linux-kernel - Re: [PATCH] kernel.h: fix new warnings for container

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Date:   Wed, 21 Jun 2017 10:25:27 +0100
From:   Ian Abbott <abbotti@....co.uk>
To:     Arnd Bergmann <arnd@...db.de>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     Michal Nazarewicz <mina86@...a86.com>,
        Kees Cook <keescook@...omium.org>,
        Borislav Petkov <bp@...e.de>,
        Johannes Berg <johannes.berg@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Alexander Potapenko <glider@...gle.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] kernel.h: fix new warnings for container_of()

On 20/06/17 21:09, Arnd Bergmann wrote:
> I see new warnings with gcc-7.0.1 with the modified container_of():
>
> fs/f2fs/dir.c: In function 'F2FS_I':
> fs/f2fs/f2fs.h:1122:385: note: found mismatched ssa struct pointer types: 'struct f2fs_inode_info' and 'struct inode'

Is that actually a warning, or just informational?  In any case, it 
seems like a good idea to avoid it.

>
> This seems to happen for all structures that have a zero offset
> between the member and the container structure, i.e. idential
> pointers.
>
> Reverting to an intermediate pointer avoids the warning, and using
> a void pointer instead of the target type should also avoid
> regressing on the previous patch again.
>
> Fixes: mmotm ("kernel.h: handle pointers to arrays better in container_of()")
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> ---
>  include/linux/kernel.h | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/kernel.h b/include/linux/kernel.h
> index d043adadcf33..bd6d96cf80b1 100644
> --- a/include/linux/kernel.h
> +++ b/include/linux/kernel.h
> @@ -856,10 +856,11 @@ static inline void ftrace_dump(enum ftrace_dump_mode oops_dump_mode) { }
>   *
>   */
>  #define container_of(ptr, type, member) ({				\
> +	void *__mptr = (void *)(ptr);					\
>  	BUILD_BUG_ON_MSG(!__same_type(*(ptr), ((type *)0)->member) &&	\
>  			 !__same_type(*(ptr), void),			\
>  			 "pointer type mismatch in container_of()");	\
> -	((type *)((char *)(ptr) - offsetof(type, member))); })
> +	((type *)(__mptr - offsetof(type, member))); })
>
>  /* Rebuild everything on CONFIG_FTRACE_MCOUNT_RECORD */
>  #ifdef CONFIG_FTRACE_MCOUNT_RECORD
>

Acked-by: Ian Abbott <abbotti@....co.uk>

-- 
-=( Ian Abbott @ MEV Ltd.    E-mail: <abbotti@....co.uk> )=-
-=(                          Web: http://www.mev.co.uk/  )=-