[PATCH] batman-adv: fix random jitter calculation

[PATCH] batman-adv: fix random jitter calculation

[Akinobu Mita]

batadv_iv_ogm_emit_send_time() attempts to calculates a random integer
in the range of 'orig_interval +- BATADV_JITTER' by the below lines.

        msecs = atomic_read(&bat_priv->orig_interval) - BATADV_JITTER;
        msecs += (random32() % 2 * BATADV_JITTER);

But it actually gets 'orig_interval' or 'orig_interval - BATADV_JITTER'
because '%' and '*' have same precedence and associativity is
left-to-right.

This adds the parentheses at the appropriate position so that it matches
original intension.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Acked-by: Antonio Quartulli <ordex@autistici.org>
Cc: Marek Lindner <lindner_marek@yahoo.de>
Cc: Simon Wunderlich <siwu@hrz.tu-chemnitz.de>
Cc: Antonio Quartulli <ordex@autistici.org>
Cc: b.a.t.m.a.n@lists.open-mesh.org
Cc: "David S. Miller" <davem@davemloft.net>
Cc: netdev@vger.kernel.org
---

This patch is extracted from the patch series "rename random32 and
net_random to prandom" since it is a fix.

 net/batman-adv/bat_iv_ogm.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/batman-adv/bat_iv_ogm.c b/net/batman-adv/bat_iv_ogm.c
index 9f3925a..7d02ebd 100644
--- a/net/batman-adv/bat_iv_ogm.c
+++ b/net/batman-adv/bat_iv_ogm.c
@@ -123,7 +123,7 @@ batadv_iv_ogm_emit_send_time(const struct batadv_priv *bat_priv)
 	unsigned int msecs;
 
 	msecs = atomic_read(&bat_priv->orig_interval) - BATADV_JITTER;
-	msecs += (random32() % 2 * BATADV_JITTER);
+	msecs += random32() % (2 * BATADV_JITTER);
 
 	return jiffies + msecs_to_jiffies(msecs);
 }
-- 
1.7.11.7

Re: [PATCH] batman-adv: fix random jitter calculation

[David Miller]

From: Akinobu Mita <akinobu.mita@gmail.com>
Date: Wed, 26 Dec 2012 21:32:10 +0900

> batadv_iv_ogm_emit_send_time() attempts to calculates a random integer
> in the range of 'orig_interval +- BATADV_JITTER' by the below lines.
> 
>         msecs = atomic_read(&bat_priv->orig_interval) - BATADV_JITTER;
>         msecs += (random32() % 2 * BATADV_JITTER);
> 
> But it actually gets 'orig_interval' or 'orig_interval - BATADV_JITTER'
> because '%' and '*' have same precedence and associativity is
> left-to-right.
> 
> This adds the parentheses at the appropriate position so that it matches
> original intension.
> 
> Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
> Acked-by: Antonio Quartulli <ordex@autistici.org>

Applied, thanks.