linux-kernel - Re: [PATCH] seq_file: fix clang warning for NULL pointer arithmetic

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Date:   Mon, 26 Oct 2020 18:49:59 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Arnd Bergmann <arnd@...nel.org>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Tejun Heo <tj@...nel.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Arnd Bergmann <arnd@...db.de>,
        Amir Goldstein <amir73il@...il.com>, Jan Kara <jack@...e.cz>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        clang-built-linux@...glegroups.com
Subject: Re: [PATCH] seq_file: fix clang warning for NULL pointer arithmetic

On Mon, Oct 26, 2020 at 10:52:56PM +0100, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@...db.de>
> 
> Clang points out that adding something to NULL is notallowed
> in standard C:
> 
> fs/kernfs/file.c:127:15: warning: performing pointer arithmetic on a
> null pointer has undefined behavior [-Wnull-pointer-arithmetic]
>                 return NULL + !*ppos;
>                        ~~~~ ^
> fs/seq_file.c:529:14: warning: performing pointer arithmetic on a
> null pointer has undefined behavior [-Wnull-pointer-arithmetic]
>         return NULL + (*pos == 0);
> 
> Rephrase the function to do the same thing without triggering that
> warning. Linux already relies on a specific binary representation
> of NULL, so it makes no real difference here. The instance in
> kernfs was copied from single_start, so fix both at once.
> 
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> Fixes: c2b19daf6760 ("sysfs, kernfs: prepare read path for kernfs")
> Signed-off-by: Arnd Bergmann <arnd@...db.de>

Reviewed-by: Nathan Chancellor <natechancellor@...il.com>

> ---
>  fs/kernfs/file.c | 2 +-
>  fs/seq_file.c    | 2 +-
>  2 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/kernfs/file.c b/fs/kernfs/file.c
> index f277d023ebcd..b55e6ef4d677 100644
> --- a/fs/kernfs/file.c
> +++ b/fs/kernfs/file.c
> @@ -124,7 +124,7 @@ static void *kernfs_seq_start(struct seq_file *sf, loff_t *ppos)
>  		 * The same behavior and code as single_open().  Returns
>  		 * !NULL if pos is at the beginning; otherwise, NULL.
>  		 */
> -		return NULL + !*ppos;
> +		return (void *)(uintptr_t)!*ppos;
>  	}
>  }
>  
> diff --git a/fs/seq_file.c b/fs/seq_file.c
> index 31219c1db17d..d456468eb934 100644
> --- a/fs/seq_file.c
> +++ b/fs/seq_file.c
> @@ -526,7 +526,7 @@ EXPORT_SYMBOL(seq_dentry);
>  
>  static void *single_start(struct seq_file *p, loff_t *pos)
>  {
> -	return NULL + (*pos == 0);
> +	return (void *)(uintptr_t)(*pos == 0);
>  }
>  
>  static void *single_next(struct seq_file *p, void *v, loff_t *pos)
> -- 
> 2.27.0
>