Re: loop subsystem corrupted after mounting multiple btrfs sub-volumes

[Stanislav Brabec]

On Feb 26, 2016 at 21:05 Austin S. Hemmelgarn wrote:

> It's kind of interesting, but I can't reproduce _any_ of this behavior
> with either ext4 or BTRFS when I manually set up the loop devices and
> point mount(8) at those instead of using -o loop on a file. That really
> seems to indicate that this is caused by something mount(8) is doing
> when it's calling losetup.

Behavior of "-oloop" is more similar to "losetup -f /fs.img"? than to
"losetup /dev/loop0 /fs.img".

Anyway, I can reproduce without -oloop:
# losetup /dev/loop0 /btrfs.img
# mount /dev/loop0 /mnt/1
# grep /mnt /proc/self/mountinfo
107 59 0:59 /d0/dd0/ddd0/s1/d1/dd1/ddd1/s2 /mnt/1 rw,relatime shared:45 - btrfs /dev/loop0 rw,space_cache,subvolid=257,subvol=/d0/dd0/ddd0/s1/d1/dd1/ddd1/s2
# losetup /dev/loop1 /btrfs.img
# mount -osubvol=/ /dev/loop1 /mnt/2
# grep /mnt /proc/self/mountinfo
107 59 0:59 /d0/dd0/ddd0/s1/d1/dd1/ddd1/s2 /mnt/1 rw,relatime shared:45 - btrfs /dev/loop1 rw,space_cache,subvolid=257,subvol=/d0/dd0/ddd0/s1/d1/dd1/ddd1/s2
108 59 0:59 / /mnt/2 rw,relatime shared:48 - btrfs /dev/loop1 rw,space_cache,subvolid=5,subvol=/
# uname -a
Linux oct 4.4.1-1-default #1 SMP PREEMPT Mon Feb 15 11:03:27 UTC 2016 (6398c2d) x86_64 x86_64 x86_64 GNU/Linux

(Note that the system was freshly rebooted. After other experiments,
the second line of mountinfo can be missing completely.)

>> 2) mount(2) called after the reproducer returns OK but does nothing.
>>
> OK, we've determined that mount(2) is misbehaving.  That doesn't change
> the fact that mount(8) is triggering this, and therefore should itself
> be corrected.

> Assume that mount(2) gets fixed so it doesn't lose it's
> mind and /proc/self/mountinfo doesn't change.  There will still be
> issues resulting from mount(8)'s behavior:
> 1. BTRFS will lose it's mind and corrupt data when using a multi-device
> filesystem (due to the problems with duplicate FS UUID's).
> 2. XFS might have similar issues to 1 when using metadata checksumming,
> although it's more likely that it won't allow the second mount to succeed.
> 3. Most other filesystems will likely end up corrupting data.

Do I understand, that you are saying:

Yes, mounting multiple loop devices associated with one file is a
legitimate use, but mount(8) should never do it, because it has other
ugly side effects?

OK, it looks like a next task for mount(8) to fix.

-- 
Best Regards / S pozdravem,

Stanislav Brabec
software developer
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