random32: add prandom_u32_max and convert open coded users

Many functions have open coded a function that returns a random
number in range [0,N-1]. Under the assumption that we have a PRNG
such as taus113 with being well distributed in [0, ~0U] space,
we can implement such a function as uword t = (n*m')>>32, where
m' is a random number obtained from PRNG, n the right open interval
border and t our resulting random number, with n,m',t in u32 universe.

Lets go with Joe and simply call it prandom_u32_max(), although
technically we have an right open interval endpoint, but that we
have documented. Other users can further be migrated to the new
prandom_u32_max() function later on; for now, we need to make sure
to migrate reciprocal_divide() users for the reciprocal_divide()
follow-up fixup since their function signatures are going to change.

Joint work with Hannes Frederic Sowa.

Cc: Jakub Zawadzki <darkjames-ws@darkjames.pl>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/drivers/net/team/team_mode_random.c b/drivers/net/team/team_mode_random.c
index 7f032e2113437f1ca6b2952ba4ccc28e56dd20ba..cd2f692b8074e6b543a511b5f76a775a7ab17f47 100644 (file)
--- a/drivers/net/team/team_mode_random.c
+++ b/drivers/net/team/team_mode_random.c
@@ -13,20 +13,14 @@
 #include <linux/module.h>
 #include <linux/init.h>
 #include <linux/skbuff.h>
-#include <linux/reciprocal_div.h>
 #include <linux/if_team.h>
 
-static u32 random_N(unsigned int N)
-{
-       return reciprocal_divide(prandom_u32(), N);
-}
-
 static bool rnd_transmit(struct team *team, struct sk_buff *skb)
 {
        struct team_port *port;
        int port_index;
 
-       port_index = random_N(team->en_port_count);
+       port_index = prandom_u32_max(team->en_port_count);
        port = team_get_port_by_index_rcu(team, port_index);
        if (unlikely(!port))
                goto drop;

diff --git a/include/linux/random.h b/include/linux/random.h
index 4002b3df4c8566d4497a9dae222e6fc1e6e37b66..1cfce0e24dbdb09a527d914ab511e6b6280fa364 100644 (file)
--- a/include/linux/random.h
+++ b/include/linux/random.h
@@ -8,7 +8,6 @@
 
 #include <uapi/linux/random.h>
 
-
 extern void add_device_randomness(const void *, unsigned int);
 extern void add_input_randomness(unsigned int type, unsigned int code,
                                 unsigned int value);
@@ -38,6 +37,23 @@ struct rnd_state {
 u32 prandom_u32_state(struct rnd_state *state);
 void prandom_bytes_state(struct rnd_state *state, void *buf, int nbytes);
 
+/**
+ * prandom_u32_max - returns a pseudo-random number in interval [0, ep_ro)
+ * @ep_ro: right open interval endpoint
+ *
+ * Returns a pseudo-random number that is in interval [0, ep_ro). Note
+ * that the result depends on PRNG being well distributed in [0, ~0U]
+ * u32 space. Here we use maximally equidistributed combined Tausworthe
+ * generator, that is, prandom_u32(). This is useful when requesting a
+ * random index of an array containing ep_ro elements, for example.
+ *
+ * Returns: pseudo-random number in interval [0, ep_ro)
+ */
+static inline u32 prandom_u32_max(u32 ep_ro)
+{
+       return (u32)(((u64) prandom_u32() * ep_ro) >> 32);
+}
+
 /*
  * Handle minimum values for seeds
  */

diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
index 59fb3db2e71333074c106449b154ddbbc2ed814a..df3cbdd585c7ea613c4a367694c08108c5830fdd 100644 (file)
--- a/net/packet/af_packet.c
+++ b/net/packet/af_packet.c
@@ -1289,7 +1289,7 @@ static unsigned int fanout_demux_rnd(struct packet_fanout *f,
                                     struct sk_buff *skb,
                                     unsigned int num)
 {
-       return reciprocal_divide(prandom_u32(), num);
+       return prandom_u32_max(num);
 }
 
 static unsigned int fanout_demux_rollover(struct packet_fanout *f,

diff --git a/net/sched/sch_choke.c b/net/sched/sch_choke.c
index ddd73cb2d7ba4080f0e8f75ddc715cf9cb2a1587..2aee02802c2760fafabb47d6bf4229d08d8ef907 100644 (file)
--- a/net/sched/sch_choke.c
+++ b/net/sched/sch_choke.c
@@ -14,7 +14,6 @@
 #include <linux/types.h>
 #include <linux/kernel.h>
 #include <linux/skbuff.h>
-#include <linux/reciprocal_div.h>
 #include <linux/vmalloc.h>
 #include <net/pkt_sched.h>
 #include <net/inet_ecn.h>
@@ -77,12 +76,6 @@ struct choke_sched_data {
        struct sk_buff **tab;
 };
 
-/* deliver a random number between 0 and N - 1 */
-static u32 random_N(unsigned int N)
-{
-       return reciprocal_divide(prandom_u32(), N);
-}
-
 /* number of elements in queue including holes */
 static unsigned int choke_len(const struct choke_sched_data *q)
 {
@@ -233,7 +226,7 @@ static struct sk_buff *choke_peek_random(const struct choke_sched_data *q,
        int retrys = 3;
 
        do {
-               *pidx = (q->head + random_N(choke_len(q))) & q->tab_mask;
+               *pidx = (q->head + prandom_u32_max(choke_len(q))) & q->tab_mask;
                skb = q->tab[*pidx];
                if (skb)
                        return skb;