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From:   Andy Lutomirski <luto@kernel.org>
Date:   Fri, 20 Sep 2019 16:30:20 -0700
Message-ID: <CALCETrWCjGHKnKikj+YVw22Ufpmnh1TCdGPjG2RL-qzsF=wisA@mail.gmail.com>
Subject: Re: [PATCH RFC v4 1/1] random: WARN on large getrandom() waits and
 introduce getrandom2()
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Andy Lutomirski <luto@kernel.org>,
        "Ahmed S. Darwish" <darwish.07@gmail.com>,
        Lennart Poettering <mzxreary@0pointer.de>,
        "Theodore Y. Ts'o" <tytso@mit.edu>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        "Alexander E. Patrakov" <patrakov@gmail.com>,
        Michael Kerrisk <mtk.manpages@gmail.com>,
        Willy Tarreau <w@1wt.eu>,
        Matthew Garrett <mjg59@srcf.ucam.org>,
        lkml <linux-kernel@vger.kernel.org>,
        Ext4 Developers List <linux-ext4@vger.kernel.org>,
        Linux API <linux-api@vger.kernel.org>,
        linux-man <linux-man@vger.kernel.org>
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On Fri, Sep 20, 2019 at 3:44 PM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> On Fri, Sep 20, 2019 at 1:51 PM Andy Lutomirski <luto@kernel.org> wrote:
> >
> > To be clear, when I say "blocking", I mean "blocks until we're ready,
> > but we make sure we're ready in a moderately timely manner".
>
> .. an I want a pony.
>
> The problem is that you start from an assumption that we simply can't
> seem to do.

Eh, fair enough, I wasn't thinking about platforms without fast clocks.

I'm very nervous about allowing getrandom(..., 0) to fail with
-EAGAIN, though.  On a very, very brief search, I didn't find any
programs that would incorrectly assume it worked, but I can easily
imagine programs crashing, and that might be bad, too.  At the end of
the day, most user programmers who call getrandom() really did notice
that we flubbed the ABI, and either they were too lazy to fall back to
/dev/urandom, or they didn't want to for some reason, or they
genuinely want the blocking behavior.  And people who work with little
embedded systems without good clocks that basically can't generate
random numbers already know this, and they have little scripts to help
out.

So I think that just improving the
getrandom()-is-blocking-on-x86-and-arm behavior, adding GRND_INSECURE
and GRND_SECURE_BLOCKING, and adding the warning if 0 is passed is
good enough.  I suppose we could also have separate
GRND_SECURE_BLOCKING and GRND_SECURE_BLOCK_FOREVER.  We could also say
that, if you want to block forever, you should poll() on /dev/random
(with my patches applied, where this actually does what users would
want).

--Andy