linux-kernel - Re: [PATCH, RFC] random: introduce getrandom(2) system call

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Date:	Wed, 23 Jul 2014 14:10:16 +0200
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	George Spelvin <linux@...izon.com>, tytso@....edu
Cc:	linux-crypto@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH, RFC] random: introduce getrandom(2) system call

On Wed, Jul 23, 2014, at 13:52, George Spelvin wrote:
> I keep wishing for a more general solution.  For example, some way to
> have a "spare" extra fd that could be accessed with a special O_NOFAIL
> flag.
> 
> That would allow any number of library functions to not fail, such as
> logging from nasty corner cases.
> 
> But you'd have to provide one per thread, and block non-fatal signals
> while it was open, so you don't get reentrancy problems.  Ick.
> 
> 
> This overly-specialized system call (and worse yet, a blocking
> system call that you can't put into a poll() loop) just feels ugly
> to me.  Is it *absolutely* necessary?

One point that often came up besides fd exhaustion is missing
/dev/u?random device nodes in chroot environments.

I also thought about a more general interface, like e.g. an
opennod(dev_t device, int flags) call but all those ideas ended up being
very complex changes besides having design issues. getrandom is simple
and solves a real problem.

The only problem I see, that we allow access to /dev/random without
checking any permission bits like we did on opening /dev/random before
and we cannot restrict applications to deplete the whole entropy pool.

> For example, how about simply making getentropy() a library function that
> aborts if it can't open /dev/urandom?  If you're suffering fd exhaustion,
> you're being DoSed already.

Maybe applications want to mitigate fd exhaustion.

Bye,
Hannes
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