Re: [PATCH 41/48] objtool/klp: Rewrite symbol correlation algorithm

Song Liu
2026-04-25T00:53:59Z

On Wed, Apr 22, 2026 at 9:04 PM Josh Poimboeuf <jpoimboe@kernel.org> wrote:
>
> Rewrite the symbol correlation code, using a tiered list of
> deterministic strategies in a loop. For duplicately named symbols, each
> tier applies a filter with the goal of finding a 1:1 deterministic
> correlation between the original and patched version of the symbol.
>
> Overall this works much better than the existing algorithm, particularly
> with LTO kernels.

I found it is hard to follow all the matching algorithms here. Could you
please add some examples for each case: different levels in find_twin(),
match in find_twin_suffixed(), and match in find_twin_positional()?

Also a few nitpicks below.

> Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
> ---
[...]
> +static struct symbol *find_twin(struct elfs *e, struct symbol *sym1)
> +{
> + struct symbol *name_last = NULL, *scope_last = NULL,
> + *file_last = NULL, *csum_last = NULL;
> + unsigned int name_orig = 0, name_patched = 0;
> + unsigned int scope_orig = 0, scope_patched = 0;
> + unsigned int file_orig = 0, file_patched = 0;
> + unsigned int csum_orig = 0, csum_patched = 0;
> + struct symbol *sym2, *match = NULL;
> +
> + /* Count orig candidates */
> + for_each_sym_by_demangled_name(e->orig, sym1->demangled_name, sym2) {
> + if (sym2->twin || sym1->type != sym2->type || dont_correlate(sym2) ||
> + (!maybe_same_file(sym1, sym2)))
> continue;
>
> - count++;
> - result = sym2;
> + /* Level 1: name match (widest filter) */
> + name_orig++;
> +
> + /* Level 2: scope (scope changes allowed) */
> + if (is_tu_local_sym(sym1) != is_tu_local_sym(sym2))

is_tu_local_sym(sym1) is called many times, shall we add a variable
for it?

> + continue;
> + scope_orig++;
> +
> + /* Level 3: file (scope changes disallowed) */
> + if (!same_file(sym1, sym2))
> + continue;
> + file_orig++;
> +
> + /* Level 4: checksum (unchanged symbols) */
> + if (sym1->len != sym2->len || !sym1->csum.checksum ||
> + sym1->csum.checksum != sym2->csum.checksum)
> + continue;
> + csum_orig++;
> }
>
> - if (count > 1) {
> - ERROR("Multiple (%d) correlation candidates for %s", count, sym->name);
> - return -1;
> + /* Count patched candidates */
> + for_each_sym_by_demangled_name(e->patched, sym1->demangled_name, sym2) {
> + if (sym2->twin || sym1->type != sym2->type || dont_correlate(sym2))
> + continue;
> +
> + /* Level 1 */
> + if (!maybe_same_file(sym1, sym2))
> + continue;

This for_each_sym_by_demangled_name() has two "if () continue" while the
first one has one. Maybe keep them the same (just for symmetry)?

Thanks,
Song

> + name_patched++;
> + name_last = sym2;
> + …