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Date:   Tue, 26 Jul 2022 13:20:33 +0200
From:   "Jason A. Donenfeld" <Jason@zx2c4.com>
To:     Florian Weimer <fweimer@redhat.com>
Cc:     libc-alpha@sourceware.org,
        Adhemerval Zanella Netto <adhemerval.zanella@linaro.org>,
        Cristian =?utf-8?Q?Rodr=C3=ADguez?= <crrodriguez@opensuse.org>,
        Paul Eggert <eggert@cs.ucla.edu>, linux-crypto@vger.kernel.org
Subject: Re: [PATCH v2] arc4random: simplify design for better safety
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Hey Florian,

On Tue, Jul 26, 2022 at 01:12:28PM +0200, Florian Weimer wrote:
> >> What happens if /dev/random is actually /dev/urandom?  Will the poll
> >> call fail?
> >
> > Yes. I'm unsure if you're asking this because it'd be a nice
> > simplification to only have to open one fd, or because you're worried
> > about confusion. I don't think the confusion problem is one we should
> > take too seriously, but if you're concerned, we can always fstat and
> > check the maj/min. Seems a bit much, though.
> 
> Turning /dev/random into /dev/urandom (e.g. with a symbolic link) used
> to be the only way to get some applications working because they tried
> to read from /dev/random at a higher rate than the system was estimating
> entropy coming in.  We may have to do something differently here if the
> failing poll causes too much breakage.

The "backup plan" would be to sleep-loop-read /proc/sys/kernel/random/entropy_avail
until it passes a certain threshold one time. This might also work on even older
kernels than the poll() trick. But that's pretty darn ugly, so it's not
obvious to me where the cut-off in frustration is, when we throw our
hands up and decide the ugliness is worth it compared to whatever
problems we happen to be facing at the time with the poll() technique.
But at least there is an alternative, should we need it.

Jason