-------- Original Message --------
Subject: btrfs check segfaults after flipping 2 Bytes
From: Niklas Fischer <nik...@niklasfi.de>
To: <linux-btrfs@vger.kernel.org>
Date: 2014年10月02日 04:29
Hello,
I was trying to determine how btrfs reacts to disk errors, when I
discovered, that flipping two Bytes, supposedly inside of a file can
render the filesystem unusable. Here is what I did:
1. dd if=/dev/zero of=/dev/sdg2 bs=1M
2. mkfs.btrfs /dev/sdg2
3. mount /dev/sdg2 /tmp/btrfs
4. echo "hello world this is some text" > /tmp/btrfs/hello
5. umount /dev/sdg2
this should result in this image [1]
in the following steps /dev/sdg2 is altered. Basically, we want to apply
s/world/wirld/g
6. dd if=/dev/sdg2 of=/tmp/sdg2 bs=1M
7. edit /tmp/sdg2 in ghex, find & replace all occurences of "world" with
"wirld" (two occurences found), save as /tmp/sdg2_new
2 problems here.
[1] csum mismatch
As already mentioned by Ducan and Brendan, the csum does not match.
What makes thing much worse, since small file's extent is inlined, the
data is stored in metadata tree blocks,
and the file system is almost empty so the inline extent lies in the
*root* leaf of fs_tree.
These two unfortunate facts makes the whole fs_tree corrupted(only one
leaf, and its cusm dismatch),
which cause btrfs-progs segfault.
The good news is that, the bug in btrfs-progs is already fixed by Wang's
patch:
https://patchwork.kernel.org/patch/4254631/
So at least, btrfs-progs will not segfault anymore.
[2] two occurences?
So you definitely changed something you should not touch... maybe
another tree root?
Anyway, almost everything in btrfs is csums, so any manual modification
may cause problem.
8. dd if=/tmp/sdg2_new of=/dev/sdg2 bs=1M
/dev/sdg2 now looks like this [2]
now, when I try mounting /dev/sdg2:
"""22:21 root@localhost ~#
LANG=EN mount /dev/sdg2 /tmp/btrfs/
mount: mount /dev/sdg2 on /tmp/btrfs failed: Cannot allocate memory"""
ok, so let us try btrfs check
"""22:21 root@localhost ~#
LANG=EN btrfs check /dev/sdg2
checksum verify failed on 4222976 found F22E71BD wanted CE334502
checksum verify failed on 4222976 found F22E71BD wanted CE334502
Csum didn't match
Checking filesystem on /dev/sdg2
UUID: ea2843f1-e8aa-420f-acf8-6ef13f6c6753
checking extents
Speicherzugriffsfehler (Speicherabzug geschrieben)"""
that last line reads "Segfault, memory dumped". Is this behaviour expected?
Kind regards,
Niklas
[1] tgz: https://gigamove.rz.rwth-aachen.de/d/id/R5CKikxbYTBdqs
raw: https://gigamove.rz.rwth-aachen.de/d/id/VgMXAicS7CGEjt
[2] tgz: https://gigamove.rz.rwth-aachen.de/d/id/cEebNYne9ppFy4
raw: https://gigamove.rz.rwth-aachen.de/d/id/ATJa7wpfEWdfL2
BTW, [2]'s tgz is in fact raw, and raw is tgz....
Thanks,
Qu
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