Re: [PATCH v2 49/53] objtool/klp: Fix position-dependent checksums for non-relocated jumps/calls

[Song Liu]

On Fri, May 1, 2026 at 5:09 AM Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
>
> When computing klp checksums, instructions with non-relocated jump/call
> destination offsets are problematic because the offset values can change
> when surrounding code has moved, causing the function to be incorrectly
> marked as changed.
>
> Specifically, that includes jumps from alternatives to the end of the
> alternative, which from objtool's perspective are jumps to the end of
> the alternative instruction block in the original function.
>
> Note that 'jump_dest' jumps don't include sibling calls (those use
> call_dest), nor do they include jumps to/from .cold sub functions (those
> are cross-section and need a reloc).
>
> Fix it by hashing the opcode bytes (excluding the immediate operand)
> along with a position-independent representation of the destination.
> For calls, use the function name, and for jumps, use the destination's
> offset within its function.
>
> [Note the "9 bit hole" comment was wrong: it has been 8 bits since
> commit 70589843b36f ("objtool: Add option to trace function validation")
> added the 'trace' field.  Adding the 4-bit 'immediate_len' field now
> leaves a 4-bit hole.]
>
> Fixes: 0d83da43b1e1 ("objtool/klp: Add --checksum option to generate per-function checksums")
> Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>

Acked-by: Song Liu <song@xxxxxxxxxx>