Tweak the new evutil_weakrand_() code

Make its state actually get seeded.

Document it more thoroughly.

Turn its state into a structure.

Fix a bug in evutil_weakrand_range_() where it could return the top of
the range.

Change its return type to ev_int32_t.

Add a quick unit test to make sure that the value of
evutil_weakrand_range_() is in range.

bufferevent-internal.h

@@ -105,8 +105,8 @@ struct bufferevent_rate_limit_group {
 	 * to refill. */
 	struct event master_refill_event;
 
-	/** Seed for weak random number generator. */
+	/** Seed for weak random number generator. Protected by 'lock' */
-	ev_uint32_t weakrand_seed;
+	struct evutil_weakrand_state weakrand_seed;
 
 	/** Lock to protect the members of this group.  This lock should nest
 	 * within every bufferevent lock: if you are holding this lock, do

bufferevent_ratelim.c

@@ -438,7 +438,10 @@ bev_refill_callback_(evutil_socket_t fd, short what, void *arg)
 	BEV_UNLOCK(&bev->bev);
 }
 
-/** Helper: grab a random element from a bufferevent group. */
+/** Helper: grab a random element from a bufferevent group.
+ *
+ * Requires that we hold the lock on the group.
+ */
 static struct bufferevent_private *
 bev_group_random_element_(struct bufferevent_rate_limit_group *group)
 {
@@ -660,6 +663,9 @@ bufferevent_rate_limit_group_new(struct event_base *base,
 
 	bufferevent_rate_limit_group_set_min_share(g, 64);
 
+	evutil_weakrand_seed_(&g->weakrand_seed,
+	    (ev_uint32_t) ((now.tv_sec + now.tv_usec) + (ev_intptr_t)g));
+
 	return g;
 }
 

event-internal.h

@@ -291,8 +291,9 @@ struct event_base {
 	/** A function used to wake up the main thread from another thread. */
 	int (*th_notify_fn)(struct event_base *base);
 
-	/* Saved seed for weak random number generator. */
+	/** Saved seed for weak random number generator. Some backends use
-	ev_uint32_t weakrand_seed;
+	 * this to produce fairness among sockets. Protected by th_base_lock. */
+	struct evutil_weakrand_state weakrand_seed;
 };
 
 struct event_config_entry {

evutil.c

@@ -2274,21 +2274,50 @@ evutil_getenv_(const char *varname)
 }
 
 ev_uint32_t
-evutil_weakrand_(ev_uint32_t* seed)
+evutil_weakrand_seed_(struct evutil_weakrand_state *state, ev_uint32_t seed)
 {
-	*seed = ((*seed) * 1103515245 + 12345) & 0x7fffffff;
+	if (seed == 0) {
-	return (*seed);
+		struct timeval tv;
+		evutil_gettimeofday(&tv, NULL);
+		seed = (ev_uint32_t)tv.tv_sec + (ev_uint32_t)tv.tv_usec;
+#ifdef _WIN32
+		seed += (ev_uint32_t) _getpid();
+#else
+		seed += (ev_uint32_t) getpid();
+#endif
+	}
+	state->seed = seed;
+	return seed;
 }
 
-ev_uint32_t
+ev_int32_t
-evutil_weakrand_range_(ev_uint32_t* seed, ev_uint32_t top)
+evutil_weakrand_(struct evutil_weakrand_state *state)
 {
-	ev_uint32_t divisor, result;
+	/* This RNG implementation is a linear congruential generator, with
+	 * modulus 2^31, multiplier 1103515245, and addend 12345.  It's also
+	 * used by OpenBSD, and by Glibc's TYPE_0 RNG.
+	 *
+	 * The linear congruential generator is not an industrial-strength
+	 * RNG!  It's fast, but it can have higher-order patterns.  Notably,
+	 * the low bits tend to have periodicity.
+	 */
+	state->seed = ((state->seed) * 1103515245 + 12345) & 0x7fffffff;
+	return (ev_int32_t)(state->seed);
+}
 
-	divisor = EV_INT32_MAX / top;
+ev_int32_t
-	do
+evutil_weakrand_range_(struct evutil_weakrand_state *state, ev_int32_t top)
-		result = evutil_weakrand_(seed) / divisor;
+{
-	while (result > top);
+	ev_int32_t divisor, result;
+
+	/* We can't just do weakrand() % top, since the low bits of the LCG
+	 * are less random than the high ones.  (Specifically, since the LCG
+	 * modulus is 2^N, every 2^m for m<N will divide the modulus, and so
+	 * therefore the low m bits of the LCG will have period 2^m.) */
+	divisor = EVUTIL_WEAKRAND_MAX / top;
+	do {
+		result = evutil_weakrand_(state) / divisor;
+	} while (result >= top);
 	return result;
 }
 

poll.c

@@ -93,6 +93,8 @@ poll_init(struct event_base *base)
 
 	evsig_init_(base);
 
+	evutil_weakrand_seed_(&base->weakrand_seed, 0);
+
 	return (pollop);
 }
 

select.c

@@ -121,6 +121,8 @@ select_init(struct event_base *base)
 
 	evsig_init_(base);
 
+	evutil_weakrand_seed_(&base->weakrand_seed, 0);
+
 	return (sop);
 }
 

test/regress_buffer.c

@@ -699,7 +699,7 @@ test_evbuffer_add_file(void *ptr)
 	struct event *rev=NULL, *wev=NULL;
 	struct event_base *base = testdata->base;
 	evutil_socket_t pair[2] = {-1, -1};
-	static ev_uint32_t seed = 123456789U;
+	struct evutil_weakrand_state seed = { 123456789U };
 
 	/* This test is highly parameterized based on substrings of its
 	 * argument.  The strings are: */

test/regress_util.c

@@ -829,7 +829,7 @@ test_evutil_rand(void *arg)
 	char buf2[32];
 	int counts[256];
 	int i, j, k, n=0;
-	static ev_uint32_t seed = 12346789U;
+	struct evutil_weakrand_state seed = { 12346789U };
 
 	memset(buf2, 0, sizeof(buf2));
 	memset(counts, 0, sizeof(counts));
@@ -869,6 +869,13 @@ test_evutil_rand(void *arg)
 		}
 	}
 
+	evutil_weakrand_seed_(&seed, 0);
+	for (i = 0; i < 10000; ++i) {
+		ev_int32_t r = evutil_weakrand_range_(&seed, 9999);
+		tt_int_op(0, <=, r);
+		tt_int_op(r, <, 9999);
+	}
+
 	/* for (i=0;i<256;++i) { printf("%3d %2d\n", i, counts[i]); } */
 end:
 	;

util-internal.h

@@ -254,8 +254,33 @@ int evutil_resolve_(int family, const char *hostname, struct sockaddr *sa,
 
 const char *evutil_getenv_(const char *name);
 
-ev_uint32_t evutil_weakrand_(ev_uint32_t* seed);
+/* Structure to hold the state of our weak random number generator.
-ev_uint32_t evutil_weakrand_range_(ev_uint32_t* seed, ev_uint32_t top);
+ */
+struct evutil_weakrand_state {
+	ev_uint32_t seed;
+};
+
+#define EVUTIL_WEAKRAND_MAX EV_INT32_MAX
+
+/* Initialize the state of a week random number generator based on 'seed'.  If
+ * the seed is 0, construct a new seed based on not-very-strong platform
+ * entropy, like the PID and the time of day.
+ *
+ * This function, and the other evutil_weakrand* functions, are meant for
+ * speed, not security or statistical strength.  If you need a RNG which an
+ * attacker can't predict, or which passes strong statistical tests, use the
+ * evutil_secure_rng* functions instead.
+ */
+ev_uint32_t evutil_weakrand_seed_(struct evutil_weakrand_state *state, ev_uint32_t seed);
+/* Return a pseudorandom value between 0 and EVUTIL_WEAKRAND_MAX inclusive.
+ * Updates the state in 'seed' as needed -- this value must be protected by a
+ * lock.
+ */
+ev_int32_t evutil_weakrand_(struct evutil_weakrand_state *seed);
+/* Return a pseudorandom value x such that 0 <= x < top. top must be no more
+ * than EVUTIL_WEAKRAND_MAX. Updates the state in 'seed' as needed -- this
+ * value must be proteced by a lock */
+ev_int32_t evutil_weakrand_range_(struct evutil_weakrand_state *seed, ev_int32_t top);
 
 /* Evaluates to the same boolean value as 'p', and hints to the compiler that
  * we expect this value to be false. */

win32select.c

@@ -202,6 +202,8 @@ win32_init(struct event_base *base)
 	if (evsig_init_(base) < 0)
 		winop->signals_are_broken = 1;
 
+	evutil_weakrand_seed_(&base->weakrand_seed, 0);
+
 	return (winop);
  err:
 	XFREE(winop->readset_in);