2006-08-19 00:35:19

by Josh Triplett

Subject: [PATCH] rcu: Fix sign bug making rcu_random always return the same sequence

rcu_random uses a counter rrs_count to occasionally mix data from
get_random_bytes into the state of its pseudorandom generator. However, the
rrs_counter gets declared as an unsigned long, and rcu_random checks for
--rrs_count < 0, so this code will never mix any real random data into the
state, and will thus always return the same sequence of random numbers.

Also, change the return value of rcu_random from long to unsigned long, to
avoid potential issues caused by the use of the % operator, which can return
negative values for negative left operands.

Signed-off-by: Josh Triplett <[email protected]>
---
kernel/rcutorture.c | 4 ++--
1 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/rcutorture.c b/kernel/rcutorture.c
index 8b09c95..ef6a124 100644
--- a/kernel/rcutorture.c
+++ b/kernel/rcutorture.c
@@ -146,7 +146,7 @@ rcu_torture_free(struct rcu_torture *p)

struct rcu_random_state {
unsigned long rrs_state;
- unsigned long rrs_count;
+ long rrs_count;
};

#define RCU_RANDOM_MULT 39916801 /* prime */
@@ -159,7 +159,7 @@ #define DEFINE_RCU_RANDOM(name) struct r
* Crude but fast random-number generator. Uses a linear congruential
* generator, with occasional help from get_random_bytes().
*/
-static long
+static unsigned long
rcu_random(struct rcu_random_state *rrsp)
{
long refresh;
--
1.4.1.1

2006-08-19 00:49:58

by Paul E. McKenney

Subject: Re: [PATCH] rcu: Fix sign bug making rcu_random always return the same sequence

On Fri, Aug 18, 2006 at 05:35:17PM -0700, Josh Triplett wrote:
> rcu_random uses a counter rrs_count to occasionally mix data from
> get_random_bytes into the state of its pseudorandom generator. However, the
> rrs_counter gets declared as an unsigned long, and rcu_random checks for
> --rrs_count < 0, so this code will never mix any real random data into the
> state, and will thus always return the same sequence of random numbers.
>
> Also, change the return value of rcu_random from long to unsigned long, to
> avoid potential issues caused by the use of the % operator, which can return
> negative values for negative left operands.

Acked-by: Paul E. McKenney <[email protected]>
> Signed-off-by: Josh Triplett <[email protected]>
> ---
> kernel/rcutorture.c | 4 ++--
> 1 files changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/rcutorture.c b/kernel/rcutorture.c
> index 8b09c95..ef6a124 100644
> --- a/kernel/rcutorture.c
> +++ b/kernel/rcutorture.c
> @@ -146,7 +146,7 @@ rcu_torture_free(struct rcu_torture *p)
>
> struct rcu_random_state {
> unsigned long rrs_state;
> - unsigned long rrs_count;
> + long rrs_count;
> };
>
> #define RCU_RANDOM_MULT 39916801 /* prime */
> @@ -159,7 +159,7 @@ #define DEFINE_RCU_RANDOM(name) struct r
> * Crude but fast random-number generator. Uses a linear congruential
> * generator, with occasional help from get_random_bytes().
> */
> -static long
> +static unsigned long
> rcu_random(struct rcu_random_state *rrsp)
> {
> long refresh;
> --
> 1.4.1.1
>
>