crash when mounting subvolume in a subdirectory

crash when mounting subvolume in a subdirectory

[Michael Niederle]

Hi!

I'm not sure whether this *should* be possible, but I think it *shouldn't*
crash:

I created a snapshot of the root directory within a subdirectory:

# mount /dev/sde2 /mnt
# cd /mnt
# mkdir save
# btrfs subvolume snapshot . save/snap1
# umount /mnt

Then I tried to mount the snapshot:

# mount -o subvol=save/snap1 /dev/sde2 /mnt

This inevitably leads to a segfault in the btrfs-driver crashing the whole
system. I tried this with kernel versions 2.6.32 and 2.6.37.rc4.

If I create the subvolume within the root directory of the btrfs volume
everything works fine.

I'm using btrfs for nearly a year by now (since the release of 2.6.32) and am
using subvolumes within subdirectories since then but never tried to directly
mount one until today, when my main btrfs volume crashed (by a hardware failure
or due to a bug in 2.6.36 - I don't know).

If you cannot reproduce this behaviour I can try to send you the kernel log
(not so easy, because the system crashes and I will have to write it down by
hand).

Greetings, Michael

Re: crash when mounting subvolume in a subdirectory

[Li Zefan]

Michael Niederle wrote:
> Hi!
> 
> I'm not sure whether this *should* be possible, but I think it *shouldn't*
> crash:
> 
> I created a snapshot of the root directory within a subdirectory:
> 
> # mount /dev/sde2 /mnt
> # cd /mnt
> # mkdir save
> # btrfs subvolume snapshot . save/snap1
> # umount /mnt
> 
> Then I tried to mount the snapshot:
> 
> # mount -o subvol=save/snap1 /dev/sde2 /mnt
> 
> This inevitably leads to a segfault in the btrfs-driver crashing the whole
> system. I tried this with kernel versions 2.6.32 and 2.6.37.rc4.
> 
> If I create the subvolume within the root directory of the btrfs volume
> everything works fine.
> 
> I'm using btrfs for nearly a year by now (since the release of 2.6.32) and am
> using subvolumes within subdirectories since then but never tried to directly
> mount one until today, when my main btrfs volume crashed (by a hardware failure
> or due to a bug in 2.6.36 - I don't know).
> 
> If you cannot reproduce this behaviour I can try to send you the kernel log
> (not so easy, because the system crashes and I will have to write it down by
> hand).

It's currently not allowed to mount a subvolume which is not created in
the root directory of the default subvolume, so you should have failed
to mount, but you hit a bug..

I've fixed it, and will send out the patch in minutes.

Re: crash when mounting subvolume in a subdirectory

[C Anthony Risinger]

On Mon, Dec 6, 2010 at 7:40 PM, Li Zefan <lizf@cn.fujitsu.com> wrote:
> Michael Niederle wrote:
>> Hi!
>>
>> I'm not sure whether this *should* be possible, but I think it *shouldn't*
>> crash:
>
> It's currently not allowed to mount a subvolume which is not created in
> the root directory of the default subvolume, so you should have failed
> to mount, but you hit a bug..
>
> I've fixed it, and will send out the patch in minutes.

i did the same thing, except for a slightly different reason and
slightly more accidentally on purpose:

http://www.spinics.net/lists/linux-btrfs/msg07190.html

buuuuut, i ended up with a corrupted FS (that i still have a dump of,
happily waiting for a way to mount it... *hint* :-) so if your are
still up and running then luck is on your side, and don't do it again
;-)

C Anthony