objtool/klp: Add test for alternative replacement code in checksums

checksum_update_insn() walks insn->alts after hashing the instruction
itself: the alternative's type, and where the replacement forms a group,
its feature number and every instruction in it.  A patch which edits only
the replacement -- code that runs on some CPUs and not others -- still has
to move the function's checksum.

When it does not, klp diff calls the function unchanged and leaves it out.
The patch ships the old replacement, and the bug is fixed only on machines
whose CPU takes the other arm.  Which machines those are depends on the
feature bit, so it presents as a machine-specific bug rather than a missing
patch.

insn->alts is built by objtool's check pass, not by the compiler, so the
pair goes through that first.  --mcount is the action used: it is the
cheapest one that does not also need --link.

Two of the three paths are isolated.  Skipping the alts walk and dropping
the feature hash both make this fail, and the second of those only exists
inside the alt_group branch, so reaching it proves the grouped path is
taken.  The alternative's type is hashed but not varied here -- the fixture
emits one kind of alternative -- so that line is covered without being
isolated, as is the in_alt recursion guard, which wants nested
alternatives.

Assisted-by: Claude:claude-opus-4
Based-on-test-by: Joe Lawrence <joe.lawrence@redhat.com>
Assisted-by: Claude:claude-opus-5
Signed-off-by: Song Liu <song@kernel.org>
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://patch.msgid.link/20260916184351.2720310-53-song@kernel.org

0 files changed, 0 insertions, 0 deletions