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>
This commit is contained in:
Daniel Borkmann 2014-01-22 02:29:39 +01:00 committed by David S. Miller
parent 6cd28f044b
commit f337db64af
4 changed files with 20 additions and 17 deletions

View File

@ -13,20 +13,14 @@
#include <linux/module.h> #include <linux/module.h>
#include <linux/init.h> #include <linux/init.h>
#include <linux/skbuff.h> #include <linux/skbuff.h>
#include <linux/reciprocal_div.h>
#include <linux/if_team.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) static bool rnd_transmit(struct team *team, struct sk_buff *skb)
{ {
struct team_port *port; struct team_port *port;
int port_index; 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); port = team_get_port_by_index_rcu(team, port_index);
if (unlikely(!port)) if (unlikely(!port))
goto drop; goto drop;

View File

@ -8,7 +8,6 @@
#include <uapi/linux/random.h> #include <uapi/linux/random.h>
extern void add_device_randomness(const void *, unsigned int); extern void add_device_randomness(const void *, unsigned int);
extern void add_input_randomness(unsigned int type, unsigned int code, extern void add_input_randomness(unsigned int type, unsigned int code,
unsigned int value); unsigned int value);
@ -38,6 +37,23 @@ struct rnd_state {
u32 prandom_u32_state(struct rnd_state *state); u32 prandom_u32_state(struct rnd_state *state);
void prandom_bytes_state(struct rnd_state *state, void *buf, int nbytes); 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 * Handle minimum values for seeds
*/ */

View File

@ -1289,7 +1289,7 @@ static unsigned int fanout_demux_rnd(struct packet_fanout *f,
struct sk_buff *skb, struct sk_buff *skb,
unsigned int num) unsigned int num)
{ {
return reciprocal_divide(prandom_u32(), num); return prandom_u32_max(num);
} }
static unsigned int fanout_demux_rollover(struct packet_fanout *f, static unsigned int fanout_demux_rollover(struct packet_fanout *f,

View File

@ -14,7 +14,6 @@
#include <linux/types.h> #include <linux/types.h>
#include <linux/kernel.h> #include <linux/kernel.h>
#include <linux/skbuff.h> #include <linux/skbuff.h>
#include <linux/reciprocal_div.h>
#include <linux/vmalloc.h> #include <linux/vmalloc.h>
#include <net/pkt_sched.h> #include <net/pkt_sched.h>
#include <net/inet_ecn.h> #include <net/inet_ecn.h>
@ -77,12 +76,6 @@ struct choke_sched_data {
struct sk_buff **tab; 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 */ /* number of elements in queue including holes */
static unsigned int choke_len(const struct choke_sched_data *q) 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; int retrys = 3;
do { 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]; skb = q->tab[*pidx];
if (skb) if (skb)
return skb; return skb;