Re: [PATCH v3] elf: Remove ELF_RTYPE_CLASS_EXTERN_PROTECTED_DATA

[Fangrui Song <maskray@google.com>]

On 2022-06-07, H.J. Lu wrote:
>On Tue, Jun 7, 2022 at 11:21 AM Fangrui Song <maskray@google.com> wrote:
>>
>> On 2022-06-07, H.J. Lu wrote:
>> >On Tue, Jun 7, 2022 at 6:25 AM Szabolcs Nagy via Libc-alpha
>> ><libc-alpha@sourceware.org> wrote:
>> >>
>> >> The 06/01/2022 10:56, Fangrui Song wrote:
>> >> > If an executable has copy relocations for extern protected data, that
>> >> > can only work if the library containing the definition is built with
>> >> > assumptions (a) the compiler emits GOT-generating relocations (b) the
>> >> > linker produces R_*_GLOB_DAT instead of R_*_RELATIVE.  Otherwise the
>> >> > library uses its own definition directly and the executable accesses a
>> >> > stale copy.  Note: the GOT relocations defeat the purpose of protected
>> >> > visibility as an optimization, but allow rtld to make the executable and
>> >> > library use the same copy when copy relocations are present, but it
>> >> > turns out this never worked perfectly.
>> >> >
>> >> > ELF_RTYPE_CLASS_EXTERN_PROTECTED_DATA has strange semantics when both
>> >> > a.so and b.so define protected var and the executable copy relocates
>> >> > var: b.so accesses its own copy even with GLOB_DAT.  The behavior change
>> >> > is from commit 62da1e3b00b51383ffa7efc89d8addda0502e107 (x86) and then
>> >> > copied to nios2 (ae5eae7cfc9c4a8297ff82ec6b794faca1976ecc) and arc
>> >> > (0e7d930c4c11de896fe807f67fa1eb756c9c1e05).
>> >> >
>> >> > Without ELF_RTYPE_CLASS_EXTERN_PROTECTED_DATA, b.so accesses the copy
>> >> > relocated data like a.so.
>> >> >
>> >> > ELF_RTYPE_CLASS_EXTERN_PROTECTED_DATA has another effect in the absence
>> >> > of copy relocations: when a protected data symbol is defined in multiple
>> >> > objects, the code tries to bind the relocation locally.  Without
>> >> > ELF_RTYPE_CLASS_EXTERN_PROTECTED_DATA, STV_PROTECTED is handled in the
>> >> > same way as STV_DEFAULT: if ld produces GLOB_DAT (some ports of GNU ld),
>> >> > the relocation will bind to the first definition; otherwise (e.g.
>> >> > ld.lld) ld does the binding locally and ld.so doesn't help.
>> >> >
>> >>
>> >> i think we should not change the interposition semantics.
>> >> we should go back to the old behaviour where only copy
>> >> relocs were broken (and there was an expensive workaround
>> >> to deal with protected symbol interposition).
>> >>
>> >> i think you want to revert the elf/dl-lookup.c changes of
>> >>
>> >>   commit 62da1e3b00b51383ffa7efc89d8addda0502e107
>> >>   Author:     H.J. Lu <hjl.tools@gmail.com>
>> >>   CommitDate: 2015-03-31 05:16:57 -0700
>> >>
>> >>   Add ELF_RTYPE_CLASS_EXTERN_PROTECTED_DATA to x86
>> >>
>> >
>> >I am OK to remove support of copy relocation against protected
>> >symbols since it doesn't work properly.
>>
>> Thanks.
>>
>> >My only question is if
>> >ld.so should issue a warning or an error when seeing a copy
>> >relocation against a protected symbol.   Copy relocation against
>> >protected symbol defeats the purpose of protected symbol.
>>
>> The check already exists (_dl_check_protected_symbol) but currently
>> relies on GNU_PROPERTY_1_NEEDED_INDIRECT_EXTERN_ACCESS (only implemented
>> for x86, and adoption is low on x86).
>>
>> For ELF_RTYPE_CLASS_COPY, I think the GNU_PROPERTY_1_NEEDED_INDIRECT_EXTERN_ACCESS
>> check can be removed.
>
>Will removal of GNU_PROPERTY_1_NEEDED_INDIRECT_EXTERN_ACCESS
>check cause many run-time errors?
>> (
>> Since GCC 5, x86-64 -fpie has HAVE_LD_PIE_COPYRELOC.
>> When neither -m[no]direct-extern-access is specified, HAVE_LD_PIE_COPYRELOC takes effect.
>> The executable does not have GNU_PROPERTY_1_NEEDED_INDIRECT_EXTERN_ACCESS
>> but the incompatibility exists.
>> It just kinda works because GCC and GNU ld cooperate to produce a GLOB_DAT in the DSO.
>> )
>>
>> For ELF_RTYPE_CLASS_PLT, the pointer equality does not matter much in
>> practice:
>>
>> * protected visibility adoption is very low due to various problems.
>> * Taking a function address in the executable and expecting it to match the address in a DSO is rare.
>> * Many users use ICF and by and large don't care about function addresses to some extent.
>>
>> I think having the warning under GNU_PROPERTY_1_NEEDED_INDIRECT_EXTERN_ACCESS is fine.
>> (
>> * x86-32 -fno-pic uses R_386_PC32 as a jump instruction, which is
>>    indistinguishable from an address-taken operation
>>    https://maskray.me/blog/2021-01-09-copy-relocations-canonical-plt-entries-and-protected#branch-instructions-on-x86
>> )
>
>An error with GNU_PROPERTY_1_NEEDED_INDIRECT_EXTERN_ACCESS
>and a warning without?

This plan sounds good, when we create a separate patch enhancing the
diagnostics.

x86-32 may need a exception (i.e. no warning) for ELF_RTYPE_CLASS_PLT to handle R_386_PC32.

>> >> > It's extremely unlikely anyone relies on the
>> >> > ELF_RTYPE_CLASS_EXTERN_PROTECTED_DATA behavior, so let's remove it: this
>> >> > removes a check in the symbol lookup code.
>> >> >
>> >> > --
>> >> > Changes from v1:
>> >> > * Reword commit message as suggested by Szabolcs Nagy
>> >> >
>> >> > Changes from v2:
>> >> > * Explain interposition behavior
>> >> > ---
>> >> >  elf/dl-lookup.c             | 90 -------------------------------------
>> >> >  sysdeps/arc/dl-sysdep.h     | 21 ---------
>> >> >  sysdeps/generic/ldsodefs.h  | 12 +----
>> >> >  sysdeps/i386/dl-machine.h   |  3 +-
>> >> >  sysdeps/nios2/dl-sysdep.h   | 21 ---------
>> >> >  sysdeps/x86/dl-lookupcfg.h  |  4 --
>> >> >  sysdeps/x86_64/dl-machine.h |  8 +---
>> >> >  7 files changed, 4 insertions(+), 155 deletions(-)
>> >> >  delete mode 100644 sysdeps/arc/dl-sysdep.h
>> >> >  delete mode 100644 sysdeps/nios2/dl-sysdep.h
>> >> >
>> >> > diff --git a/elf/dl-lookup.c b/elf/dl-lookup.c
>> >> > index a42f6d5390..41d108e0b8 100644
>> >> > --- a/elf/dl-lookup.c
>> >> > +++ b/elf/dl-lookup.c
>> >> ...
>> >> > @@ -854,43 +801,6 @@ _dl_lookup_symbol_x (const char *undef_name, struct link_map *undef_map,
>> >> >        return 0;
>> >> >      }
>> >> >
>> >> > -  int protected = (*ref
>> >> > -                && ELFW(ST_VISIBILITY) ((*ref)->st_other) == STV_PROTECTED);
>> >> > -  if (__glibc_unlikely (protected != 0))
>> >> > -    {
>> >> > -      /* It is very tricky.  We need to figure out what value to
>> >> > -      return for the protected symbol.  */
>> >> > -      if (type_class == ELF_RTYPE_CLASS_PLT)
>> >> > -     {
>> >> > -       if (current_value.s != NULL && current_value.m != undef_map)
>> >> > -         {
>> >> > -           current_value.s = *ref;
>> >> > -           current_value.m = undef_map;
>> >> > -         }
>> >> > -     }
>> >> > -      else
>> >> > -     {
>> >> > -       struct sym_val protected_value = { NULL, NULL };
>> >> > -
>> >> > -       for (scope = symbol_scope; *scope != NULL; i = 0, ++scope)
>> >> > -         if (do_lookup_x (undef_name, new_hash, &old_hash, *ref,
>> >> > -                          &protected_value, *scope, i, version, flags,
>> >> > -                          skip_map,
>> >> > -                          (ELF_RTYPE_CLASS_EXTERN_PROTECTED_DATA
>> >> > -                           && ELFW(ST_TYPE) ((*ref)->st_info) == STT_OBJECT
>> >> > -                           && type_class == ELF_RTYPE_CLASS_EXTERN_PROTECTED_DATA)
>> >> > -                          ? ELF_RTYPE_CLASS_EXTERN_PROTECTED_DATA
>> >> > -                          : ELF_RTYPE_CLASS_PLT, NULL) != 0)
>> >> > -           break;
>> >> > -
>> >> > -       if (protected_value.s != NULL && protected_value.m != undef_map)
>> >> > -         {
>> >> > -           current_value.s = *ref;
>> >> > -           current_value.m = undef_map;
>> >> > -         }
>> >> > -     }
>> >> > -    }
>> >> > -
>> >>
>> >> i think we should keep this part without the
>> >> ELF_RTYPE_CLASS_EXTERN_PROTECTED_DATA bit.
>> >
>> >
>> >
>> >--
>> >H.J.
>
>
>
>-- 
>H.J.