Re: [PATCH v7 2/6] Add GLIBC_ABI_DT_RELR for DT_RELR support

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

On 31/03/2022 13:38, H.J. Lu wrote:
> The EI_ABIVERSION field of the ELF header in executables and shared
> libraries can be bumped to indicate the minimum ABI requirement on the
> dynamic linker.  However, EI_ABIVERSION in executables isn't checked by
> the Linux kernel ELF loader nor the existing dynamic linker.  Executables
> will crash mysteriously if the dynamic linker doesn't support the ABI
> features required by the EI_ABIVERSION field.  The dynamic linker should
> be changed to check EI_ABIVERSION in executables.
> 
> Add a glibc version, GLIBC_ABI_DT_RELR, to indicate DT_RELR support so
> that the existing dynamic linkers will issue an error on executables with
> GLIBC_ABI_DT_RELR dependency.  Issue an error if there is a DT_RELR entry
> without GLIBC_ABI_DT_RELR dependency nor GLIBC_PRIVATE definition.
> 
> Support __placeholder_only_for_empty_version_map as the placeholder symbol
> used only for empty version map to generate GLIBC_ABI_DT_RELR without any
> symbols.

LGTM, one small suggestion below.

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

> ---
>  elf/Makefile         | 14 ++++++++++++--
>  elf/Versions         |  5 +++++
>  elf/dl-version.c     | 33 +++++++++++++++++++++++++++++++--
>  include/link.h       |  6 ++++++
>  scripts/abilist.awk  |  2 ++
>  scripts/versions.awk |  7 ++++++-
>  6 files changed, 62 insertions(+), 5 deletions(-)
> 
> diff --git a/elf/Makefile b/elf/Makefile
> index c96924e9c2..09d3d88336 100644
> --- a/elf/Makefile
> +++ b/elf/Makefile
> @@ -1105,8 +1105,12 @@ $(eval $(call include_dsosort_tests,dso-sort-tests-1.def))
>  $(eval $(call include_dsosort_tests,dso-sort-tests-2.def))
>  endif
>  
> -check-abi: $(objpfx)check-abi-ld.out
> -tests-special += $(objpfx)check-abi-ld.out
> +check-abi: $(objpfx)check-abi-ld.out \
> +	   $(objpfx)check-abi-version-libc.out
> +tests-special += \
> +  $(objpfx)check-abi-ld.out \
> +  $(objpfx)check-abi-version-libc.out \
> +# tests-special
>  update-abi: update-abi-ld
>  update-all-abi: update-all-abi-ld
>  
> @@ -2725,3 +2729,9 @@ $(objpfx)tst-p_align3: $(objpfx)tst-p_alignmod3.so
>  $(objpfx)tst-p_align3.out: tst-p_align3.sh $(objpfx)tst-p_align3
>  	$(SHELL) $< $(common-objpfx) '$(test-program-prefix)'; \
>  	$(evaluate-test)
> +
> +$(objpfx)check-abi-version-libc.out: $(common-objpfx)libc.so
> +	LC_ALL=C $(READELF) -V -W $< \
> +		| sed -ne '/.gnu.version_d/, /.gnu.version_r/ p' \
> +		| grep GLIBC_ABI_DT_RELR > $@; \
> +	$(evaluate-test)

Ok.

> diff --git a/elf/Versions b/elf/Versions
> index 8bed855d8c..a9ff278de7 100644
> --- a/elf/Versions
> +++ b/elf/Versions
> @@ -23,6 +23,11 @@ libc {
>    GLIBC_2.35 {
>      _dl_find_object;
>    }
> +  GLIBC_ABI_DT_RELR {
> +    # This symbol is used only for empty version map and will be removed
> +    # by scripts/versions.awk.
> +    __placeholder_only_for_empty_version_map;
> +  }
>    GLIBC_PRIVATE {
>      # functions used in other libraries
>      __libc_early_init;

Ok.

> diff --git a/elf/dl-version.c b/elf/dl-version.c
> index b47bd91727..720ec596a5 100644
> --- a/elf/dl-version.c
> +++ b/elf/dl-version.c
> @@ -214,12 +214,20 @@ _dl_check_map_versions (struct link_map *map, int verbose, int trace_mode)
>  	      while (1)
>  		{
>  		  /* Match the symbol.  */
> +		  const char *string = strtab + aux->vna_name;
>  		  result |= match_symbol (DSO_FILENAME (map->l_name),
>  					  map->l_ns, aux->vna_hash,
> -					  strtab + aux->vna_name,
> -					  needed->l_real, verbose,
> +					  string, needed->l_real, verbose,
>  					  aux->vna_flags & VER_FLG_WEAK);
>  
> +		  if (map->l_abi_version == lav_none
> +		      /* 0xfd0e42: _dl_elf_hash ("GLIBC_ABI_DT_RELR").  */
> +		      && aux->vna_hash == 0xfd0e42
> +		      && __glibc_likely (strcmp (string,
> +						 "GLIBC_ABI_DT_RELR")
> +					 == 0))
> +		    map->l_abi_version = lav_dt_relr_ref;
> +
>  		  /* Compare the version index.  */
>  		  if ((unsigned int) (aux->vna_other & 0x7fff) > ndx_high)
>  		    ndx_high = aux->vna_other & 0x7fff;

Ok.

> @@ -253,6 +261,16 @@ _dl_check_map_versions (struct link_map *map, int verbose, int trace_mode)
>        ent = (ElfW(Verdef) *) (map->l_addr + def->d_un.d_ptr);
>        while (1)
>  	{
> +	  /* 0x0963cf85: _dl_elf_hash ("GLIBC_PRIVATE").  */
> +	  if (ent->vd_hash == 0x0963cf85)
> +	    {
> +	      ElfW(Verdaux) *aux = (ElfW(Verdaux) *) ((char *) ent
> +						      + ent->vd_aux);
> +	      if (__glibc_likely (strcmp ("GLIBC_PRIVATE",
> +					  strtab + aux->vda_name) == 0))
> +		map->l_abi_version = lav_private_def;
> +	    }
> +
>  	  if ((unsigned int) (ent->vd_ndx & 0x7fff) > ndx_high)
>  	    ndx_high = ent->vd_ndx & 0x7fff;
>  

Ok, maybe remove the XXX below now that we can't really avoid the loop anymore.

> @@ -352,6 +370,17 @@ _dl_check_map_versions (struct link_map *map, int verbose, int trace_mode)
>  	}
>      }
>  
> +  /* Issue an error if there is a DT_RELR entry without GLIBC_ABI_DT_RELR
> +     dependency nor GLIBC_PRIVATE definition.  */
> +  if (map->l_info[DT_RELR] != NULL
> +      && __glibc_unlikely (map->l_abi_version == lav_none))
> +    {
> +      _dl_exception_create
> +	(&exception, DSO_FILENAME (map->l_name),
> +	 N_("DT_RELR without GLIBC_ABI_DT_RELR dependency"));
> +      goto call_error;
> +    }
> +
>    return result;
>  }
>  

Ok.

> diff --git a/include/link.h b/include/link.h
> index 03db14c7b0..8ec5e35cf2 100644
> --- a/include/link.h
> +++ b/include/link.h
> @@ -177,6 +177,12 @@ struct link_map
>  	lt_library,		/* Library needed by main executable.  */
>  	lt_loaded		/* Extra run-time loaded shared object.  */
>        } l_type:2;
> +    enum			/* ABI dependency of this object.  */
> +      {
> +	lav_none,		/* No ABI dependency.  */
> +	lav_dt_relr_ref,	/* Need GLIBC_ABI_DT_RELR.  */
> +	lav_private_def		/* Define GLIBC_PRIVATE.  */
> +      } l_abi_version:2;
>      unsigned int l_relocated:1;	/* Nonzero if object's relocations done.  */
>      unsigned int l_init_called:1; /* Nonzero if DT_INIT function called.  */
>      unsigned int l_global:1;	/* Nonzero if object in _dl_global_scope.  */

Ok.

> diff --git a/scripts/abilist.awk b/scripts/abilist.awk
> index 24a34ccbed..6cc7af6ac8 100644
> --- a/scripts/abilist.awk
> +++ b/scripts/abilist.awk
> @@ -55,6 +55,8 @@ $2 == "g" || $2 == "w" && (NF == 7 || NF == 8) {
>    # caused STV_HIDDEN symbols to appear in .dynsym, though that is useless.
>    if (NF > 7 && $7 == ".hidden") next;
>  
> +  if (version ~ /^GLIBC_ABI_/ && !include_abi_version) next;
> +
>    if (version == "GLIBC_PRIVATE" && !include_private) next;
>  
>    desc = "";

Ok.

> diff --git a/scripts/versions.awk b/scripts/versions.awk
> index 357ad1355e..d70b07bd1a 100644
> --- a/scripts/versions.awk
> +++ b/scripts/versions.awk
> @@ -185,8 +185,13 @@ END {
>  	closeversion(oldver, veryoldver);
>  	veryoldver = oldver;
>        }
> -      printf("%s {\n  global:\n", $2) > outfile;
>        oldver = $2;
> +      # Skip the placeholder symbol used only for empty version map.
> +      if ($3 == "__placeholder_only_for_empty_version_map;") {
> +	printf("%s {\n", $2) > outfile;
> +	continue;
> +      }
> +      printf("%s {\n  global:\n", $2) > outfile;
>      }
>      printf("   ") > outfile;
>      for (n = 3; n <= NF; ++n) {

Ok.