linux-kernel - [PATCH] objtool: check that module init/exit function is an indirect call target

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Message-Id: <20230118105215.B9DA960514@lion.mk-sys.cz>
Date:   Wed, 18 Jan 2023 11:52:15 +0100 (CET)
From:   Michal Kubecek <mkubecek@...e.cz>
To:     Josh Poimboeuf <jpoimboe@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org
Subject: [PATCH] objtool: check that module init/exit function is an indirect
 call target

Some out-of-tree modules still do not use module_init() / module_exit()
macros and simply create functions with magic names init_module() and
cleanup_module() instead. As a result, these functions are not recognized
as indirect call targets by objtool and such module fails to load into an
IBT enabled kernel.

This old way is not even documented any more but it is cleaner to issue
a warning than to let the module fail on load without obvious reason.

Signed-off-by: Michal Kubecek <mkubecek@...e.cz>
---
 tools/objtool/Documentation/objtool.txt | 8 ++++++++
 tools/objtool/check.c                   | 7 +++++++
 2 files changed, 15 insertions(+)

diff --git a/tools/objtool/Documentation/objtool.txt b/tools/objtool/Documentation/objtool.txt
index 8a671902a187..8e53fc6735ef 100644
--- a/tools/objtool/Documentation/objtool.txt
+++ b/tools/objtool/Documentation/objtool.txt
@@ -410,6 +410,14 @@ the objtool maintainers.
    can remove this warning by putting the ANNOTATE_INTRA_FUNCTION_CALL
    directive right before the call.
 
+12. file.o: warning: func(): not an indirect call target
+
+   This means that objtool is running with --ibt and a function expected
+   to be an indirect call target is not. In particular, this happens for
+   init_module() or cleanup_module() if a module relies on these special
+   names and does not use module_init() / module_exit() macros to create
+   them.
+
 
 If the error doesn't seem to make sense, it could be a bug in objtool.
 Feel free to ask the objtool maintainer for help.
diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index 4b7c8b33069e..0afa4f0ffa67 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -854,8 +854,15 @@ static int create_ibt_endbr_seal_sections(struct objtool_file *file)
 	list_for_each_entry(insn, &file->endbr_list, call_node) {
 
 		int *site = (int *)sec->data->d_buf + idx;
+		struct symbol *sym = insn->sym;
 		*site = 0;
 
+		if (opts.module && sym && sym->type == STT_FUNC &&
+		    insn->offset == sym->offset &&
+		    (!strcmp(sym->name, "init_module") ||
+		     !strcmp(sym->name, "cleanup_module")))
+			WARN("%s(): not an indirect call target", sym->name);
+
 		if (elf_add_reloc_to_insn(file->elf, sec,
 					  idx * sizeof(int),
 					  R_X86_64_PC32,
-- 
2.38.1