Re: [PATCH 3/4] ipv4: igmp: encode multicast exponential fields

[Nikolay Aleksandrov]

On 26/03/2026 17:07, Ujjal Roy wrote:

In IGMP, QQIC and MRC fields are not currently encoded
when generating query packets. Since the receiver of the
query interprets these fields using the IGMPv3 floating-
point decoding logic, any raw interval value that exceeds
the linear threshold is currently parsed incorrectly as
an exponential value, leading to an incorrect interval
calculation.

Encode and assign the corresponding protocol fields during
query generation. Introduce the logic to dynamically
calculate the exponent and mantissa using bit-scan (fls).
This ensures QQIC and MRC fields (8-bit) are properly
encoded when transmitting query packets with intervals
that exceed their respective linear threshold value of
128 (for QQI/MRT).

RFC 3376: if QQIC/MRC >= 128, the QQIC/MRC field represents
a floating-point value as follows:
      0 1 2 3 4 5 6 7
     +-+-+-+-+-+-+-+-+
     |1| exp | mant  |
     +-+-+-+-+-+-+-+-+

Signed-off-by: Ujjal Roy <royuj...@gmail.com>
---
  include/linux/igmp.h      | 80 +++++++++++++++++++++++++++++++++++++++
  net/bridge/br_multicast.c | 14 +++----
  2 files changed, 86 insertions(+), 8 deletions(-)

[snip]

diff --git a/net/bridge/br_multicast.c b/net/bridge/br_multicast.c
index 1438c023db62..1de6242413e0 100644
--- a/net/bridge/br_multicast.c
+++ b/net/bridge/br_multicast.c
@@ -939,7 +939,7 @@ static struct sk_buff *br_ip4_multicast_alloc_query(struct 
net_bridge_mcast *brm
        struct sk_buff *skb;
        struct igmphdr *ih;
        struct ethhdr *eth;
-       unsigned long lmqt;
+       unsigned long lmqt, mrt;
        struct iphdr *iph;
        u16 lmqt_srcs = 0;

please keep reverse xmas tree order

@@ -1004,15 +1004,15 @@ static struct sk_buff *br_ip4_multicast_alloc_query(struct net_bridge_mcast *brm

        skb_put(skb, 24);

skb_set_transport_header(skb, skb->len);

+       mrt = group ? brmctx->multicast_last_member_interval :
+                     brmctx->multicast_query_response_interval;
        *igmp_type = IGMP_HOST_MEMBERSHIP_QUERY;

switch (brmctx->multicast_igmp_version) {

        case 2:
                ih = igmp_hdr(skb);
                ih->type = IGMP_HOST_MEMBERSHIP_QUERY;
-               ih->code = (group ? brmctx->multicast_last_member_interval :
-                                   brmctx->multicast_query_response_interval) /
-                          (HZ / IGMP_TIMER_SCALE);
+               ih->code = mrt / (HZ / IGMP_TIMER_SCALE);
                ih->group = group;
                ih->csum = 0;
                csum = &ih->csum;
@@ -1021,11 +1021,9 @@ static struct sk_buff 
*br_ip4_multicast_alloc_query(struct net_bridge_mcast *brm
        case 3:
                ihv3 = igmpv3_query_hdr(skb);
                ihv3->type = IGMP_HOST_MEMBERSHIP_QUERY;
-               ihv3->code = (group ? brmctx->multicast_last_member_interval :
-                                     
brmctx->multicast_query_response_interval) /
-                            (HZ / IGMP_TIMER_SCALE);
+               ihv3->code = igmpv3_mrc(mrt / (HZ / IGMP_TIMER_SCALE));
                ihv3->group = group;
-               ihv3->qqic = brmctx->multicast_query_interval / HZ;
+               ihv3->qqic = igmpv3_qqic(brmctx->multicast_query_interval / HZ);
                ihv3->nsrcs = htons(lmqt_srcs);
                ihv3->resv = 0;
                ihv3->suppress = sflag;