GH-1076: Fix use of getrandom()

The availability and use of getrandom() actually caused unrandom and
deterministic results in terms of Zeek's random number generation.

src/util.cc

@@ -1105,16 +1105,15 @@ void init_random_seed(const char* read_file, const char* write_file,
 	else if ( use_empty_seeds )
 		seeds_done = true;
 
-#ifdef HAVE_GETRANDOM
-	if ( ! seeds_done )
-		{
-		ssize_t nbytes = getrandom(buf.data(), sizeof(buf), 0);
-		seeds_done = nbytes == ssize_t(sizeof(buf));
-		}
-#endif
-
 	if ( ! seeds_done )
 		{
+#ifdef HAVE_GETRANDOM
+		// getrandom() guarantees reads up to 256 bytes are always successful,
+		assert(sizeof(buf) < 256);
+		auto nbytes = getrandom(buf.data(), sizeof(buf), 0);
+		assert(nbytes == sizeof(buf));
+		pos += nbytes;
+#else
 		// Gather up some entropy.
 		gettimeofday((struct timeval *)(buf.data() + pos), 0);
 		pos += sizeof(struct timeval) / sizeof(uint32_t);
@@ -1141,9 +1140,10 @@ void init_random_seed(const char* read_file, const char* write_file,
 				// systems due to a lack of entropy.
 				errno = 0;
 			}
+#endif
 
 		if ( pos < KeyedHash::SEED_INIT_SIZE )
-			reporter->FatalError("Could not read enough random data from /dev/urandom. Wanted %d, got %lu", KeyedHash::SEED_INIT_SIZE, pos);
+			reporter->FatalError("Could not read enough random data. Wanted %d, got %lu", KeyedHash::SEED_INIT_SIZE, pos);
 
 		if ( ! seed )
 			{

testing/btest/Baseline/bifs.rand/out.3

@@ -0,0 +1 @@
+F

testing/btest/bifs/rand.zeek

@@ -1,13 +1,19 @@
 #
 # @TEST-EXEC: zeek -b %INPUT >out
 # @TEST-EXEC: zeek -b %INPUT do_seed=F >out.2
+# @TEST-EXEC: unset ZEEK_SEED_FILE && zeek -b %INPUT real_random=T >out.3
 # @TEST-EXEC: btest-diff out
 # @TEST-EXEC: btest-diff out.2
+# @TEST-EXEC: btest-diff out.3
 
 const do_seed = T &redef;
+const real_random = F &redef;
 
 event zeek_init()
 	{
+	if ( real_random )
+		return;
+
 	local a = rand(1000);
 	local b = rand(1000);
 	local c = rand(1000);
@@ -27,3 +33,38 @@ event zeek_init()
 	print e;
 	print f;
 	}
+
+event zeek_init() &priority=-10
+	{
+	if ( ! real_random )
+		return;
+
+	local v1: vector of count = vector();
+	local v2: vector of count = vector();
+	local i = 0;
+
+	while ( i < 20 )
+		{
+		v1 += rand(65535);
+		i += 1;
+		}
+
+	i = 0;
+
+	while ( i < 20 )
+		{
+		v2 += rand(65535);
+		i += 1;
+		}
+
+	# Note: this is expected to be F with high probability, but
+	# technically could all be the same because, well, that's a
+	# valid "random" sequence, too
+	print all_set(v1 == v2);
+
+	if ( all_set(v1 == v2) )
+		{
+		print v1;
+		print v2;
+		}
+	}