Re: [PATCH v3] elf: Remove ELF_RTYPE_CLASS_EXTERN_PROTECTED_DATA

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

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?

>
> >> > 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.