From: Dave Chinner <david@fromorbit.com>
To: "J. Bruce Fields" <bfields@fieldses.org>
Cc: David Howells <dhowells@redhat.com>,
linux-fsdevel@vger.kernel.org, linux-afs@vger.kernel.org,
linux-nfs@vger.kernel.org, samba-technical@lists.samba.org,
linux-kernel@vger.kernel.org, linux-ext4@vger.kernel.org
Subject: Re: [PATCH 1/6] statx: Add a system call to make enhanced file info available
Date: Mon, 9 May 2016 11:45:43 +1000 [thread overview]
Message-ID: <20160509014543.GS18496@dastard> (raw)
In-Reply-To: <20160506182923.GB13350@fieldses.org>
[ OT, but I'll reply anyway :P ]
On Fri, May 06, 2016 at 02:29:23PM -0400, J. Bruce Fields wrote:
> On Thu, May 05, 2016 at 08:56:02AM +1000, Dave Chinner wrote:
> > In the latest XFS filesystem format, we randomise the generation
> > value during every inode allocation to make it hard to guess the
> > handle of adjacent inodes from an existing ino+gen pair, or even
> > from life time to life time of the same inode.
>
> The one thing I wonder about is whether that increases the probability
> of a filehandle collision (where you accidentally generate the same
> filehandle for two different files).
Not possible - inode number is still different between the two
files. i.e. ino+gen makes the handle unique, not gen.
> If the generation number is a 32-bit counter per inode number (is that
> actually the way filesystems work?), then it takes 2^32 reuses of the
> inode number to hit the same filehandle.
4 billion unlink/create operations that hit the same inode number
are going to take some time. I suspect someone will notice the load
generated by an attmept to brute force this sort of thing ;)
> If you choose it randomly then
> you expect a collision after about 2^16 reuses.
I'm pretty sure that a random search will need to, on average,
search half the keyspace before a match is found (i.e. 2^31
attempts, not 2^16).
> > > If the caller didn't ask for them, then they may be approximated. For
> > > example, NFS won't waste any time updating them from the server, unless
> > > as a byproduct of updating something requested.
> >
> > I would suggest that exposing them from the NFS server is something
> > we most definitely don't want to do because they are the only thing
> > that keeps remote users from guessing filehandles with ease....
>
> The first line of defense is not to depend on unguessable filehandles.
> (Don't export sudirectories unless you're willing to export the whole
> filesystem; and don't depend on directory permissions to keep children
> secret.)
Defense in depth also says "don't make it easy to guess filehandles"
because not everyone knows this is a problem. In many cases, users
may not even know what consitutes a "filesystem" because their NFS
server appliance only defines "exports". The underlying
implementation may, in fact, be "everything exported from a single
filesystem" and so the user has no choice in the matter....
Dave.
--
Dave Chinner
david@fromorbit.com
next prev parent reply other threads:[~2016-05-09 1:45 UTC|newest]
Thread overview: 52+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-04-29 12:57 [RFC][PATCH 0/6] Enhanced file stat system call David Howells
2016-04-29 12:57 ` [PATCH 1/6] statx: Add a system call to make enhanced file info available David Howells
2016-05-02 22:46 ` Andreas Dilger
2016-05-03 15:53 ` David Howells
2016-05-04 22:56 ` Dave Chinner
2016-05-05 0:09 ` NeilBrown
2016-05-05 19:48 ` Jeff Layton
2016-05-06 18:07 ` J. Bruce Fields
2016-05-05 20:04 ` David Howells
2016-05-06 1:39 ` Dave Chinner
2016-05-06 18:29 ` J. Bruce Fields
2016-05-09 1:45 ` Dave Chinner [this message]
2016-05-09 2:46 ` J. Bruce Fields
2016-05-04 23:56 ` NeilBrown
2016-05-08 8:35 ` Christoph Hellwig
2016-05-09 12:02 ` Jeff Layton
2016-05-10 7:00 ` Christoph Hellwig
2016-05-10 13:21 ` Jeff Layton
2016-05-09 12:57 ` David Howells
2016-05-09 13:23 ` Trond Myklebust
2016-05-10 7:04 ` Christoph Hellwig
2016-05-10 8:25 ` David Howells
2016-05-12 9:11 ` Christoph Hellwig
2016-05-13 15:28 ` Arnd Bergmann
2016-05-23 8:22 ` Christoph Hellwig
2016-05-23 9:33 ` David Howells
2016-05-18 10:55 ` David Howells
2016-05-09 13:00 ` David Howells
2016-05-09 13:38 ` David Howells
2016-05-10 7:08 ` Christoph Hellwig
2016-05-10 8:43 ` David Howells
2016-05-12 9:12 ` Christoph Hellwig
2016-05-09 13:40 ` David Howells
2016-04-29 12:57 ` [PATCH 2/6] statx: AFS: Return enhanced file attributes David Howells
2016-04-29 12:57 ` [PATCH 3/6] statx: Ext4: " David Howells
2016-05-02 22:48 ` Andreas Dilger
2016-05-03 20:24 ` David Howells
2016-05-08 8:38 ` Christoph Hellwig
2016-04-29 12:58 ` [PATCH 4/6] statx: NFS: " David Howells
2016-05-02 22:48 ` Andreas Dilger
2016-04-29 12:58 ` [PATCH 5/6] statx: Make windows attributes available for CIFS, NTFS and FAT to use David Howells
2016-05-02 22:52 ` Andreas Dilger
2016-10-03 21:03 ` Steve French
2016-05-03 20:23 ` David Howells
2016-05-08 8:39 ` Christoph Hellwig
2016-04-29 12:58 ` [PATCH 6/6] statx: CIFS: Return enhanced attributes David Howells
2016-04-30 21:05 ` [RFC][PATCH 0/6] Enhanced file stat system call Jeff Layton
2016-05-04 13:46 ` Arnd Bergmann
[not found] ` <CAH2r5mvWHwmqkzyfjHro__dWf-9ZAskPHgHDGS22jNzXZVQ4iA@mail.gmail.com>
2016-05-06 2:00 ` Steve French
2016-05-09 13:09 ` Arnd Bergmann
2016-05-13 14:28 ` Richard Sharpe
2016-05-13 15:08 ` Arnd Bergmann
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