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

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.

Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
Acked-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: Thadeu Lima de Souza Cascardo

tools/objtool/check.c

@@ -1308,7 +1308,9 @@ static void add_return_call(struct objtool_file *file, struct instruction *insn)
 	insn->type = INSN_RETURN;
 	insn->retpoline_safe = true;
 
-	list_add_tail(&insn->call_node, &file->return_thunk_list);
+	/* Skip the non-text sections, specially .discard ones */
+	if (insn->sec->text)
+		list_add_tail(&insn->call_node, &file->return_thunk_list);
 }
 
 static bool same_function(struct instruction *insn1, struct instruction *insn2)