From: George Spelvin lkml@sdf.org
[ Upstream commit fd0c42c4dea54335967c5a86f15fc064235a2797 ]
and change to pseudorandom numbers, as this is a traffic dithering operation that doesn't need crypto-grade.
The previous code operated in 4 steps:
1. Generate a random byte 0 <= rand_tq <= 255 2. Multiply it by BATADV_TQ_MAX_VALUE - tq 3. Divide by 255 (= BATADV_TQ_MAX_VALUE) 4. Return BATADV_TQ_MAX_VALUE - rand_tq
This would apperar to scale (BATADV_TQ_MAX_VALUE - tq) by a random value between 0/255 and 255/255.
But! The intermediate value between steps 3 and 4 is stored in a u8 variable. So it's truncated, and most of the time, is less than 255, after which the division produces 0. Specifically, if tq is odd, the product is always even, and can never be 255. If tq is even, there's exactly one random byte value that will produce a product byte of 255.
Thus, the return value is 255 (511/512 of the time) or 254 (1/512 of the time).
If we assume that the truncation is a bug, and the code is meant to scale the input, a simpler way of looking at it is that it's returning a random value between tq and BATADV_TQ_MAX_VALUE, inclusive.
Well, we have an optimized function for doing just that.
Fixes: 3c12de9a5c75 ("batman-adv: network coding - code and transmit packets if possible") Signed-off-by: George Spelvin lkml@sdf.org Signed-off-by: Sven Eckelmann sven@narfation.org Signed-off-by: Simon Wunderlich sw@simonwunderlich.de Signed-off-by: Sasha Levin sashal@kernel.org --- net/batman-adv/network-coding.c | 9 +-------- 1 file changed, 1 insertion(+), 8 deletions(-)
diff --git a/net/batman-adv/network-coding.c b/net/batman-adv/network-coding.c index a7b5cf08d3630..09549885cd147 100644 --- a/net/batman-adv/network-coding.c +++ b/net/batman-adv/network-coding.c @@ -1012,15 +1012,8 @@ static struct batadv_nc_path *batadv_nc_get_path(struct batadv_priv *bat_priv, */ static u8 batadv_nc_random_weight_tq(u8 tq) { - u8 rand_val, rand_tq; - - get_random_bytes(&rand_val, sizeof(rand_val)); - /* randomize the estimated packet loss (max TQ - estimated TQ) */ - rand_tq = rand_val * (BATADV_TQ_MAX_VALUE - tq); - - /* normalize the randomized packet loss */ - rand_tq /= BATADV_TQ_MAX_VALUE; + u8 rand_tq = prandom_u32_max(BATADV_TQ_MAX_VALUE + 1 - tq);
/* convert to (randomized) estimated tq again */ return BATADV_TQ_MAX_VALUE - rand_tq;