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From: Trent Piepho <tpiepho@impinj.com>
To: "tytso@mit.edu" <tytso@mit.edu>
Cc: "jannh@google.com" <jannh@google.com>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
"sultanxda@gmail.com" <sultanxda@gmail.com>
Subject: Re: Linux messages full of `random: get_random_u32 called from`
Date: Fri, 18 May 2018 22:56:18 +0000 [thread overview]
Message-ID: <1526684178.31570.26.camel@impinj.com> (raw)
In-Reply-To: <20180518023209.GD15263@thunk.org>
On Thu, 2018-05-17 at 22:32 -0400, Theodore Y. Ts'o wrote:
> On Fri, May 18, 2018 at 01:27:03AM +0000, Trent Piepho wrote:
> > I've hit this on an embedded system. mke2fs hangs trying to format a
> > persistent writable filesystem, which is where the random seed to
> > initialize the kernel entropy pool would be stored, because it wants 16
> > bytes of non-cryptographic random data for a filesystem UUID, and util-
> > linux libuuid calls getrandom(16, 0) - no GRND_RANDOM flag - and this
> > hangs for over four minutes.
>
> This is fixed in util-linux 2.32. It ships with the following commits:
I feel like "fix" might overstate the result a bit.
This ends up taking a full second to make each UUID. Having gone to
great effort to make an iMX25 complete userspace startup in 250 ms, a
full second, per UUID, in early startup is pretty appalling.
Let's look at what we're doing after this fix:
Want non-cryptographic random data for UUID, ask kernel for it.
Kernel has non-cryptographic random data, won't give it to us.
Wait one second for cryptographic random data, which we didn't need.
Give up and create our own random data, which is non-cryptographic and
even worse than what the kernel could have given us from the start.
util-linux falls back to rand() seeded with the pid, uid, tv_sec, and
tv_usec from gettimeofday(). Pretty bad on an embedded system with no
RTC and worse than what the kernel in crng_init 1 state can give us.
What took microseconds now takes a seconds. We have lower quality
random data than we had before.
Seems like two steps backward. Can't we do better?
How about adding a flag to getrandom() that allows the kernel to return
low-quality data if high-quality data would require blocking?
It would seem to be a fact that there will be users of non-
cryptographic random data in early boot. What is the best practice for
that? To fall back to each user trying "to find randomly-looking
things on an 1990s Unix." That doesn't seem good to me. But what's
the better way?
next prev parent reply other threads:[~2018-05-18 22:56 UTC|newest]
Thread overview: 65+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-04-26 4:11 Sultan Alsawaf
2018-04-26 5:00 ` Theodore Y. Ts'o
2018-04-26 5:05 ` Sultan Alsawaf
2018-04-26 7:32 ` Theodore Y. Ts'o
2018-04-26 15:17 ` Sultan Alsawaf
2018-04-26 19:25 ` Theodore Y. Ts'o
2018-04-26 20:22 ` Sultan Alsawaf
2018-04-26 20:47 ` Christian Brauner
2018-04-27 0:00 ` Theodore Y. Ts'o
2018-04-27 15:38 ` Jason A. Donenfeld
2018-04-27 19:14 ` Theodore Y. Ts'o
2018-04-26 23:56 ` Theodore Y. Ts'o
2018-04-27 5:20 ` Sultan Alsawaf
2018-04-27 20:10 ` Theodore Y. Ts'o
2018-04-27 22:59 ` Sultan Alsawaf
2018-04-29 14:32 ` Pavel Machek
2018-04-29 17:05 ` Sultan Alsawaf
2018-04-29 18:41 ` Pavel Machek
2018-04-29 20:20 ` Sultan Alsawaf
2018-04-29 21:18 ` Pavel Machek
2018-04-29 21:34 ` Sultan Alsawaf
2018-04-29 22:05 ` Theodore Y. Ts'o
2018-04-29 22:26 ` Sultan Alsawaf
2018-04-29 22:43 ` Jason A. Donenfeld
2018-04-29 22:49 ` Sultan Alsawaf
2018-04-30 0:11 ` Theodore Y. Ts'o
2018-04-30 4:34 ` Sultan Alsawaf
2018-04-30 16:11 ` Theodore Y. Ts'o
2018-05-01 19:53 ` Pavel Machek
2018-04-29 22:43 ` Pavel Machek
2018-04-30 0:32 ` Laura Abbott
2018-04-30 21:12 ` Jeremy Cline
2018-05-01 11:52 ` Justin Forbes
2018-05-01 12:55 ` Theodore Y. Ts'o
2018-05-01 22:35 ` Justin Forbes
2018-05-02 0:02 ` Theodore Y. Ts'o
2018-05-02 12:09 ` Justin Forbes
2018-05-02 16:26 ` Theodore Y. Ts'o
2018-05-02 17:49 ` Laura Abbott
2018-05-02 22:25 ` Theodore Y. Ts'o
2018-05-03 6:19 ` Pavel Machek
2018-05-03 12:23 ` Justin Forbes
2018-05-02 0:43 ` Sultan Alsawaf
2018-05-02 0:56 ` Theodore Y. Ts'o
2018-05-02 1:11 ` Sultan Alsawaf
2018-04-29 18:30 ` Sultan Alsawaf
2018-04-29 20:08 ` Theodore Y. Ts'o
2018-05-18 1:27 ` Trent Piepho
2018-05-18 2:32 ` Theodore Y. Ts'o
2018-05-18 22:56 ` Trent Piepho [this message]
2018-05-18 23:22 ` Theodore Y. Ts'o
2018-05-21 18:39 ` Trent Piepho
2018-04-29 14:29 ` Pavel Machek
-- strict thread matches above, loose matches on Subject: below --
2018-04-24 11:48 Paul Menzel
2018-04-24 13:56 ` Theodore Y. Ts'o
2018-04-24 14:30 ` Paul Menzel
2018-04-24 15:49 ` Theodore Y. Ts'o
2018-04-24 15:56 ` Paul Menzel
2018-04-25 7:41 ` Theodore Y. Ts'o
2018-04-26 3:48 ` Paul Menzel
2018-04-29 14:22 ` Pavel Machek
2018-04-29 23:02 ` Dave Jones
2018-04-29 23:07 ` Dave Jones
2018-04-30 0:21 ` Theodore Y. Ts'o
2018-04-26 5:51 ` Pavel Machek
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