EXT4 partition changes every time it's mounted. (How to stop this?)

EXT4 partition changes every time it's mounted. (How to stop this?)

[Lee Davis]

Hi folks,

I'm setting up a Linux system with full disk encryption. I've written a script (to run on every boot) which will verify that neither the MBR or /boot partition (ext4) has been modified.

Problem is that every time I boot up the system, my /boot partition hashes differently.

Trying to solve this, I set the /boot partition to mount as read only in fstab. I then took individual hashes of every file on /boot. After I rebooted none of the files had changed (and there were no new files), yet the filesystem hashed as a whole had changed.

I'm guessing there is something behind the scenes which is changed/updated each time the filesystem is mounted? If so, can it be disabled?

Any advice would be greatly appreciated!

Cheers,
Lee

Re: EXT4 partition changes every time it's mounted. (How to stop this?)

[Theodore Tso]

On May 21, 2011, at 3:45 PM, Lee Davis wrote:

> I'm setting up a Linux system with full disk encryption. I've written a script (to run on every boot) which will verify that neither the MBR or /boot partition (ext4) has been modified.
> 
> Problem is that every time I boot up the system, my /boot partition hashes differently.
> 
> I'm guessing there is something behind the scenes which is changed/updated each time the filesystem is mounted? If so, can it be disabled?

Yes, there is a last mounted time in the superblock.   If you mount the file system read-only, then not only will that field not be changed, but you can guarantee that nothing else will change...

-- Ted

Re: EXT4 partition changes every time it's mounted. (How to stop this?)

[Lee Davis]

Hi Ted,

I've checked again, and despite being mounted read-only, the filesystem is definitely changing every time I reboot.

The mount options are: ro,relatime,commit=0

I've taken three images of the filesystem (rebooting between each image), and with a bit of research I've managed to discover which fields in the superblock are being changed. They are: s_wtime and s_kbytes_written. 

Here's how they're being changed:

0x430 (s_wtime): 06C8D94D -> 68D0D94D -> B9D7D94D

0x578 (s_kbytes_written): 42C8000000000000 -> 45C8000000000000 -> 48C8000000000000

These two fields are the only things that are changing in the entire filesystem.


Any ideas on what might be causing this and/or how to disable it?

Much appreciated,
Lee

P.S. I just thought to test it after unmounting/re-mounting, and nothing was changed. So it appears to only get changed when I boot the system.

It's a fresh Ubuntu Natty installation, with GRUB on the MBR, an ext4 boot partition, and an encrypted ext4 root partition (which I set up through the installer). I can't think what might be writing to the boot partition.





----- Original Message -----
From: Theodore Tso <tytso@MIT.EDU>
To: Lee Davis <l.davis37@ymail.com>
Cc: "linux-ext4@vger.kernel.org" <linux-ext4@vger.kernel.org>
Sent: Sunday, 22 May 2011 9:47 PM
Subject: Re: EXT4 partition changes every time it's mounted. (How to stop this?)


On May 21, 2011, at 3:45 PM, Lee Davis wrote:

> I'm setting up a Linux system with full disk encryption. I've written a script (to run on every boot) which will verify that neither the MBR or /boot partition (ext4) has been modified.
> 
> Problem is that every time I boot up the system, my /boot partition hashes differently.
> 
> I'm guessing there is something behind the scenes which is changed/updated each time the filesystem is mounted? If so, can it be disabled?

Yes, there is a last mounted time in the superblock.   If you mount the file system read-only, then not only will that field not be changed, but you can guarantee that nothing else will change...

-- Ted
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Re: EXT4 partition changes every time it's mounted. (How to stop this?)

[Lee Davis]

I've just tested this on a clean Ubuntu installation without any crypto -- exact same thing happens. Should I log this as an ext4 bug?

Regards,
Lee


----- Original Message -----
From: Lee Davis <l.davis37@ymail.com>
To: Theodore Tso <tytso@MIT.EDU>
Cc: "linux-ext4@vger.kernel.org" <linux-ext4@vger.kernel.org>
Sent: Monday, 23 May 2011 2:39 PM
Subject: Re: EXT4 partition changes every time it's mounted. (How to stop this?)

Hi Ted,

I've checked again, and despite being mounted read-only, the filesystem is definitely changing every time I reboot.

The mount options are: ro,relatime,commit=0

I've taken three images of the filesystem (rebooting between each image), and with a bit of research I've managed to discover which fields in the superblock are being changed. They are: s_wtime and s_kbytes_written. 

Here's how they're being changed:

0x430 (s_wtime): 06C8D94D -> 68D0D94D -> B9D7D94D

0x578 (s_kbytes_written): 42C8000000000000 -> 45C8000000000000 -> 48C8000000000000

These two fields are the only things that are changing in the entire filesystem.


Any ideas on what might be causing this and/or how to disable it?

Much appreciated,
Lee

P.S. I just thought to test it after unmounting/re-mounting, and nothing was changed. So it appears to only get changed when I boot the system.

It's a fresh Ubuntu Natty installation, with GRUB on the MBR, an ext4 boot partition, and an encrypted ext4 root partition (which I set up through the installer). I can't think what might be writing to the boot partition.





----- Original Message -----
From: Theodore Tso <tytso@MIT.EDU>
To: Lee Davis <l.davis37@ymail.com>
Cc: "linux-ext4@vger.kernel.org" <linux-ext4@vger.kernel.org>
Sent: Sunday, 22 May 2011 9:47 PM
Subject: Re: EXT4 partition changes every time it's mounted. (How to stop this?)


On May 21, 2011, at 3:45 PM, Lee Davis wrote:

> I'm setting up a Linux system with full disk encryption. I've written a script (to run on every boot) which will verify that neither the MBR or /boot partition (ext4) has been modified.
> 
> Problem is that every time I boot up the system, my /boot partition hashes differently.
> 
> I'm guessing there is something behind the scenes which is changed/updated each time the filesystem is mounted? If so, can it be disabled?

Yes, there is a last mounted time in the superblock.   If you mount the file system read-only, then not only will that field not be changed, but you can guarantee that nothing else will change...

-- Ted
--
To unsubscribe from this list: send the line "unsubscribe linux-ext4" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html

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