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Date:   Mon, 7 Nov 2022 12:18:04 +0100
From:   "Jason A. Donenfeld" <Jason@zx2c4.com>
To:     Harald Freudenberger <freude@linux.ibm.com>
Cc:     herbert@gondor.apana.org.au, linux-crypto@vger.kernel.org,
        Ard Biesheuvel <ardb@kernel.org>,
        Holger Dengler <dengler@linux.ibm.com>
Subject: Re: [PATCH] hw_random: treat default_quality as a maximum and
 default to 1024
Message-ID: <Y2jpbDnEqlOT6RoB@zx2c4.com>
References: <20221104154230.52836-1-Jason@zx2c4.com>
 <a0863b503b22b42fb8129b6847188a2e@linux.ibm.com>
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Hi Harald,

On Mon, Nov 07, 2022 at 10:24:42AM +0100, Harald Freudenberger wrote:
> Well, I am not sure if this is the right way to go. So by default a
> hw rng which does not implement the registration correctly is
> rewarded with the implicit assumption that it produces 100% of
> entropy.
> I see your point - a grep through the kernel code gives the impression
> that a whole bunch of registrations is done with an empty quality
> field. What about assuming a default quality of 50% if the field
> is not filled ?

The vast majority of hardware RNGs do *not* work this way. The
reasonable assumption is to assume that a hardware RNG provides fully
random bits, unless the documentation leads the driver author to specify
something less.

Really, just quit with all the nutty mailing list stuff here. Next: "how
about 74.4% because that matches the vibrations of cedar trees"... If
you want this to be different on a particular kernel, you can set your
exact value as a command line. This patch here is simply about a
sensible default.

Jason