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From:   Siddh Raman Pant <code@siddh.me>
To:     Gao Xiang <hsiangkao@linux.alibaba.com>, Chao Yu <chao@kernel.org>,
        Yue Hu <huyue2@coolpad.com>,
        Jeffle Xu <jefflexu@linux.alibaba.com>
Cc:     linux-erofs <linux-erofs@lists.ozlabs.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        syzbot+a8e049cd3abd342936b6@syzkaller.appspotmail.com
Message-ID: <20221209102151.311049-1-code@siddh.me>
Subject: [PATCH v2] erofs/zmap.c: Fix incorrect offset calculation
Date:   Fri,  9 Dec 2022 15:51:51 +0530
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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=3DReproC&x=3D1037a6b28=
80000
Kernel config: https://syzkaller.appspot.com/text?tag=3DKernelConfig&x=3De2=
021a61197ebe02
Dashboard link: https://syzkaller.appspot.com/bug?extid=3Da8e049cd3abd34293=
6b6

Reported-by: syzbot+a8e049cd3abd342936b6@syzkaller.appspotmail.com
Suggested-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Signed-off-by: Siddh Raman Pant <code@siddh.me>
---
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 *i=
node, loff_t offset,
 =09=09iomap->type =3D IOMAP_HOLE;
 =09=09iomap->addr =3D IOMAP_NULL_ADDR;
 =09=09/*
-=09=09 * No strict rule how to describe extents for post EOF, yet
-=09=09 * we need do like below. Otherwise, iomap itself will get
+=09=09 * No strict rule on how to describe extents for post EOF, yet
+=09=09 * we need to do like below. Otherwise, iomap itself will get
 =09=09 * into an endless loop on post EOF.
+=09=09 *
+=09=09 * Calculate the effective offset by subtracting extent start
+=09=09 * (map.m_la) from the requested offset, and add it to length.
+=09=09 * (NB: offset >=3D map.m_la always)
 =09=09 */
 =09=09if (iomap->offset >=3D inode->i_size)
-=09=09=09iomap->length =3D length + map.m_la - offset;
+=09=09=09iomap->length =3D length + offset - map.m_la;
 =09}
 =09iomap->flags =3D 0;
 =09return 0;
--=20
2.35.1