Re: [PATCH net-next v2 4/5] ipv6: mld: encode multicast exponential fields

Ido Schimmel
2026-04-01T07:29:27Z

On Mon, Mar 30, 2026 at 07:16:10PM +0000, Ujjal Roy wrote:
> In MLD, QQIC and MRC fields are not currently encoded when

s/currently/correctly/

> generating query packets. Since the receiver of the query
> interprets these fields using the MLDv2 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.

[...]

> +static inline u16 mldv2_mrc(unsigned long mrd)
> +{
> + u16 mc_man, mc_exp;
> +
> + /* RFC3810: MRC < 32768 is literal */
> + if (mrd < MLD_MRC_MIN_THRESHOLD)
> + return (u16)mrd;
> +
> + /* Saturate at max representable (mant = 0xFFF, exp = 7) -> 8387584 */
> + if (mrd >= MLD_MRC_MAX_THRESHOLD)
> + return 0xFFFF;
> +
> + mc_exp = (u16)(fls(mrd) - 16);
> + mc_man = (u16)((mrd >> (mc_exp + 3)) & 0x0FFF);
> +
> + return (0x8000 | (mc_exp << 12) | mc_man);
> +}

[...]

> diff --git a/net/bridge/br_multicast.c b/net/bridge/br_multicast.c
> index 27010744d7ae..c2d144f6a86e 100644
> --- a/net/bridge/br_multicast.c
> +++ b/net/bridge/br_multicast.c
> @@ -1181,7 +1181,7 @@ static struct sk_buff *br_ip6_multicast_alloc_query(struct net_bridge_mcast *brm
> break;
> case 2:
> mld2q = (struct mld2_query *)icmp6_hdr(skb);
> - mld2q->mld2q_mrc = htons((u16)jiffies_to_msecs(interval));
> + mld2q->mld2q_mrc = htons((u16)jiffies_to_msecs(mldv2_mrc(interval)));

This looks wrong. mldv2_mrc() is supposed to receive the maximum
response delay in milliseconds, but you are passing jiffies.

> mld2q->mld2q_type = ICMPV6_MGM_QUERY;
> mld2q->mld2q_code = 0;
> mld2q->mld2q_cksum = 0;
> @@ -1190,7 +1190,7 @@ static struct sk_buff *br_ip6_multicast_alloc_query(struct net_bridge_mcast *brm
> mld2q->mld2q_suppress = sflag;
> mld2q->mld2q_qrv = 2;
> mld2q->mld2q_nsrcs = htons(llqt_srcs);
> - mld2q->mld2q_qqic = brmctx->multicast_query_interval / HZ;
> + mld2q->mld2q_qqic = mldv2_qqic(brmctx->multicast_query_interval / HZ);
> mld2q->mld2q_mca = *group;
> csum = &mld2q->mld2q_cksum;
> csum_start = (void *)mld2q;
> --
> 2.43.0
>