objtool: skip non-text sections when adding return-thunk sites

Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2090231
CVE: CVE-2022-23816
CVE: CVE-2022-23825
CVE: CVE-2022-29900
CVE: CVE-2022-29901
Upstream Status: RHEL only

The .discard.text section is added in order to reserve BRK, with a
temporary function just so it can give it a size. This adds a relocation to
the return thunk, which objtool will add to the .return_sites section.
Linking will then fail as there are references to the .discard.text
section.

Do not add instructions from non-text sections to the list of return thunk
calls, avoiding the reference to .discard.text.

Originally-from: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
Suggested-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Waiman Long <longman@redhat.com>

Author: Waiman-Long

tools/objtool/check.c

@@ -1231,7 +1231,8 @@ static void add_return_call(struct objtool_file *file, struct instruction *insn,
 	insn->type = INSN_RETURN;
 	insn->retpoline_safe = true;
 
-	if (add)
+	/* Skip the non-text sections, specially .discard ones */
+	if (add && insn->sec->text)
 		list_add_tail(&insn->call_node, &file->return_thunk_list);
 }