objtool/klp: Don't match local symbols against exports

While cloning a reloc, klp diff calls find_export() to determine whether
the referenced symbol is exported.  That decides whether the reference
needs a klp reloc, which object the klp symbol belongs to, and whether
the symbol's data needs to be copied into the patch module.

But find_export() matches purely on symbol name, so a static function or
variable which happens to share its name with an export is mistaken for
a reference to that export:

  - klp_reloc_needed() creates a klp reloc pointing at the exporting
    module's symbol rather than the local one.  For a vmlinux export it
    skips the klp reloc altogether, leaving a normal reloc which the
    module loader resolves to the vmlinux symbol.

  - clone_reloc() treats the symbol as external and clones it without
    its data, leaving a dangling reference.

  - validate_special_section_klp_reloc() attributes a static branch or
    call key to the wrong module, and for a vmlinux export skips the
    unsupported-key check entirely.

Exports are always global, so ignore local symbols in find_export().

Fixes: dd590d4d57 ("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/dafc7c017d1f62e2190a20dc84a52fbfc4c70210.1786138493.git.jpoimboe@kernel.org
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>

tools/objtool/klp-diff.c

@@ -1102,6 +1102,9 @@ static struct export *find_export(struct symbol *sym)
 {
 	struct export *export;
 
+	if (is_local_sym(sym))
+		return NULL;
+
 	hash_for_each_possible(exports, export, hash, str_hash(sym->name)) {
 		if (!strcmp(export->sym, sym->name))
 			return export;