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

[Song Liu]

On Wed, Apr 22, 2026 at 9:04 PM Josh Poimboeuf <jpoimboe@xxxxxxxxxx> 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@xxxxxxxxxx>
> ---
[...]
> +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;
> +

[...]