Commit 6acd87d5 authored Dec 09, 2022 by Siddh Raman Pant Committed by Gao Xiang Jan 10, 2023

erofs/zmap.c: Fix incorrect offset calculation

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@syzkaller.appspotmail.com>
Suggested-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Signed-off-by: Siddh Raman Pant <code@siddh.me>
Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Link: https://lore.kernel.org/r/20221209102151.311049-1-code@siddh.me


Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>

parent 88603b6d

fs/erofs/zmap.c

+7 −3

Original line number	Diff line number	Diff line
		@@ -793,12 +793,16 @@ 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;