Re: random(3) in OpenBSD

[Corinna Vinschen]

Hi Bruno,

On Nov 14 19:33, Bruno Haible wrote:
> [CCing bug-gnulib]
> 
> Hi Corinna,
> 
> > Looking into all the BSD variants of random(), I found that OpenBSD
> > defaults the random() function to return non-deterministic output.  It
> > works like this: 
> > 
> > - A global var random_deterministic defaults to 0.
> > - Calls to srandom() set random_deterministic to 0 and ignore their
> >   input.
> > - Calls to srandom_deterministic() sets random_deterministic to 1 and
> >   seeds the usual PRNG.
> > - initstate() and setstate() set random_deterministic to 1.
> > - random() returns either the next value from the PRNG if
> >   random_deterministic is 1, or the next value generated by a call to
> >   arc4random() if random_deterministic is 0.  So by default, it
> >   returns arc4random() values.
> > 
> > With that behaviour, OpenBSD appears to stray from POSIX deliberately.
> 
> Yup, I observed this too when I found that the gnulib MT test for random()
> yielded strange results on OpenBSD. [1]
> 
> > I was wondering if you want to add this as incompatibility to your
> > document at
> > https://git.savannah.gnu.org/gitweb/?p=gnulib.git;a=blob;f=doc/posix-functions/random.texi
> > perhaps.
> 
> I documented it as an incompatibility for the srandom() function. But you are
> right, it's also an incompatibility for the random() function, since POSIX [2]
> says "random() shall produce by default a sequence of numbers that can be
> duplicated by calling srandom() with 1 as the seed".

Yes, that's the problem.  However, I think your addition to random.texi
isn't quite right.

> +When @samp{srandom} was not called, this function returns a non-deterministic
> +sequence rather than a deterministic sequence of numbers on some platforms:
> +OpenBSD 7.4.

In fact, as I outlined above, srandom(3) does *not* initiate a seed for
the deterministic PRNG.  It just sets the global var random_deterministic
to 0.

In turn, random(3) will call arc4random(), which typically generates
non-deterministic values from getentropy(3) or from /dev/random.

An application which requires deterministic, reproducible values from
a PRNG has to call initstate(), setstate(), or the OpenBSD-specific
function srandom_deterministic().


HTH,
Corinna