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

[Florian Weimer <fweimer at redhat dot com>]

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