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

Nikolay Aleksandrov
2026-03-27T12:19:26Z

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 <royujjal@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;