sch_netem: Bug fixing in calculating Netem interval

In Kernel 4.15.0+, Netem does not work properly.

Netem setup:

tc qdisc add dev h1-eth0 root handle 1: netem delay 10ms 2ms

Result:

PING 172.16.101.2 (172.16.101.2) 56(84) bytes of data.
64 bytes from 172.16.101.2: icmp_seq=1 ttl=64 time=22.8 ms
64 bytes from 172.16.101.2: icmp_seq=2 ttl=64 time=10.9 ms
64 bytes from 172.16.101.2: icmp_seq=3 ttl=64 time=10.9 ms
64 bytes from 172.16.101.2: icmp_seq=5 ttl=64 time=11.4 ms
64 bytes from 172.16.101.2: icmp_seq=6 ttl=64 time=11.8 ms
64 bytes from 172.16.101.2: icmp_seq=4 ttl=64 time=4303 ms
64 bytes from 172.16.101.2: icmp_seq=10 ttl=64 time=11.2 ms
64 bytes from 172.16.101.2: icmp_seq=11 ttl=64 time=10.3 ms
64 bytes from 172.16.101.2: icmp_seq=7 ttl=64 time=4304 ms
64 bytes from 172.16.101.2: icmp_seq=8 ttl=64 time=4303 ms

Patch:

(rnd % (2 * sigma)) - sigma was overflowing s32. After applying the
patch, I found following output which is desirable.

PING 172.16.101.2 (172.16.101.2) 56(84) bytes of data.
64 bytes from 172.16.101.2: icmp_seq=1 ttl=64 time=21.1 ms
64 bytes from 172.16.101.2: icmp_seq=2 ttl=64 time=8.46 ms
64 bytes from 172.16.101.2: icmp_seq=3 ttl=64 time=9.00 ms
64 bytes from 172.16.101.2: icmp_seq=4 ttl=64 time=11.8 ms
64 bytes from 172.16.101.2: icmp_seq=5 ttl=64 time=8.36 ms
64 bytes from 172.16.101.2: icmp_seq=6 ttl=64 time=11.8 ms
64 bytes from 172.16.101.2: icmp_seq=7 ttl=64 time=8.11 ms
64 bytes from 172.16.101.2: icmp_seq=8 ttl=64 time=10.0 ms
64 bytes from 172.16.101.2: icmp_seq=9 ttl=64 time=11.3 ms
64 bytes from 172.16.101.2: icmp_seq=10 ttl=64 time=11.5 ms
64 bytes from 172.16.101.2: icmp_seq=11 ttl=64 time=10.2 ms

Reviewed-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/net/sched/sch_netem.c b/net/sched/sch_netem.c
index 7bbc13b8ca47f46ea1ae05e1568a72792a9cea2a..7c179addebcd2967e9fb9c280d8bbe64e33ebe5d 100644 (file)
--- a/net/sched/sch_netem.c
+++ b/net/sched/sch_netem.c
@@ -327,7 +327,7 @@ static s64 tabledist(s64 mu, s32 sigma,
 
        /* default uniform distribution */
        if (dist == NULL)
-               return (rnd % (2 * sigma)) - sigma + mu;
+               return ((rnd % (2 * sigma)) + mu) - sigma;
 
        t = dist->table[rnd % dist->size];
        x = (sigma % NETEM_DIST_SCALE) * t;