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

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Date:   Tue, 27 Oct 2020 10:44:50 +0000
From:   Christoph Hellwig <hch@...radead.org>
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>,
        Nathan Chancellor <natechancellor@...il.com>,
        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

> 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;

Yikes.  This is just horrible, why bnot the completely obvious:

	if (ops->seq_start) {
		...
		return next;
	}

	if (*ppos)
		return NULL;
	return ppos; /* random cookie */

>  static void *single_start(struct seq_file *p, loff_t *pos)
>  {
> -	return NULL + (*pos == 0);
> +	return (void *)(uintptr_t)(*pos == 0);

Same here.