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From: "J. Bruce Fields" <bfields@fieldses.org>
To: Dave Chinner <david@fromorbit.com>
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: Sun, 8 May 2016 22:46:17 -0400	[thread overview]
Message-ID: <20160509024617.GA4646@fieldses.org> (raw)
In-Reply-To: <20160509014543.GS18496@dastard>

On Mon, May 09, 2016 at 11:45:43AM +1000, Dave Chinner wrote:
> [ 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).

Yeah, but I was wondering whether you could somehow get into the
situation where clients between then are caching N distinct filehandles
with the same inode number.  Then a collision becomes likely around
2^16, by the usual birthday paradox rule-of-thumb.

Uh, but now that I think of it that's irrelevant.  At most one of those
filehandles actually refers to a still-existing file.  Any attempt to
use the other 2^16-1 should return -ESTALE.  So collisions among that
set don't matter, it's only collisions involving the existing file that
are interesting.  So, nevermind, I can't see a practical way to hit a
problem here....

--b.

  reply	other threads:[~2016-05-09  2:46 UTC|newest]

Thread overview: 76+ 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
2016-05-02 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  0:09       ` NeilBrown
2016-05-05 19:48       ` Jeff Layton
2016-05-06 18:07         ` J. Bruce Fields
2016-05-06 18:07           ` J. Bruce Fields
2016-05-05 20:04       ` David Howells
2016-05-05 20:04         ` David Howells
2016-05-06  1:39         ` Dave Chinner
2016-05-06  1:39           ` Dave Chinner
2016-05-06  1:39           ` Dave Chinner
2016-05-06 18:29     ` J. Bruce Fields
2016-05-09  1:45       ` Dave Chinner
2016-05-09  2:46         ` J. Bruce Fields [this message]
2016-05-04 23:56   ` NeilBrown
2016-05-08  8:35   ` Christoph Hellwig
2016-05-08  8:35     ` Christoph Hellwig
2016-05-09 12:02     ` Jeff Layton
2016-05-09 12:02       ` Jeff Layton
2016-05-10  7:00       ` Christoph Hellwig
2016-05-10  7:00         ` Christoph Hellwig
2016-05-10 13:21         ` Jeff Layton
2016-05-10 13:21           ` Jeff Layton
2016-05-09 12:57   ` David Howells
2016-05-09 12:57     ` David Howells
2016-05-09 13:23     ` Trond Myklebust
2016-05-09 13:23       ` Trond Myklebust
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
2016-05-13 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 13:00     ` 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
2016-05-03 20:24     ` David Howells
2016-05-08  8:38   ` Christoph Hellwig
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
2016-10-03 21:03       ` Steve French
2016-05-03 20:23   ` David Howells
2016-05-08  8:39   ` Christoph Hellwig
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
2016-04-30 21:05   ` Jeff Layton
2016-05-04 13:46 ` Arnd Bergmann
2016-05-04 13:46   ` Arnd Bergmann
2016-05-05 22:54   ` Steve French
2016-05-06  2:00     ` Steve French
2016-05-09 13:09       ` Arnd Bergmann
2016-05-09 13:09         ` Arnd Bergmann
2016-05-13 14:28         ` Richard Sharpe
2016-05-13 14:28           ` Richard Sharpe
2016-05-13 15:08           ` Arnd Bergmann

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