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Re: [PATCH] Use getentropy() for seeding PRNG


On Mon, Aug 13, 2018 at 5:36 PM, Fritz Reese <fritzoreese@gmail.com> wrote:

> On Fri, Aug 3, 2018 at 9:19 AM Janne Blomqvist
> <blomqvist.janne@gmail.com> wrote:
> >
> > The getentropy function, found on Linux, OpenBSD, and recently also
> > FreeBSD, can be used to get random bytes to initialize the PRNG.  It
> > is similar to the traditional way of reading from /dev/urandom, but
> > being a system call rather than a special file, it doesn't suffer from
> > problems like running out of file descriptors, or failure when running
> > in a container where /dev/urandom is not available.
> >
> > Regtested on x86_64-pc-linux-gnu, Ok for trunk?
>
> Actually, getentropy() is similar to reading from /dev/random, where
> getrandom() is similar to reading from /dev/urandom.


No, getentropy is similar to getrandom with the flags argument == 0. Which
is similar to reading /dev/urandom, except that just after boot if enough
entropy hasn't yet been gathered, it may block instead of returning some
not-quite-random data. But once it has been initialized, it will never
block again.

I agree that reading from /dev/random is overkill, but this patch isn't
doing the equivalent of that.


> Since the
> original behavior of getosrandom() is to read from /dev/urandom, I
> think it is better to use getrandom() for consistent semantics.
>
> Furthermore, getentropy() may block to achieve an appropriate degree
> of randomness, since it is intended for secure use.


The only time this might happen is just after boot, after that the entropy
never drains (in contrast to /dev/random). So unless you're planning to
write an init daemon in Fortran, this shouldn't matter.



-- 
Janne Blomqvist


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