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Date:   Tue, 27 Oct 2020 10:44:50 +0000
From:   Christoph Hellwig <hch@infradead.org>
To:     Arnd Bergmann <arnd@kernel.org>
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Tejun Heo <tj@kernel.org>,
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Subject: Re: [PATCH] seq_file: fix clang warning for NULL pointer arithmetic
Message-ID: <20201027104450.GA8864@infradead.org>
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> 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.