Random number generation bugs

[jsbarber]

Issue 1:
There is code in init_random_seed that checks if HAVE_GETRANDOM is defined and, when set, calls getrandom to fill a buffer with random data. However, the code then bypasses the code block that uses the buffer to create the actual seed value from the random data. It then calls bro_srandom with a seed value of 0 and a "true" second argument (which causes bro_rand_determistic to be set). Hence, the seed is always set the same. See https://try.zeek.org/#/tryzeek/saved/440256:

# Issues 1 - 3
event zeek_init()
	{
	local i = 0;
	while (i < 20)
		{
		print rand(65535);
		i += 1;
		}
	}

Issue 2:
I presume this would be considered by design, but bro_rand_deterministic always get set and hence the system random function is never used; instead the built-in bro_prng is used. (Which, at a minimum contradicts the documentation.)

Issue 3:
When the seed is 0, bro_prng returns the same one value (0x7fffffff) continuously.

Issue 4:
bro_prng also violates the constraint that its return value should be less than or equal to the system RAND_MAX. This in turn causes the BiF rand function to violate its specification: it can return a value greater than the max argument. (Due to issue 3, this only occurs if you re-seed the generator.)

For issue 4, https://try.zeek.org/#/tryzeek/saved/440259:

event zeek_init()
	{
	srand(0x7d662799A);
	local i = 0;
	while (i < 40)
		{
		local rn = rand(65535);
		if (rn > 65535)
			print "ACK", rn;
		i += 1;
		}
	}

[jsiwek]

@jsbarber Thanks for reporting those, #1081 should address the issues if you'd like to confirm. Particularly:

• Issues 1 + 2: dba7643
• Issue 3: 887b53b
• Issue 4: 0f4eb9a

[jsbarber]

@jsiwek Looks good to me. Thanks!