linux-ext4 - Re: [PATCH 1/2 v3] EXT4: Secure Delete: Zero out file data

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Message-Id: <9EC28987-9A72-4753-8822-A138C5F0E622@dilger.ca>
Date:	Mon, 4 Jul 2011 12:19:00 -0600
From:	Andreas Dilger <adilger@...ger.ca>
To:	Amir Goldstein <amir73il@...il.com>
Cc:	Allison Henderson <achender@...ux.vnet.ibm.com>,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH 1/2 v3] EXT4: Secure Delete: Zero out file data

On 2011-07-04, at 11:44 AM, Amir Goldstein wrote:
> On Mon, Jul 4, 2011 at 8:19 PM, Allison Henderson
> <achender@...ux.vnet.ibm.com> wrote:
>> 
>> Thx all for the reviews!  It sounds like the zero out code is in the right
>> spot then.  We are thinking about adding an optimization too, where we use
>> use secure discard instead of the sb_issue_zeroout, but only if the device
>> supports it.  I was thinking about putting that code some where in the
>> commit call back because that is where the existing discard code is, but
>> maybe that's not such a good place to put it then?  What does everyone
>> think?  Thx!
> 
> I already stated my opinion about the need for 2-phase secure delete.
> If you have to choose between security (zeroout pre commit) and the
> atomicity of the unlink() command (zeroout post commit), then it's
> a question of policy.
> Is there any other FS (or OS) that implements secure delete?
> Perhaps we could follow its semantics.

One thing we did ages ago, before extent-mapped files made unlink so
fast, was to move the blocks from unlinked files and truncated-to-zero
files to a delete queue in the main transaction, and then do the unlink
via a separate thread.

This facility could be resurrected (a version of the patch was posted to
linux-ext4 at http://www.spinics.net/lists/linux-ext4/msg06178.html) to
do the block zeroing/discard in the context of the unlink thread.  It
could be structured so that sync/fsync on the file waits for background
zeroing to complete, so that apps doing secure delete + fsync will be
sure that the file is safely erased.  The fsync would be needed for this
in any case, otherwise even an inline async zero-fill could fail if the
system crashes before the blocks are actually flushed to disk.

Cheers, Andreas

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