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

[Nikolay Aleksandrov <razor@blackwall.org>]

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;