Re: getrandom and getentropy

[Kurt Roeckx <kurt%roeckx.be@localhost>]

On Sat, May 02, 2020 at 03:38:43PM +0000, Taylor R Campbell wrote:
> > Date: Sat, 2 May 2020 11:10:44 +0200
> > From: Kurt Roeckx <kurt%roeckx.be@localhost>
> > 
> > On Fri, May 01, 2020 at 07:19:09PM +0000, Taylor R Campbell wrote:
> > > 
> > > The alias getentropy(p,n) := getrandom(p,n,GRND_INSECURE)
> > 
> > At several places in your document you imply this. But
> > getentropy(p,n) is more like getrandom(p,n,0). That is, it also
> > waits until it's seeded, it only blocks a single time.
> > 
> > I hink we've previously talked about it, and you said the OpenBSD
> > manpage doesn't mention anything related to it. But it's implied
> > behaviour for OpenBSD, they never had an interface where you can
> > get random numbers before it's properly seeded.
> 
> I reviewed the OpenBSD implementation at
> 
> https://cvsweb.openbsd.org/src/sys/dev/rnd.c?rev=1.204&content-type=text/x-cvsweb-markup
> 
> and I see no evidence of blocking.  Where does it block?
> 
> On OpenBSD, /dev/random and /dev/urandom are the same -- they both
> never block, according to <https://man.openbsd.org/urandom>.
> 
> I reviewed the OpenBSD commit logs, and I don't see any evidence of
> any blocking paths since 2010 when /dev/srandom was removed:
> 
> https://cvsweb.openbsd.org/src/sys/dev/rnd.c?rev=1.103&content-type=text/x-cvsweb-markup
> 
> Generally whether something can block or not is an important part of
> the API contract, and it seems to me that from the beginning when
> OpenBSD introduced getentropy it has never blocked.

It's my understanding that it never blocks because the bootloader
provides entropy. Be time time the first user can call genentropy,
it has already been seeded.


Kurt