[PATCH v2] erofs/zmap.c: Fix incorrect offset calculation

[Gao Xiang]

On Fri, Dec 09, 2022 at 03:51:51PM +0530, Siddh Raman Pant via Linux-erofs wrote:
> Effective offset to add to length was being incorrectly calculated,
> which resulted in iomap->length being set to 0, triggering a WARN_ON
> in iomap_iter_done().
> 
> Fix that, and describe it in comments.
> 
> This was reported as a crash by syzbot under an issue about a warning
> encountered in iomap_iter_done(), but unrelated to erofs.
> 
> 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+a8e049cd3abd342936b6 at syzkaller.appspotmail.com
> Suggested-by: Gao Xiang <hsiangkao at linux.alibaba.com>
> Signed-off-by: Siddh Raman Pant <code at siddh.me>

It looks good to me!

Reviewed-by: Gao Xiang <hsiangkao at linux.alibaba.com>

Thanks,
Gao Xiang

> ---
> Changes since v2:
> - Fix the calculation instead of bailing out.
> 
>  fs/erofs/zmap.c | 10 +++++++---
>  1 file changed, 7 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/erofs/zmap.c b/fs/erofs/zmap.c
> index 0bb66927e3d0..a171e4caba3c 100644
> --- a/fs/erofs/zmap.c
> +++ b/fs/erofs/zmap.c
> @@ -790,12 +790,16 @@ static int z_erofs_iomap_begin_report(struct inode *inode, loff_t offset,
>  		iomap->type = IOMAP_HOLE;
>  		iomap->addr = IOMAP_NULL_ADDR;
>  		/*
> -		 * No strict rule how to describe extents for post EOF, yet
> -		 * we need do like below. Otherwise, iomap itself will get
> +		 * No strict rule on how to describe extents for post EOF, yet
> +		 * we need to do like below. Otherwise, iomap itself will get
>  		 * into an endless loop on post EOF.
> +		 *
> +		 * Calculate the effective offset by subtracting extent start
> +		 * (map.m_la) from the requested offset, and add it to length.
> +		 * (NB: offset >= map.m_la always)
>  		 */
>  		if (iomap->offset >= inode->i_size)
> -			iomap->length = length + map.m_la - offset;
> +			iomap->length = length + offset - map.m_la;
>  	}
>  	iomap->flags = 0;
>  	return 0;
> -- 
> 2.35.1
> 
>