2.4: "access beyond end of device" after ext2 mount

[Mario Holbe <Mario.Holbe@TU-Ilmenau.DE>]

Hello,

mounting an ext2 (ext3 as well) filesystem seems to modify the
block device's EOF behaviour: before the mount the device returned
EOF, after the mount it doesn't anymore:

[on a fresh booted system]
root@darkside:~# uname -a
Linux darkside 2.4.27 #1 Sat Jan 15 17:07:20 CET 2005 i686 GNU/Linux
root@darkside:~# dd if=/dev/hdg7 of=/dev/null
9992366+0 records in
9992366+0 records out
5116091392 bytes transferred in 87,157394 seconds (58699453 bytes/sec)
root@darkside:~# mke2fs /dev/hdg7
...
Block size=4096 (log=2)
Fragment size=4096 (log=2)
625248 inodes, 1249045 blocks
...
root@darkside:~# dd if=/dev/hdg7 of=/dev/null
9992366+0 records in
9992366+0 records out
5116091392 bytes transferred in 87,439332 seconds (58510184 bytes/sec)
root@darkside:~# mount -t ext2 -o ro /dev/hdg7 /mnt
root@darkside:~# umount /dev/hdg7
root@darkside:~# dd if=/dev/hdg7 of=/dev/null
attempt to access beyond end of device
22:07: rw=0, want=4996184, limit=4996183
dd: reading `/dev/hdg7': Input/output error
9992360+0 records in
9992360+0 records out
5116088320 bytes transferred in 87,145443 seconds (58707468 bytes/sec)
root@darkside:~# bc
1249045 * 4
4996180
1249045 * 4 * 2
9992360

Could somebody please explain this to me? Is this intentional?
I could partly imagine the ext2 mount shrinking the block device's
boundaries to the filesystem boundaries - for security reasons perhaps,
even if I personally think this isn't the best idea at all, since it
would violate layers encapsulation.
However, I have no idea why a) if so, this is not reverted at least
on umount and why b) the EOF behaviour of the block device changes -
before there were an EOF sent to the application (dd) while after there
isn't, but an I/O error instead.

Kernel is Debian's kernel-source-2.4.27 + kernel-patch-2.4-i2c +
kernel-patch-2.4-lm-sensors.

It seems there were already some mails regarding this issue suggesting
this could have shown up between 2.4.24 and 2.4.25, but it seems they
were misunderstood, for example:

From: "ProNIC Solutions" <mot@pronicsolutions.com>
Subject: 2.4.25: attempt to access beyond end of device
Date: Tue, 16 Mar 2004 09:06:31 -0500
Message-ID: <005201c40b5f$e21d34a0$0600a8c0@p17>

From: "Peter S. Mazinger" <ps.m@gmx.net>
Subject: BUG in 2.4.25-rc1: attempt to access beyond end of device
Date: Fri, 6 Feb 2004 13:53:40 +0100 (CET)
Message-ID: <Pine.LNX.4.44.0402061347160.27376-100000@lnx.bridge.intra>


regards
   Mario
-- 
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