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

[Fangrui Song]

On 2022-06-08, H.J. Lu wrote:
>On Tue, Jun 7, 2022 at 8:53 PM Fangrui Song <maskray@google.com> wrote:
>>
>> On 2022-06-07, H.J. Lu wrote:
>> >On Tue, Jun 7, 2022 at 4:53 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).  Report a warning instead.  While here, 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.
>> >> ---
>> >>  sysdeps/generic/dl-protected.h | 46 ++++++++++++++++++----------------
>> >>  1 file changed, 25 insertions(+), 21 deletions(-)
>> >>
>> >> diff --git a/sysdeps/generic/dl-protected.h b/sysdeps/generic/dl-protected.h
>> >> index 88cb8ec917..ed40d9fea9 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 (undef_map == NULL || undef_map->l_type != lt_executable)
>> >> +    return;
>> >> +
>> >> +  if (type_class & ELF_RTYPE_CLASS_COPY)
>> >>      {
>> >> -      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"));
>> >> +      /* 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);
>> >> +
>> >> +      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"));
>> >>      }
>> >> +  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);
>> >
>> >Should there be an error for GNU_PROPERTY_1_NEEDED_INDIRECT_EXTERN_ACCESS?
>>
>> I lean toward a warning, as bullet point 2 in my commit message
>> explains.
>
>It is due to R_386_PC32.   Can we make the error optional and enable it
>for x86-64?

Do you mean that the branch should call call _dl_signal_error in the
presence of GNU_PROPERTY_1_NEEDED_INDIRECT_EXTERN_ACCESS?

else if ((type_class & ELF_RTYPE_CLASS_PLT) && ref->st_value != 0
         && ref->st_shndx == SHN_UNDEF)

>> In addition, this check requires that the executable with a non-zero
>> st_value has at least one JUMP_SLOT relocation.
>>
>> In the following setup the executable does not have JUMP_SLOT, so
>> there is no diagnostic, with or without the patch.
>
>We can pass symbol definition and check STT_FUNC.

My point is that the check only kicks in when there is a dynamic
relocation using ELF_RTYPE_CLASS_PLT (typically JUMP_SLOT).  If the
executable just takes the address but doesn't call the function, the
branch will not be executed at all.

That said, I am flexible and can add the wrong if you feel strong about
it.  To be clear, do you indicate that the error should require
!defined(__i386__) ?

>> // a.c -fno-pic -no-pie
>> #include <stdio.h>
>> int foo(void);
>> int main(void) { printf("%p\n", foo);
>>
>> // b.c -fpic -shared
>> int foo(void) { return 3; }
>> asm(".protected foo");
>>
>> >>  }
>> >>
>> >>  #endif /* _DL_PROTECTED_H */
>> >> --
>> >> 2.36.1.255.ge46751e96f-goog
>> >>
>> >
>> >
>> >--
>> >H.J.
>
>
>
>-- 
>H.J.