[PATCH v2] elf: Refine direct extern access diagnostics to protected symbol

[H.J. Lu]

On Wed, Jun 8, 2022 at 12:37 PM Fangrui Song via Libc-alpha
<libc-alpha@sourceware.org> wrote:
>
> Refine commit 349b0441dab375099b1d7f6909c1742286a67da9:
>
> 1. Copy relocations for extern protected data do not work properly,
> regardless whether GNU_PROPERTY_1_NEEDED_INDIRECT_EXTERN_ACCESS is used.
> It makes sense to produce a warning unconditionally.  When the defining
> shared object has GNU_PROPERTY_1_NEEDED_INDIRECT_EXTERN_ACCESS, report
> an error to satisfy the "no copy relocations" enforcement intended by
> this GNU property.
>
> 2. Non-zero value of an undefined function symbol may break pointer
> equality, but may be benign in many cases (many programs don't take the
> address in the shared object then compare it with the address in the
> executable).  Reword the diagnostic to be clearer.
>
> 3. Remove the unneeded condition !(undef_map->l_1_needed &
> GNU_PROPERTY_1_NEEDED_INDIRECT_EXTERN_ACCESS). If the executable has
> GNU_PROPERTY_1_NEEDED_INDIRECT_EXTERN_ACCESS (can only occur in error
> cases), the diagnostic should be emitted as well.
>
> --
> Changes from v1:
> * Keep 2 as an error in the presence of GNU_PROPERTY_1_NEEDED_INDIRECT_EXTERN_ACCESS
>   as requested by H.J. Lu.
> ---
>  sysdeps/generic/dl-protected.h | 50 ++++++++++++++++++----------------
>  1 file changed, 27 insertions(+), 23 deletions(-)
>
> diff --git a/sysdeps/generic/dl-protected.h b/sysdeps/generic/dl-protected.h
> index 88cb8ec917..38386b5200 100644
> --- a/sysdeps/generic/dl-protected.h
> +++ b/sysdeps/generic/dl-protected.h
> @@ -26,29 +26,33 @@ _dl_check_protected_symbol (const char *undef_name,
>                             const struct link_map *map,
>                             int type_class)
>  {
> -  if (undef_map != NULL
> -      && undef_map->l_type == lt_executable
> -      && !(undef_map->l_1_needed
> -          & GNU_PROPERTY_1_NEEDED_INDIRECT_EXTERN_ACCESS)
> -      && (map->l_1_needed
> -         & GNU_PROPERTY_1_NEEDED_INDIRECT_EXTERN_ACCESS))
> -    {
> -      if ((type_class & ELF_RTYPE_CLASS_COPY))
> -       /* Disallow copy relocations in executable against protected
> -          data symbols in a shared object which needs indirect external
> -          access.  */
> -       _dl_signal_error (0, map->l_name, undef_name,
> -                         N_("copy relocation against non-copyable protected symbol"));
> -      else if (ref->st_value != 0
> -              && ref->st_shndx == SHN_UNDEF
> -              && (type_class & ELF_RTYPE_CLASS_PLT))
> -       /* Disallow non-zero symbol values of undefined symbols in
> -          executable, which are used as the function pointer, against
> -          protected function symbols in a shared object with indirect
> -          external access.  */
> -       _dl_signal_error (0, map->l_name, undef_name,
> -                         N_("non-canonical reference to canonical protected function"));
> -    }
> +  if (undef_map == NULL || undef_map->l_type != lt_executable)
> +    return;
> +
> +  if (type_class & ELF_RTYPE_CLASS_COPY)
> +    /* Disallow copy relocations in executable against protected
> +       data symbols in a shared object which needs indirect external
> +       access.  */
> +    _dl_error_printf ("warning: copy relocation against non-copyable "
> +                     "protected symbol `%s' in `%s'\n",
> +                     undef_name, map->l_name);
> +  else if ((type_class & ELF_RTYPE_CLASS_PLT) && ref->st_value != 0
> +          && ref->st_shndx == SHN_UNDEF)
> +    /* Disallow non-zero symbol values of undefined symbols in
> +       executable, which are used as the function pointer, against
> +       protected function symbols in a shared object with indirect
> +       external access.  */
> +    _dl_error_printf (
> +       "warning: direct reference to "
> +       "protected function `%s' in `%s' may break pointer equality\n",
> +       undef_name, map->l_name);
> +  else
> +    return;
> +
> +  if (map->l_1_needed & GNU_PROPERTY_1_NEEDED_INDIRECT_EXTERN_ACCESS)
> +    _dl_signal_error (
> +       0, map->l_name, undef_name,
> +       N_ ("error due to GNU_PROPERTY_1_NEEDED_INDIRECT_EXTERN_ACCESS"));
>  }
>
>  #endif /* _DL_PROTECTED_H */
> --
> 2.36.1.255.ge46751e96f-goog
>

LGTM.

Thanks.

-- 
H.J.