[RFC PATCH] erofs/zmap.c: Bail out when no further region remains

[Gao Xiang]

On Mon, Nov 14, 2022 at 05:33:49PM +0530, Siddh Raman Pant wrote:
> The following calculation of iomap->length on line 798 in
> z_erofs_iomap_begin_report() can yield 0:
> 	if (iomap->offset >= inode->i_size)
> 		iomap->length = length + map.m_la - offset;
> 
> This triggers a WARN_ON in iomap_iter_done() (see line 34 of
> fs/iomap/iter.c).
> 
> Hence, return error when this scenario is encountered.
> 
> ============================================================
> 
> This was reported as a crash by syzbot under an issue about
> warning encountered in iomap_iter_done(), but unrelated to
> erofs. Hence, not adding issue hash in Reported-by line.
> 
> C reproducer: https://syzkaller.appspot.com/text?tag=ReproC&x=1037a6b2880000
> Kernel config: https://syzkaller.appspot.com/text?tag=KernelConfig&x=e2021a61197ebe02
> Dashboard link: https://syzkaller.appspot.com/bug?extid=a8e049cd3abd342936b6
> 
> Reported-by: syzbot at syzkaller.appspotmail.com
> Signed-off-by: Siddh Raman Pant <code at siddh.me>
> ---
>  fs/erofs/zmap.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/fs/erofs/zmap.c b/fs/erofs/zmap.c
> index 0bb66927e3d0..bad852983eb9 100644
> --- a/fs/erofs/zmap.c
> +++ b/fs/erofs/zmap.c
> @@ -796,6 +796,9 @@ static int z_erofs_iomap_begin_report(struct inode *inode, loff_t offset,
>  		 */
>  		if (iomap->offset >= inode->i_size)
>  			iomap->length = length + map.m_la - offset;
> +
> +		if (iomap->length == 0)

I just wonder if we should return -EINVAL for post-EOF cases or
IOMAP_HOLE with arbitrary length?

Thanks,
Gao Xiang

> +			return -EINVAL;
>  	}
>  	iomap->flags = 0;
>  	return 0;
> -- 
> 2.35.1
>