linux-kernel - Re: [PATCH 2/2] random: add fork_event sysctl for polling VM forks

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Date:   Mon, 2 May 2022 18:51:19 +0200
From:   Lennart Poettering <mzxreary@...inter.de>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     linux-kernel@...r.kernel.org, linux-crypto@...r.kernel.org,
        Dominik Brodowski <linux@...inikbrodowski.net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Theodore Ts'o <tytso@....edu>,
        Alexander Graf <graf@...zon.com>,
        Colm MacCarthaigh <colmmacc@...zon.com>,
        Torben Hansen <htorben@...zon.co.uk>,
        Jann Horn <jannh@...gle.com>
Subject: Re: [PATCH 2/2] random: add fork_event sysctl for polling VM forks

On Mo, 02.05.22 18:12, Jason A. Donenfeld (Jason@...c4.com) wrote:

> > > In order to inform userspace of virtual machine forks, this commit adds
> > > a "fork_event" sysctl, which does not return any data, but allows
> > > userspace processes to poll() on it for notification of VM forks.
> > >
> > > It avoids exposing the actual vmgenid from the hypervisor to userspace,
> > > in case there is any randomness value in keeping it secret. Rather,
> > > userspace is expected to simply use getrandom() if it wants a fresh
> > > value.
> >
> > Wouldn't it make sense to expose a monotonic 64bit counter of detected
> > VM forks since boot through read()? It might be interesting to know
> > for userspace how many forks it missed the fork events for. Moreover it
> > might be interesting to userspace to know if any fork happened so far
> > *at* *all*, by checking if the counter is non-zero.
>
> "Might be interesting" is different from "definitely useful". I'm not
> going to add this without a clear use case. This feature is pretty
> narrowly scoped in its objectives right now, and I intend to keep it
> that way if possible.

Sure, whatever. I mean, if you think it's preferable to have 3 API
abstractions for the same concept each for it's special usecase, then
that's certainly one way to do things. I personally would try to
figure out a modicum of generalization for things like this. But maybe
that' just me…

I can just tell you, that in systemd we'd have a usecase for consuming
such a generation counter: we try to provide stable MAC addresses for
synthetic network interfaces managed by networkd, so we hash them from
/etc/machine-id, but otoh people also want them to change when they
clone their VMs. We could very nicely solve this if we had a
generation counter easily accessible from userspace, that starts at 0
initially. Because then we can hash as we always did when the counter
is zero, but otherwise use something else, possibly hashed from the
generation counter.

But anyway, I understand you are not interested in
generalization/other usecases, so I'll shut up.

Lennart

--
Lennart Poettering, Berlin