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Subject: Re: Linux 5.3-rc8
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     "Ahmed S. Darwish" <darwish.07@gmail.com>,
        "Theodore Y. Ts'o" <tytso@mit.edu>,
        Andreas Dilger <adilger.kernel@dilger.ca>,
        Jan Kara <jack@suse.cz>, Ray Strode <rstrode@redhat.com>,
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        zhangjs <zachary@baishancloud.com>, linux-ext4@vger.kernel.org,
        Lennart Poettering <lennart@poettering.net>,
        lkml <linux-kernel@vger.kernel.org>
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From:   "Alexander E. Patrakov" <patrakov@gmail.com>
Message-ID: <8c2a47cc-a519-ad94-5d9a-18bb03ba2fd7@gmail.com>
Date:   Sat, 14 Sep 2019 22:09:23 +0500
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14.09.2019 21:52, Linus Torvalds =D0=BF=D0=B8=D1=88=D0=B5=D1=82:
> On Sat, Sep 14, 2019 at 9:35 AM Alexander E. Patrakov
> <patrakov@gmail.com> wrote:
>>
>> Let me repeat: not -EINVAL, please. Please find some other error code,=

>> so that the application could sensibly distinguish between this case
>> (low quality entropy is in the buffer) and the "kernel is too dumb" ca=
se
>> (and no entropy is in the buffer).
>=20
> I'm not convinced we want applications to see that difference.
>=20
> The fact is, every time an application thinks it cares, it has caused
> problems. I can just see systemd saying "ok, the kernel didn't block,
> so I'll just do
>=20
>     while (getrandom(x) =3D=3D -ENOENTROPY)
>         sleep(1);
>=20
> instead. Which is still completely buggy garbage.

OK, I understand this viewpoint. But then still, -EINVAL is not the=20
answer, because a hypothetical evil version of systemd will use -EINVAL=20
as -ENOENTROPY (with flags =3D=3D 0 and a reasonable buffer size, there i=
s=20
simply no other reason for the kernel to return -EINVAL). Yes I=20
understand that this is a complete reverse of my previous argument.

> The fact is, we can't guarantee entropy in general. It's probably
> there is practice, particularly with user space saving randomness from
> last boot etc, but that kind of data may be real entropy, but the
> kernel cannot *guarantee* that it is.
>=20
> And people don't like us guaranteeing that rdrand/rdseed is "real
> entropy" either, since they don't trust the CPU hw either.
>=20
> Which means that we're all kinds of screwed. The whole "we guarantee
> entropy" model is broken.

I agree here. Given that you suggested "to just fill the buffer and=20
return 0" in the previous mail (well, I think you really meant "return=20
buflen", otherwise ENOENTROPY =3D=3D 0 and your previous objection applie=
s),=20
let's do just that. As a bonus, it saves applications from the complex=20
dance with retrying via /dev/urandom and finally brings a reliable API=20
(modulo old and broken kernels) to get random numbers (well, as random=20
as possible right now) without needing a file descriptor.

--=20
Alexander E. Patrakov


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