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

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Date:	Fri, 18 Aug 2006 17:50:46 -0700
From:	"Paul E. McKenney" <paulmck@...ibm.com>
To:	Josh Triplett <josht@...ibm.com>
Cc:	linux-kernel@...r.kernel.org, Andrew Morton <akpm@...l.org>,
	Dipkanar Sarma <dipankar@...ibm.com>
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 <paulmck@...ibm.com>
> Signed-off-by: Josh Triplett <josh@...edesktop.org>
> ---
>  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
> 
> 
-
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