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

[Fangrui Song <maskray@google.com>]

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(-)

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2.36.0.464.gb9c8b46e94-goog