linux-kernel - Re: [PATCH] kcsan: fix debugfs initcall return type

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Message-ID: <YJ8BS9fs5qrtQIzg@elver.google.com>
Date:   Sat, 15 May 2021 01:01:31 +0200
From:   Marco Elver <elver@...gle.com>
To:     Arnd Bergmann <arnd@...nel.org>
Cc:     "Paul E. McKenney" <paulmck@...nel.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        kasan-dev <kasan-dev@...glegroups.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH] kcsan: fix debugfs initcall return type

On Fri, May 14, 2021 at 11:16PM +0200, Arnd Bergmann wrote:
> On Fri, May 14, 2021 at 10:18 PM Paul E. McKenney <paulmck@...nel.org> wrote:
> > On Fri, May 14, 2021 at 01:11:05PM -0700, Nathan Chancellor wrote:
> 
> > > You can see my response to Marco here:
> > >
> > > https://lore.kernel.org/r/ad7fa126-f371-5a24-1d80-27fe8f655b05@kernel.org/
> > >
> > > Maybe some improved wording might look like
> > >
> > > clang with CONFIG_LTO_CLANG points out that an initcall function should
> > > return an 'int' due to the changes made to the initcall macros in commit
> > > 3578ad11f3fb ("init: lto: fix PREL32 relocations"):
> >
> > OK, so the naive reading was correct, thank you!
> >
> > > ...
> > >
> > > Arnd, do you have any objections?
> >
> > In the meantime, here is what I have.  Please let me know of any needed
> > updates.
> >
> 
> Looks good to me, thanks for the improvements!

FWIW, this prompted me to see if I can convince the compiler to complain
in all configs. The below is what I came up with and will send once the
fix here has landed. Need to check a few other config+arch combinations
(allyesconfig with gcc on x86_64 is good).

Thanks,
-- Marco

------ >8 ------

>From 96c1c4e9902e96485268909d5ea8f91b9595e187 Mon Sep 17 00:00:00 2001
From: Marco Elver <elver@...gle.com>
Date: Fri, 14 May 2021 21:08:50 +0200
Subject: [PATCH] init: verify that function is initcall_t at compile-time

In the spirit of making it hard to misuse an interface, add a
compile-time assertion in the CONFIG_HAVE_ARCH_PREL32_RELOCATIONS case
to verify the initcall function matches initcall_t, because the inline
asm bypasses any type-checking the compiler would otherwise do. This
will help developers catch incorrect API use in all configurations.

A recent example of this is:
https://lkml.kernel.org/r/20210514140015.2944744-1-arnd@kernel.org

Signed-off-by: Marco Elver <elver@...gle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Arnd Bergmann <arnd@...db.de>
Cc: Joe Perches <joe@...ches.com>
Cc: Masahiro Yamada <masahiroy@...nel.org>
Cc: Miguel Ojeda <ojeda@...nel.org>
Cc: Nathan Chancellor <nathan@...nel.org>
Cc: "Paul E. McKenney" <paulmck@...nel.org>
---
 include/linux/init.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/include/linux/init.h b/include/linux/init.h
index 045ad1650ed1..d82b4b2e1d25 100644
--- a/include/linux/init.h
+++ b/include/linux/init.h
@@ -242,7 +242,8 @@ extern bool initcall_debug;
 	asm(".section	\"" __sec "\", \"a\"		\n"	\
 	    __stringify(__name) ":			\n"	\
 	    ".long	" __stringify(__stub) " - .	\n"	\
-	    ".previous					\n");
+	    ".previous					\n");	\
+	static_assert(__same_type(initcall_t, &fn));
 #else
 #define ____define_initcall(fn, __unused, __name, __sec)	\
 	static initcall_t __name __used 			\
-- 
2.31.1.751.gd2f1c929bd-goog