linux-kernel - Re: Linux 5.3-rc8

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Date:   Mon, 16 Sep 2019 10:07:27 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     "Theodore Y. Ts'o" <tytso@....edu>
Cc:     Lennart Poettering <mzxreary@...inter.de>,
        "Alexander E. Patrakov" <patrakov@...il.com>,
        "Ahmed S. Darwish" <darwish.07@...il.com>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Jan Kara <jack@...e.cz>, Ray Strode <rstrode@...hat.com>,
        William Jon McCann <mccann@....edu>,
        zhangjs <zachary@...shancloud.com>, linux-ext4@...r.kernel.org,
        lkml <linux-kernel@...r.kernel.org>
Subject: Re: Linux 5.3-rc8

On Mon, Sep 16, 2019 at 10:00 AM Theodore Y. Ts'o <tytso@....edu> wrote:
>
> How /dev/random blocks is very different from how getrandom(2) blocks.
> Getrandom(2) blocks until the CRNG, and then it never blocks again.

Yes and no.

getrandom() very much blocks exactly like /dev/random, when you give
it the GRND_RANDOM flag.

Which is completely broken, and was already known to be broken. So
that flag is just plain stupid.

And getrandom() does *not* block like /dev/urandom does (ie not at
all), which was actually useful, and very widely used.

So you really have the worst of both worlds.

Yes, getrandom(0) does what /dev/random _should_ have done, and what
getrandom(GRND_RANDOM) should be but isn't.

But by making the choice it did, we now have three useless flag
combinations, and we lack one people _want_ and need.

And this design mistake very much caused the particular bug we are now hitting.

                  Linus