linux-ext4 - [Bug 25832] kernel crashes when a mounted ext3/4 file system is physically removed

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Message-Id: <201104260122.p3Q1Ms1X009789@demeter1.kernel.org>
Date:	Tue, 26 Apr 2011 01:22:54 GMT
From:	bugzilla-daemon@...zilla.kernel.org
To:	linux-ext4@...r.kernel.org
Subject: [Bug 25832] kernel crashes when a mounted ext3/4 file system is
 physically removed

https://bugzilla.kernel.org/show_bug.cgi?id=25832

--- Comment #75 from rocko <rockorequin@...mail.com>  2011-04-26 01:22:52 ---
Created an attachment (id=55522)
 --> (https://bugzilla.kernel.org/attachment.cgi?id=55522)
null pointer deference hit while running without a desktop

I just now managed to crash the kernel *without* a desktop running. Running the
VM in recovery mode (ie console with no desktop), I tried two ways:

a) I modified the script to mount the device after binding it. With no other
modifications, the kernel did not crash with around 50 bind/mount/unbind
attempts (which is not conclusive but seems a reasonable number of tests to
try). Note that with this setup, the device kept getting a new drive letter on
each bind, ie /dev/sdb, /dev/sdc, /dev/sdd, etc, whereas with a desktop running
it is assigned each time to /dev/sdb.

b) I modified the script to umount the device immediately after the subsequent
unbind, ie the process is bind, mount on /tmp/usb, unbind, umount /tmp/usb. It
crashed with the null pointer dereference first time (log attached just in
case).

So the umount might be the key to the issue. I should think desktop's
auto-mounting code would also be trying to umount devices once it realises
they're no longer there.

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