linux-ext4 - Kernel 3.3.8 breaks accidental ext3 mount of extended partition

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Date:	Mon, 18 Jun 2012 07:59:30 +0200
From:	Torsten Hilbrich <torsten.hilbrich@...unet.com>
To:	jmoyer@...hat.com
CC:	linux-ext4@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>
Subject: Kernel 3.3.8 breaks accidental ext3 mount of extended partition

Hello,

a software that tries to mount each existing partition as ext3 file system started to fail when updating from v3.3.7 to v3.3.8.

The applications then hangs-up in the mount syscall, here is a snapshot of its stack at this moment:

[<ffffffff81060c6a>] __cond_resched+0x2a/0x40
[<ffffffff81134b3f>] __getblk+0x1bf/0x270
[<ffffffff811362b3>] __bread+0x13/0xb0
[<ffffffff81182222>] ext3_fill_super+0x132/0x1b00
[<ffffffff8110891a>] mount_bdev+0x1aa/0x1f0
[<ffffffff8117ffc5>] ext3_mount+0x15/0x20
[<ffffffff81107fc3>] mount_fs+0x43/0x1a0
[<ffffffff81123102>] vfs_kern_mount+0x72/0x100
[<ffffffff81123882>] do_kern_mount+0x52/0x110
[<ffffffff8112519a>] do_mount+0x25a/0x7d0
[<ffffffff811257a8>] sys_mount+0x98/0xf0
[<ffffffff814f3e92>] system_call_fastpath+0x16/0x1b

The expected behaviour (which was still there in v3.3.7) is that the mount syscall fails because the partition contains no valid ext3 file system.

I have create more snapshot of the stack in the following pastebin: http://pastebin.com/99x9EpnM

Using bisecting I found the following commit to be the cause of the issue:

commit 3735b0a1d73af536484ddefef4d8438dd468c4a6
Author: Jeff Moyer <jmoyer@...hat.com>
Date:   Fri May 11 16:34:10 2012 +0200

    block: don't mark buffers beyond end of disk as mapped

    commit 080399aaaf3531f5b8761ec0ac30ff98891e8686 upstream.

Reverting this commit brought back the desired behaviour of the mount syscall failing because of invalid superblock.

It seems to be that ext3_fill_super is unable to read the super block because there is no blocked for an extended partition.

The same issue can be found in v3.4, where the upstream commit (080399aa) was released.

This problem was found and debugged with an amd64 build of the linux kernel. The native ext3 support was used:

CONFIG_EXT3_FS=y
# CONFIG_EXT3_DEFAULTS_TO_ORDERED is not set
CONFIG_EXT3_FS_XATTR=y
# CONFIG_EXT3_FS_POSIX_ACL is not set
# CONFIG_EXT3_FS_SECURITY is not set

I will rewrite the application causing this issue to check the file system type before trying to mount the partition to work around this problem for now.

	Torsten
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