On Do, 2014-07-17 at 08:52 -0400, Theodore Ts'o wrote:
> On Thu, Jul 17, 2014 at 12:57:07PM +0200, Hannes Frederic Sowa wrote:
> >
> > Btw. couldn't libressl etc. fall back to binary_sysctl
> > kernel.random.uuid and seed with that as a last resort? We have it
> > available for few more years.
>
> Yes, they could. But trying to avoid more uses of binary_sysctl seems
> to be a good thing, I think. The other thing is that is that this
> interface provides is the ability to block until the entropy pool is
> initialized, which isn't a big deal for x86 systems, but might be
> useful as a gentle forcing function to force ARM systems to figure out
> good ways of making sure the entropy pools are initialized (i.e., by
> actually providing !@#!@ cycle counter) without breaking userspace
> compatibility --- since this is a new interface.
I am not questioning this new interface - I like it - just wanted to
mention there is already a safe fallback for LibreSSL in the way they
already seem to do it in OpenBSD (via sysctl).
>
> > > + if (count > 256)
> > > + return -EINVAL;
> > > +
> >
> > Why this "arbitrary" limitation? Couldn't we just check for > SSIZE_MAX
> > or to be more conservative to INT_MAX?
>
> I'm not wedded to this limitation. OpenBSD's getentropy(2) has an
> architected arbitrary limit of 128 bytes. I haven't made a final
> decision if the right answer is to hard code some value, or make this
> limit be configurable, or remote the limit entirely (which in practice
> would be SSIZE_MAX or INT_MAX).
>
> The main argument I can see for putting in a limit is to encourage the
> "proper" use of the interface. In practice, anything larger than 128
> probably means the interface is getting misused, either due to a bug
> or some other kind of oversight.
>
> For example, when I started instrumenting /dev/urandom, I caught
> Google Chrome pulling 4k out of /dev/urandom --- twice --- at startup
> time. It turns out it was the fault of the NSS library, which was
> using fopen() to access /dev/urandom. (Sigh.)
In the end people would just recall getentropy in a loop and fetch 256
bytes each time. I don't think the artificial limit does make any sense.
I agree that this allows a potential misuse of the interface, but
doesn't a warning in dmesg suffice?
It also makes it easier to port applications from open("/dev/*random"),
read(...) to getentropy() by reusing the same limits.
I would vote for warning (at about 256 bytes) + no limit.
Thanks,
Hannes
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