      TEST_VERIFY (!has_relr);
> +#endif
> +    }
> +
> +  for (int i = 0; i < array_length (arr); i++)
> +    TEST_VERIFY ((arr[i] == 0 && val[i] == 0)
> +		 || (arr[i] == &o && val[i] == 1)
> +		 || (arr[i] == &x && val[i] == 2));
> +
> +  return 0;
> +}
> +
> +#include <support/test-driver.c>

Ok.