Commit f337db64 authored by Daniel Borkmann's avatar Daniel Borkmann Committed by David S. Miller

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 <>
Cc: Eric Dumazet <>
Signed-off-by: default avatarHannes Frederic Sowa <>
Signed-off-by: default avatarDaniel Borkmann <>
Signed-off-by: default avatarDavid S. Miller <>
parent 6cd28f04
......@@ -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;
......@@ -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
......@@ -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,
......@@ -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;
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