[PATCH 0/3] Simplify ELF_RTYPE_CLASS_EXTERN_PROTECTED_DATA and revert aarch64/arm's extern protected data handling

[H.J. Lu]

On Sat, Apr 30, 2022 at 11:06 PM Fangrui Song via Libc-alpha
<libc-alpha@sourceware.org> wrote:
>
> Say both a.so and b.so define protected var and the executable copy
> relocates var. ELF_RTYPE_CLASS_EXTERN_PROTECTED_DATA has strange
> semantics: a.so accesses the copy in the executable while b.so accesses
> its own. This behavior requires that (a) the compiler emits
> GOT-generating relocations (b) the linker produces GLOB_DAT instead of
> RELATIVE.
>
> Without the ELF_RTYPE_CLASS_EXTERN_PROTECTED_DATA code, b.so's GLOB_DAT
> will bind to the executable (normal behavior).
>
> For aarch64/arm it makes sense to restore the original behavior and don't
> pay the ELF_RTYPE_CLASS_EXTERN_PROTECTED_DATA cost. The behavior is very
> unlikely used by anyone.
>
> * Clang code generation treats STV_PROTECTED the same way as STV_HIDDEN:
>   no GOT-generating relocation in the first place.
> * gold and lld reject copy relocation on a STV_PROTECTED symbol.
> * Nowadays -fpie/-fpic modes are popular. GCC/Clang's codegen uses
>   GOT-generating relocation when accessing an default visibility
>   external symbol which avoids copy relocation.
>
> Fangrui Song (3):
>   elf: Remove ELF_RTYPE_CLASS_EXTERN_PROTECTED_DATA check for
>     non-DL_EXTERN_PROTECTED_DATA ports
>   Revert "[AArch64][BZ #17711] Fix extern protected data handling"
>   Revert "[ARM][BZ #17711] Fix extern protected data handling"
>
>  elf/dl-lookup.c              | 46 ++++++++++++------------------------
>  sysdeps/aarch64/dl-machine.h | 13 +++++-----
>  sysdeps/aarch64/dl-sysdep.h  |  2 --
>  sysdeps/arm/dl-machine.h     | 10 +++-----
>  sysdeps/arm/dl-sysdep.h      |  2 --
>  5 files changed, 24 insertions(+), 49 deletions(-)
>
> --
> 2.36.0.464.gb9c8b46e94-goog
>

Given that protected symbols never work properly with copy relocation,
can we change protected symbol handling to simply warn copy relocation
against protected data symbol and non-zero symbol values of undefined
symbols against protected function symbols?


-- 
H.J.