linux-kernel - Re: [PATCH] random: add blocking facility to urandom

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Date:	Mon, 12 Sep 2011 18:06:57 +0100
From:	Mark Brown <broonie@...nsource.wolfsonmicro.com>
To:	Jarod Wilson <jarod@...hat.com>
Cc:	Ted Ts'o <tytso@....edu>, Sasha Levin <levinsasha928@...il.com>,
	linux-crypto@...r.kernel.org, Matt Mackall <mpm@...enic.com>,
	Neil Horman <nhorman@...hat.com>,
	Herbert Xu <herbert.xu@...hat.com>,
	Steve Grubb <sgrubb@...hat.com>,
	Stephan Mueller <stephan.mueller@...ec.com>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] random: add blocking facility to urandom

On Mon, Sep 12, 2011 at 10:02:43AM -0400, Jarod Wilson wrote:
> Ted Ts'o wrote:

> >Yeah, but there are userspace programs that depend on urandom not
> >blocking... so your proposed change would break them.

> I'm already consigned to the fact this isn't going to fly, but I'm
> still curious to know examples of programs that are going to break
> here, for my own education. Its already possible for urandom reads
> to fail as the code is now (-ERESTARTSYS and -EFAULT are possible),
> so a sane program ought to already be handling error cases, though
> not -EAGAIN, which this would add.

It's not just a question of error handling existing, it's also about the
expectations the system has for the behaviour of the file - if urandom
is expected to always be able to return data an application is likely to
rely on the fact that it's effectively non-blocking anyway and not bother
setting non-blocking mode at all and so have no graceful handling for
this.
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