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

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Date:   Fri, 14 May 2021 11:40:22 -0700
From:   Nathan Chancellor <nathan@...nel.org>
To:     Marco Elver <elver@...gle.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Paul E. McKenney" <paulmck@...nel.org>
Cc:     Arnd Bergmann <arnd@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Arnd Bergmann <arnd@...db.de>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        kasan-dev <kasan-dev@...glegroups.com>,
        LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH] kcsan: fix debugfs initcall return type

On 5/14/2021 7:45 AM, Marco Elver wrote:
> On Fri, 14 May 2021 at 16:10, Greg Kroah-Hartman
> <gregkh@...uxfoundation.org> wrote:
>> On Fri, May 14, 2021 at 04:00:08PM +0200, Arnd Bergmann wrote:
>>> From: Arnd Bergmann <arnd@...db.de>
>>>
>>> clang points out that an initcall funciton should return an 'int':
>>>
>>> kernel/kcsan/debugfs.c:274:15: error: returning 'void' from a function with incompatible result type 'int'
>>> late_initcall(kcsan_debugfs_init);
>>> ~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~
>>> include/linux/init.h:292:46: note: expanded from macro 'late_initcall'
>>>   #define late_initcall(fn)               __define_initcall(fn, 7)
>>>
>>> Fixes: e36299efe7d7 ("kcsan, debugfs: Move debugfs file creation out of early init")
>>> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> [...]
>>>
>> Reviewed-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> 
> Reviewed-by: Marco Elver <elver@...gle.com>
> 
> Thanks for catching this -- it boggles my mind why gcc nor clang
> wouldn't warn about this by default...
> Is this a new clang?

KCSAN appears to only support x86_64, which also selects 
HAVE_ARCH_PREL32_RELOCATIONS, meaning that the initcalls never have 
their types validated because there is no assignment:

https://elixir.bootlin.com/linux/v5.12.4/source/include/linux/init.h#L240

In the case of CONFIG_LTO_CLANG, the initcall function is called in the 
stub function, resulting in the error that we see here.

Hopefully that makes sense :)

Cheers,
Nathan