linux-kernel - Re: [PATCH] udf: reduce leakage of blocks related to named streams

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Message-ID: <83cf0e5e-2695-8748-9269-6e4d01114285@digidescorp.com>
Date:   Fri, 9 Aug 2019 09:10:24 -0500
From:   Steve Magnani <steve.magnani@...idescorp.com>
To:     Jan Kara <jack@...e.cz>
Cc:     Jan Kara <jack@...e.com>, Steve Magnani <steve@...idescorp.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] udf: reduce leakage of blocks related to named streams

Jan -

Thanks for the feedback.

On 8/9/19 8:05 AM, Jan Kara wrote:
> On Wed 07-08-19 08:32:58,  Steven J. Magnani  wrote:
>> From: Steve Magnani <steve@...idescorp.com>
>>
>> Windows is capable of creating UDF files having named streams.
>> One example is the "Zone.Identifier" stream attached automatically
>> to files downloaded from a network. See:
>>    https://msdn.microsoft.com/en-us/library/dn392609.aspx
>>
>> Modification of a file having one or more named streams in Linux causes
>> the stream directory to become detached from the file, essentially leaking
>> all blocks pertaining to the file's streams. Worse, an attempt to delete
>> the file causes its directory entry (FID) to be deleted, but because the
>> driver believes that a hard link to the file remains, the Extended File
>> Entry (EFE) and all extents of the file itself remain allocated. Since
>> there is no hard link, after the FID has been deleted all of these blocks
>> are unreachable (leaked).
>>
>> ...
>>
>> For this case, this partial solution reduces the number of blocks leaked
>> during file deletion to just one (the EFE containing the stream data).
>>
>>
> Thanks for the patch! I was thinking about this and rather than this
> partial fix, I'd prefer to fail the last unlink of an inode with
> a named-stream directory with EOPNOTSUPP. Later we can properly handle this
> and walk the named-stream directory and remove all associated EFEs for the
> named streams. After all named-stream directories are restricted to not
> have any subdirectories, hardlinks, or anything similarly fancy so the walk
> should not be *that* hard to implement.
>
Maybe not but it's more work than I am able to take on anytime soon.
Absent a complete solution, how to handle this is a judgement call.
Since Windows seems to attach a Zone.Identifier stream to _all_ files
downloaded from a network, and since interchange between Windows and Linux
via USB Mass Storage is a somewhat common (or at least desirable) use case
for UDF, this issue seemed fairly serious from a user perspective. Leaking
all the blocks of a file on delete is pretty bad. Leaking a single block is
still bad, but much less so. One could argue that prohibiting deletion of
files with named streams is nearly as bad as leaking all the blocks - in
both cases, within Linux, none of the file's blocks can be reused.
That's pretty limiting for users. It's too limiting for my use cases.

------------------------------------------------------------------------
Steven J. Magnani               "I claim this network for MARS!
www.digidescorp.com              Earthling, return my space modulator!"

#include <standard.disclaimer>