[PATCH v2 0/5] Improve objtool jump table handling

[Ard Biesheuvel]

From: Ard Biesheuvel <ardb@xxxxxxxxxx>

Jump table handling has faded into the background a little due to the
fact that jump tables are [currently] disabled when enabling retpoline
mitigations and/or IBT on x86.

However, this is likely to come back and bite us later, so it still
needs to be addressed. Given the difficulty in identifying jump tables
from .rodata references and indirect jump instructions that often have
no obvious correlation, it would be better to do this in the compiler.

This series implements [on the objtool side] the suggestion made at GNU
Cauldron this year to annotate the indirect jump with a R_X86_64_NONE
relocation that refers to the jump table, and ensure that it is covered
by a STT_OBJECT symbol whose size accurately reflects the size of the
jump table.

This can be wired up in objtool with minimal effort. The only
complication is that indirect jumps may be direct jumps in disguise, if
they target retpoline thunks. This will result in more than one
relocation attached to the same instruction, which needs careful
handling in objtool.

Other than that, changes are rather straight-forward.

Patches #4 and #5 update the CRC32C driver, which has a jump table
implemented in assembler, to a) use a relative jump table, for
compatibility with linking in PIE mode (#4) and b) make the jump table
more difficult to identify by objtool's existing heuristics, but provide
the annotation so it can found nonetheless.

This series is labeled as v2 because patch #1 was sent out before.

Changes since v1:
- tweak logic in patch #1 to ensure that all jump table entries are
  covered by the same type of relocation
- use the corrected addend when validating IBT targets
- add patches #2 - #5

Cc: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Jan Beulich <jbeulich@xxxxxxxx>
Cc: "Jose E. Marchesi" <jemarch@xxxxxxx>
Cc: Kees Cook <kees@xxxxxxxxxx>

Ard Biesheuvel (5):
  objtool: Deal with relative jump tables correctly
  objtool: Allow arch code to discover jump table size
  objtool: Add support for annotated jump tables
  crypto: x86/crc32c - Use idiomatic relative jump table
  crypto: x86/crc32c - Tweak jump table to validate objtool logic

 arch/x86/crypto/crc32c-pcl-intel-asm_64.S | 40 +++++++-----
 tools/objtool/arch/loongarch/special.c    |  3 +-
 tools/objtool/arch/powerpc/special.c      |  3 +-
 tools/objtool/arch/x86/special.c          | 43 ++++++++-----
 tools/objtool/check.c                     | 65 +++++++++++++++-----
 tools/objtool/include/objtool/check.h     |  5 +-
 tools/objtool/include/objtool/elf.h       |  6 ++
 tools/objtool/include/objtool/special.h   |  3 +-
 8 files changed, 117 insertions(+), 51 deletions(-)

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2.47.0.rc0.187.ge670bccf7e-goog