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

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Date:	Fri, 26 Feb 2016 22:36:50 +0100
From:	Stanislav Brabec <sbrabec@...e.cz>
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	"Austin S. Hemmelgarn" <ahferroin7@...il.com>,
	linux-kernel@...r.kernel.org, Jens Axboe <axboe@...nel.dk>,
	Btrfs BTRFS <linux-btrfs@...r.kernel.org>,
	David Sterba <dsterba@...e.cz>
Subject: Re: loop subsystem corrupted after mounting multiple btrfs
 sub-volumes

On Feb 26, 2016 at 22:03 Al Viro wrote:
And I'm not sure how
> to deal with -o loop in a sane way, TBH - automagical losetup is bloody
> hard to get right.

See another reply in this thread for the idea:
Fri, 26 Feb 2016 22:00:44 +0100

> Keep in mind that loop-over-loop is also possible...

Indeed! Let's remember that mount(8) should never do it.

# losetup /dev/loop0 /dev/sda2
# losetup /dev/loop1 /dev/loop0
# losetup -l
NAME       SIZELIMIT OFFSET AUTOCLEAR RO BACK-FILE
/dev/loop0         0      0         0  0 /dev/sda2
/dev/loop1         0      0         0  0 /dev/loop0


But it actually does, if the command line is "overlooped":

oct:~ # mount -oloop /dev/loop1 /mnt

as it does exactly that:
oct:~ # losetup -l
NAME       SIZELIMIT OFFSET AUTOCLEAR RO BACK-FILE
/dev/loop0         0      0         0  0 /dev/sda2
/dev/loop1         0      0         0  0 /dev/loop0
/dev/loop2         0      0         1  0 /dev/loop1

It should definitely report error whenever trying -oloop on top of
anything else than a file. Or at least a warning.

Well, even losetup should report a warning.

-- 
Best Regards / S pozdravem,

Stanislav Brabec
software developer
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