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

[Siddh Raman Pant <code@siddh.me>]

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@syzkaller.appspotmail.com
Signed-off-by: Siddh Raman Pant <code@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)
+			return -EINVAL;
 	}
 	iomap->flags = 0;
 	return 0;
-- 
2.35.1