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Re: Proposal: new fsfs.conf properties

From: Johan Corveleyn <jcorvel_at_gmail.com>
Date: Wed, 12 Jul 2017 12:24:10 +0200

On Wed, Jul 12, 2017 at 12:09 PM, Branko Čibej <brane_at_apache.org> wrote:
> On 11.07.2017 22:50, Stefan Sperling wrote:
>> On Tue, Jul 11, 2017 at 09:11:58PM +0200, Branko Čibej wrote:
>>> Another issue I have with the proposal is the idea to use file suffixes.
>>> That's usually the wrong way to go about things (case in point: Windows
>>> does it, with didastrous results). It's much better to determine file
>>> format by inspection, such as, e.g., libmagic does. We already have
>>> optional support for libmagic in the client (to set svn:mime-type).
>> I would not feel comfortable having the server parse arbitrary data with
>> libmagic. The libmagic code is not very safe to run on untrusted input.
>> I have seen libmagic crash my svn client on several occasions even on
>> text files I wrote.
>>
>> At the client side it's a bit less dangerous because users have already
>> told svn to add the files in question to version control, and a libmagic
>> exploit running on the client machine can do less harm than a server-side one.
>>
>> Granted, commits are usually authenticated. If we did this we should at
>> least make really sure that no unauthenticated access can trigger this code.
>> Ideally, it would be sandboxed somehow if we started using it on the server.
>
> I wasn't really proposing to use libmagic on the server. My point is
> that instead of using file name suffixes (which the compression and
> deltification code don't know about), we'd do some sort of inspection
> instead. Detecting ZIP files, or gzip/bzip2/xz-compressed files, etc.,
> is fairly easy just from looking at a few bytes of headers. Same goes
> for most image and video formats.
>
> Of course, one could always concoct a file that would trick such
> inspection, but at least that's marginally harder to do than commit a
> large text file full of spaces and calling it 'spaceinvaders.jpg'. :)
>
> Random binary data is harder to detect, but we already deal with that
> after the fact by using the plain text if the delta is too large.

We could also make the process driven by a "client-side suggestion".
Driving it from the client-side also gives us the possibility to
eliminate the client-side deltification overhead.

I.e. the client has some logic (libmagic, suffix, looking at the first
100 bytes, ...) to determine that it's not worth deltifying and/or
compressing. It doesn't do deltification itself, and lets the server
know that it probably shouldn't either (or the server sees that the
client hasn't deltified, so accepts the content automatically as
"non-deltifiable / non-compressable"?).

Maybe needs a server-side config setting to make it respect or ignore
the client-side suggestion.

-- 
Johan
Received on 2017-07-12 12:24:44 CEST

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