[PATCH v3] getrandom system call wrapper [BZ #17252]

[Florian Weimer]

On 10/17/2016 02:54 PM, Torvald Riegel wrote:
> On Wed, 2016-10-12 at 17:58 +0200, Florian Weimer wrote:
>> On 09/12/2016 09:25 AM, Florian Weimer wrote:
>>> On 09/09/2016 05:23 PM, Torvald Riegel wrote:
>>>> On Fri, 2016-09-09 at 16:28 +0200, Florian Weimer wrote:
>>>>> On 09/09/2016 04:21 PM, Torvald Riegel wrote:
>>>>>> On Thu, 2016-09-08 at 13:44 +0200, Florian Weimer wrote:
>>>>>>> I have made the system call wrapper a cancellation point.  (If we
>>>>>>> implement the simpler getentropy interface, it would not be a
>>>>>>> cancellation point.)
>>>>>>
>>>>>> Why did you do that?
>>>>>
>>>>> I have to, because it can block indefinitely.
>>>>
>>>> That doesn't mean you have to make the default function a cancellation
>>>> point.  There are many POSIX functions which can block indefinitely and
>>>> which are not required to be cancellation points (eg, rwlocks only *may*
>>>> be cancellation points).
>>>>
>>>> Can the system call really block indefinitely, or only for a long time
>>>> and (ie, will return eventually)?
>>>
>>> Yes, if the system enters a deadlock condition where the waiting for
>>> randomness prevents it from accumulating additional randomness.
>>
>> This is what happens here:
>>
>>    <https://bugzilla.redhat.com/show_bug.cgi?id=1383060>
>>
>> systemd will eventually kill the blocked process and the boot continues,
>> but all network services will be missing.
>
> I don't see how cancellation would be the best solution for this
> problem.

You asked if the system call can really block indefinitely, and we now 
have empirical evidence that it can happen.

Florian