Re: [PATCH] seq_file: fix NULL pointer arithmetic warning

[Al Viro <viro@zeniv.linux.org.uk>]

On Sun, Jan 30, 2022 at 12:27:00PM -0300, Maíra Canal wrote:
> Implement conditional logic in order to replace NULL pointer arithmetic.
> 
> The use of NULL pointer arithmetic was pointed out by clang with the
> following warning:
> 
> fs/kernfs/file.c:128:15: warning: performing pointer arithmetic on a
> null pointer has undefined behavior [-Wnull-pointer-arithmetic]
>                 return NULL + !*ppos;
>                        ~~~~ ^
> fs/seq_file.c:559:14: warning: performing pointer arithmetic on a
> null pointer has undefined behavior [-Wnull-pointer-arithmetic]
>         return NULL + (*pos == 0);

NAK.  _If_ you want to bother with that at all, at least make it
use SEQ_START_TOKEN, rather than open-coding it; what's more,
kernfs_seq_start() should simply call single_start() instead of
open-coding it.  I.e.
	if (ops->seq_start) {
		void *next = ops->seq_start(sf, ppos);
		/* see the comment above kernfs_seq_stop_active() */
		if (next == ERR_PTR(-ENODEV))
			kernfs_seq_stop_active(sf, next);
		return next;
	} else {
		return single_start(sf, ppos);
	}
and
	return *ppos ? NULL : SEQ_START_TOKEN;
in single_start() itself.