objtool/klp: Ignore replacement offset of empty x86 alternatives

An x86 alternative with an empty replacement, e.g. the second entry of

  ALTERNATIVE_2("orig", "repl", ft1, "", ft2)

has a replacementlen of zero.  Its replacement offset still gets a
relocation, but the label it points at is the end of the previous
replacement, which is also the beginning of the *next* alternative's
replacement.  The value is meaningless; get_alt_entry() already ignores
it for that reason.

klp diff doesn't ignore it.  When such an alternative belongs to a
changed function, cloning its relocations drags in the unrelated
neighboring replacement, along with everything that replacement
references.  On an x86 clang/lto build an empty alternative in
meminfo_proc_show() pulled in the replacement of an alternative in
proc_kcore_init(), silently emitting a klp relocation against init text
which has long since been freed by the time the patch is applied.

Add arch_alt_ignore_new_reloc() and skip such relocations when cloning.
This has to be arch specific: on arm64 a zero-length replacement instead
identifies an alternative callback, whose replacement offset points at
the callback function and must be preserved.

Fixes: dd590d4d57eb ("objtool/klp: Introduce klp diff subcommand for diffing object files")
Acked-by: Song Liu <song@kernel.org>
Acked-by: Joe Lawrence <joe.lawrence@redhat.com>
Link: https://patch.msgid.link/7a885b70974795c3417f3358869e62aafd4ef783.1786138493.git.jpoimboe@kernel.org
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>

0 files changed, 0 insertions, 0 deletions