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From: Christoph Anton Mitterer <>
To: Zygo Blaxell <>
Cc: linux-btrfs <>
Subject: Re: Reproducer for "compressed data + hole data corruption bug, 2018 edition" still works on 4.20.7
Date: Mon, 04 Mar 2019 16:34:39 +0100
Message-ID: <> (raw)
In-Reply-To: <>


Thanks for your elaborate explanations :-)

On Fri, 2019-02-15 at 00:40 -0500, Zygo Blaxell wrote:
> The problem occurs only on reads.  Data that is written to disk will
> be OK, and can be read correctly by a fixed kernel.
> A kernel without the fix will give corrupt data on reads with no
> indication of corruption other than the changes to the data itself.
> Applications that copy data may read corrupted data and write it back
> to the filesystem.  This will make the corruption permanent in the
> copied data.

So that basically means even a cp (without refcopy) or a btrfs
send/receive could already cause permanent silent data corruption.
Of course, only if the conditions you've described below are met.

> Given the age of the bug

Since when was it in the kernel?

> Even
> if
> compression is enabled, the file data must be compressed for the bug
> to
> corrupt it.

Is there a simple way to find files (i.e. pathnames) that were actually

> 	- you never punch holes in files

Is there any "standard application" (like cp, tar, etc.) that would do

> 	- you never dedupe or clone files

What do you mean by clone? refcopy? Would btrfs snapshots or btrfs
send/receive be affected?

Or is there anything in btrfs itself which does any of the two per
default or on a typical system (i.e. I didn't use dedupe).

Also, did the bug only affect data, or could metadata also be
affected... basically should such filesystems be re-created since they
may also hold corruptions in the meta-data like trees and so on?

> > compression),... or only when specific file operations were done (I
> > did
> > e.g. cp with refcopy, but I think none of the standard tools does
> > hole-
> > punching)?
> That depends on whether you consider fallocate or qemu to be standard
> tools.

I assume you mean the fallocate(1) program,... cause I wouldn't know
whether any of cp/mv/etc. does the system call fallocate(2) per

My scenario looks about the following, and given your explanations, I'd
assume I should probably be safe:

- my normal laptop doesn't use compress, so it's safe anyway

- my cp has an alias to always have --reflink=auto

- two 8TB data archive disks, each with two backup disks to which the
  data of the two master disks is btrfs sent/received,... which were
  all mounted with compress

- typically I either cp or mv data from the laptop to these disks,
  => should then be safe as the laptop fs didn't use compress,...

- or I directly create the files on the data disks (which use compress)
  by means of wget, scp or similar from other sources
  => should be safe, too, as they probably don't do dedupe/hole
     punching by default

- or I cp/mv from them camera SD cards, which use some *FAT
  => so again I'd expect that to be fine

- on vacation I had the case that I put large amount of picture/videos
  from SD cards to some btrfs-with-compress mobile HDDs, and back home
  from these HDDs to my actual data HDDs.
  => here I do have the read / re-write pattern, so data could have
     been corrupted if it was compressed + deduped/hole-punched
     I'd guess that's anyway not the case (JPEGs/MPEGs don't compress
     well)... and AFAIU there would be no deduping/hole-punching 
     involved here

- on my main data disks, I do snapshots... and these snapshots I 
  send/receive to the other (also compress-mounted) btrfs disks.
  => could these operations involve deduping/hole-punching and thus the

Another thing:
I always store SHA512 hashsums of files as an XATTR of them (like
"directly after" creating such files).
I assume there would be no deduping/hole-punching involved till then,
so the sums should be from correct data, right?

But when I e.g. copy data from SD, to mobile btrfs-HDD and then to the
final archive HDD... corruption could in principle occur when copying
from mobile HDD to archive HDD.
In that case, would a diff between the two show me the corruption? I
guess not because the diff would likely get the same corruption on

> "Ordinary" sparse files (made by seeking forward while writing, as
> done
> by older Unix utilities including cp, tar, rsync, cpio, binutils) do
> not
> trigger this bug.  An ordinary sparse file has two distinct data
> extents
> from two different writes separated by a hole which has never
> contained
> file data.  A punched hole splits an existing single data extent into
> two
> pieces with a newly created hole between them that replaces
> previously
> existing file data.  These actions create different extent reference
> patterns and only the hole-punching one is affected by the bug.
> Files that contain no blocks full of zeros will not be affected by
> fallocate-d-style hole punching (it searches for existing zeros and
> punches holes over them--no zeros, no holes).  If the the hole
> punching
> intentionally introduces zeros where zeros did not exist before (e.g.
> qemu
> discard operations on raw image files) then it may trigger the bug.

So long story short, "normal" file operations (cp/mv, etc.) should not
trigger the bug.

qemu with discard would be a prominent example of triggering the bug,
but luckily for me, I only use this on an fs with compress disabled :-D
Any other such prominent examples?

I assume normal mv of refcopy (i.e. cp --reflink=auto) would not punch
holes and thus be not affected?

Further, I'd assume XATTRs couldn't be affected?

So what remains unanswered is send/receive:

> btrfs send and receive may be affected, but I don't use them so I
> don't
> have any experience of the bug related to these tools.  It seems from
> reading the btrfs receive code that it lacks any code capable of
> punching
> a hole, but I'm only doing a quick search for words like "punch", not
> a detailed code analysis.

Is there some other developer who possibly knows whether send/receive
would have been vulnerable to the issue?

But since I use send/receive anyway in just one direction from the
master to the backup disks... only the later could be affected.


  reply index

Thread overview: 38+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-08-23  3:11 Reproducer for "compressed data + hole data corruption bug, 2018 editiion" Zygo Blaxell
2018-08-23  5:10 ` Qu Wenruo
2018-08-23 16:44   ` Zygo Blaxell
2018-08-23 23:50     ` Qu Wenruo
2019-02-12  3:09 ` Reproducer for "compressed data + hole data corruption bug, 2018 edition" still works on 4.20.7 Zygo Blaxell
2019-02-12 15:33   ` Christoph Anton Mitterer
2019-02-12 15:35   ` Filipe Manana
2019-02-12 17:01     ` Zygo Blaxell
2019-02-12 17:56       ` Filipe Manana
2019-02-12 18:13         ` Zygo Blaxell
2019-02-13  7:24           ` Qu Wenruo
2019-02-13 17:36           ` Filipe Manana
2019-02-13 18:14             ` Filipe Manana
2019-02-14  1:22               ` Filipe Manana
2019-02-14  5:00                 ` Zygo Blaxell
2019-02-14 12:21                 ` Christoph Anton Mitterer
2019-02-15  5:40                   ` Zygo Blaxell
2019-03-04 15:34                     ` Christoph Anton Mitterer [this message]
2019-03-07 20:07                       ` Zygo Blaxell
2019-03-08 10:37                         ` Filipe Manana
2019-03-14 18:58                           ` Christoph Anton Mitterer
2019-03-14 20:22                           ` Christoph Anton Mitterer
2019-03-14 22:39                             ` Filipe Manana
2019-03-08 12:20                         ` Austin S. Hemmelgarn
2019-03-14 18:58                           ` Christoph Anton Mitterer
2019-03-14 18:58                         ` Christoph Anton Mitterer
2019-03-15  5:28                           ` Zygo Blaxell
2019-03-16 22:11                             ` Christoph Anton Mitterer
2019-03-17  2:54                               ` Zygo Blaxell
2019-02-15 12:02                   ` Filipe Manana
2019-03-04 15:46                     ` Christoph Anton Mitterer
2019-02-12 18:58       ` Andrei Borzenkov
2019-02-12 21:48         ` Chris Murphy
2019-02-12 22:11           ` Zygo Blaxell
2019-02-12 22:53             ` Chris Murphy
2019-02-13  2:46               ` Zygo Blaxell
2019-02-13  7:47   ` Roman Mamedov
2019-02-13  8:04     ` Qu Wenruo

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