Re: [PATCH V2] block: allow for_each_bvec to support zero len bvec

[Ming Lei <ming.lei@redhat.com>]

On Mon, Aug 17, 2020 at 06:00:55PM +0800, Ming Lei wrote:
> Block layer usually doesn't support or allow zero-length bvec. Since
> commit 1bdc76aea115 ("iov_iter: use bvec iterator to implement
> iterate_bvec()"), iterate_bvec() switches to bvec iterator. However,
> Al mentioned that 'Zero-length segments are not disallowed' in iov_iter.
> 
> Fixes for_each_bvec() so that it can move on after seeing one zero
> length bvec.
> 
> Signed-off-by: Ming Lei <ming.lei@redhat.com>
> Link: https://www.mail-archive.com/linux-kernel@vger.kernel.org/msg2262077.html
> Fixes: 1bdc76aea115 ("iov_iter: use bvec iterator to implement iterate_bvec()")
> Reported-by: syzbot <syzbot+61acc40a49a3e46e25ea@syzkaller.appspotmail.com>
> Tested-by: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
> Cc: Al Viro <viro@zeniv.linux.org.uk>
> Cc: Matthew Wilcox <willy@infradead.org>
> Cc: <stable@vger.kernel.org>
> ---
> V2:
> 	- fix reported-by tag
> 
>  include/linux/bvec.h | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/bvec.h b/include/linux/bvec.h
> index ac0c7299d5b8..9c4fab5f22a7 100644
> --- a/include/linux/bvec.h
> +++ b/include/linux/bvec.h
> @@ -117,11 +117,18 @@ static inline bool bvec_iter_advance(const struct bio_vec *bv,
>  	return true;
>  }
>  
> +static inline void bvec_iter_skip_zero_bvec(struct bvec_iter *iter)
> +{
> +	iter->bi_bvec_done = 0;
> +	iter->bi_idx++;
> +}
> +
>  #define for_each_bvec(bvl, bio_vec, iter, start)			\
>  	for (iter = (start);						\
>  	     (iter).bi_size &&						\
>  		((bvl = bvec_iter_bvec((bio_vec), (iter))), 1);	\
> -	     bvec_iter_advance((bio_vec), &(iter), (bvl).bv_len))
> +	     (bvl).bv_len ? bvec_iter_advance((bio_vec), &(iter),	\
> +		     (bvl).bv_len) : bvec_iter_skip_zero_bvec(&(iter)))
>  
>  /* for iterating one bio from start to end */
>  #define BVEC_ITER_ALL_INIT (struct bvec_iter)				\
> -- 
> 2.25.2
> 

Hello Jens,

Looks at least two reports can be fixed by this patch, so could you
take a look?

Thanks,
Ming