linux-kernel - Re: [patch 7/8] fdmap v2

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Date:	Fri, 8 Jun 2007 11:07:16 -0700 (PDT)
From:	Davide Libenzi <davidel@...ilserver.org>
To:	Theodore Tso <tytso@....edu>
cc:	Ulrich Drepper <drepper@...hat.com>,
	Eric Dumazet <dada1@...mosbay.com>,
	Kyle Moffett <mrmacman_g4@....com>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [patch 7/8] fdmap v2 - implement sys_socket2

On Fri, 8 Jun 2007, Theodore Tso wrote:

> On Thu, Jun 07, 2007 at 03:40:14PM -0700, Davide Libenzi wrote:
> > Yes. Files with the CLOFORK and CLOEXEC flag do not count for fork and 
> > exec copies.
> > I was also planning on doing it in __put_unused_fd(), every time 
> > fmap->count goes to zero. But get_random_int() is not as cheap as I 
> > thought. If we use a cheaper (although less secure) function to mix pid & 
> > jiffies, we could do it even in there.
> 
> Um, how cheap do you need it?  get_random_int() is actually pretty
> cheep, since it was designed to be usable by the networking stack for
> sequence numbers for TCP packets; and it's not like sys_close() or
> sys_open() is a majorly critical path, is it?  If the concern is
> increasing the potential hold time, I suppose you could have the
> exactly two callers of __put_unused_fd() (sys_close() and
> put_unused_fd()) call get_random_int() before grabbing the
> current->files->file_lock spinlock,

I'm actually using get_random_int() in the slow path (fmap creation time). 
It does a few things get_random_int(), and one of those is an MD4 transform.
This does not need to be super secure (the Unix allocator has been 
exactly predicatble for years), so maybe a cheaper combination of the 
previous base (generated with get_random_int) together with jiffies and 
pid is enough. I really would not want to put something like an MD4 
transform in that path.


- Davide


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