Re: [PATCH v6 3/5] Add GLIBC_ABI_DT_RELR for DT_RELR support

["H.J. Lu" <hjl.tools@gmail.com>]

On Tue, Mar 29, 2022 at 9:52 AM Adhemerval Zanella
<adhemerval.zanella@linaro.org> wrote:
>
>
>
> On 10/03/2022 17:03, H.J. Lu via Libc-alpha 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.
>
> I think it only make sense to add the GLIBC_ABI_DT_RELR if the ABI actually
> supports DT_RELR, otherwise if the ABI start to support old glibcs might
> wrongly assumes DT_RELR and fails with undefined behavior at runtime
> (which this patch essentially tries to avoid).

The DT_RELR set enables DT_RELR in glibc for all targets.

> > ---
> >  elf/Makefile         | 12 ++++++++++--
> >  elf/Versions         |  5 +++++
> >  elf/dl-version.c     | 33 +++++++++++++++++++++++++++++++--
> >  include/link.h       |  6 ++++++
> >  scripts/abilist.awk  |  2 ++
> >  scripts/versions.awk |  7 ++++++-
> >  6 files changed, 60 insertions(+), 5 deletions(-)
> >
> > diff --git a/elf/Makefile b/elf/Makefile
> > index fd462ba315..d3118b9777 100644
> > --- a/elf/Makefile
> > +++ b/elf/Makefile
> > @@ -1113,8 +1113,10 @@ $(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
>
> One line per definition.

Fixed in v7.

> >  update-abi: update-abi-ld
> >  update-all-abi: update-all-abi-ld
> >
> > @@ -2739,3 +2741,9 @@ $(objpfx)check-tst-relr-pie.out: $(objpfx)tst-relr-pie
> >               | sed -ne '/required from libc.so/,$$ p' \
> >               | grep GLIBC_ABI_DT_RELR > $@; \
> >       $(evaluate-test)
> > +
> > +$(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)
>
> This test should be enable iff for $(have-dt-relr) == yes.

This test checks if libc.so has the GLIBC_ABI_DT_RELR
version definition.   Since it doesn't require a new linker,
there is no need to check $(have-dt-relr).   This test
passes with an old linker.

> > 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;
> > +  }
>
> Same as before. Maybe use the same strategy we do for time64: add a config.h
> define to HAVE_DTRELR (set by configure) and use it to set the symbol if
> it is defined.
>
> This define would be used on dl-version.c to also enable the l_abi_version
> check.
>
> >    GLIBC_PRIVATE {
> >      # functions used in other libraries
> >      __libc_early_init;
> > 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;
> > @@ -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;
> >
> > @@ -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;
> >  }
> >
> > 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.  */
> > 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 = "";
> > 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) {



-- 
H.J.