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Subject: Re: Linux 5.3-rc8
To:     Linus Torvalds <torvalds@linux-foundation.org>,
        "Ahmed S. Darwish" <darwish.07@gmail.com>
Cc:     "Theodore Y. Ts'o" <tytso@mit.edu>,
        Andreas Dilger <adilger.kernel@dilger.ca>,
        Jan Kara <jack@suse.cz>, Ray Strode <rstrode@redhat.com>,
        William Jon McCann <mccann@jhu.edu>,
        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: <214fed0e-6659-def9-b5f8-a9d7a8cb72af@gmail.com>
Date:   Sat, 14 Sep 2019 21:35:12 +0500
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14.09.2019 21:30, Linus Torvalds =D0=BF=D0=B8=D1=88=D0=B5=D1=82:
> On Sat, Sep 14, 2019 at 8:02 AM Ahmed S. Darwish <darwish.07@gmail.com>=
 wrote:
>>
>> On Thu, Sep 12, 2019 at 12:34:45PM +0100, Linus Torvalds wrote:
>>>
>>> An alternative might be to make getrandom() just return an error
>>> instead of waiting. Sure, fill the buffer with "as random as we can"
>>> stuff, but then return -EINVAL because you called us too early.
>>
>> ACK, that's probably _the_ most sensible approach. Only caveat is
>> the slight change in user-space API semantics though...
>>
>> For example, this breaks the just released systemd-random-seed(8)
>> as it _explicitly_ requests blocking behvior from getrandom() here:
>>
>=20
> Actually, I would argue that the "don't ever block, instead fill
> buffer and return error instead" fixes this broken case.
>=20
>>      =3D> src/random-seed/random-seed.c:
>>      /*
>>       * Let's make this whole job asynchronous, i.e. let's make
>>       * ourselves a barrier for proper initialization of the
>>       * random pool.
>>       */
>>       k =3D getrandom(buf, buf_size, GRND_NONBLOCK);
>>       if (k < 0 && errno =3D=3D EAGAIN && synchronous) {
>>           log_notice("Kernel entropy pool is not initialized yet, "
>>                      "waiting until it is.");
>>
>>           k =3D getrandom(buf, buf_size, 0); /* retry synchronously */=

>>       }
>=20
> Yeah, the above is yet another example of completely broken garbage.
>=20
> You can't just wait and block at boot. That is simply 100%
> unacceptable, and always has been, exactly because that may
> potentially mean waiting forever since you didn't do anything that
> actually is likely to add any entropy.
>=20
>>       if (k < 0) {
>>           log_debug_errno(errno, "Failed to read random data with "
>>                           "getrandom(), falling back to "
>>                           "/dev/urandom: %m");
>=20
> At least it gets a log message.
>=20
> So I think the right thing to do is to just make getrandom() return
> -EINVAL, and refuse to block.

Let me repeat: not -EINVAL, please. Please find some other error code,=20
so that the application could sensibly distinguish between this case=20
(low quality entropy is in the buffer) and the "kernel is too dumb" case =

(and no entropy is in the buffer).


--=20
Alexander E. Patrakov


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