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

[Ujjal Roy <royujjal@gmail.com>]

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(-)

diff --git a/include/linux/igmp.h b/include/linux/igmp.h
index 3c12c0a63492..99fce6b0625f 100644
--- a/include/linux/igmp.h
+++ b/include/linux/igmp.h
@@ -110,6 +110,86 @@ struct ip_mc_list {
 
 /* IGMPV3 floating-point exponential field threshold */
 #define IGMPV3_EXP_MIN_THRESHOLD	128
+/* Max representable (mant = 0xF, exp = 7) -> 31744 */
+#define IGMPV3_EXP_MAX_THRESHOLD	31744
+
+/* V3 exponential field encoding */
+
+/*
+ * IGMPv3 QQIC/MRC 8-bit exponential field encode.
+ *
+ * RFC3376 defines only the decoding formula:
+ * QQI/MRT = (mant | 0x10) << (exp + 3)
+ *
+ * but does NOT define the encoding procedure. To derive exponent:
+ *
+ * For any value of mantissa and exponent, the decoding formula
+ * indicates that the "hidden bit" (0x10) is shifted 4 bits left
+ * to sit above the 4-bit mantissa. The RFC again shifts this
+ * entire block left by (exp + 3) to reconstruct the value.
+ * So, 'hidden bit' is the MSB which is shifted by (4 + exp + 3).
+ *
+ * Total left shift of the 'hidden bit' = 4 + (exp + 3) = exp + 7.
+ * This is the MSB at the 0-based bit position: (exp + 7).
+ * Since fls() is 1-based, fls(value) - 1 = exp + 7.
+ *
+ * Therefore:
+ *     exp  = fls(value) - 8
+ *     mant = (value >> (exp + 3)) & 0x0F
+ *
+ * Final encoding formula:
+ *     0x80 | (exp << 4) | mant
+ *
+ * Example (value = 3200):
+ *  0               1
+ *  0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
+ * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ * |0 0 0 0 1 1 0 0 1 0 0 0 0 0 0 0| (value = 3200)
+ * |        ^-^-mant^ ^..(exp+3)..^| exp = 4, mant = 9
+ * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ *
+ * Encoded:
+ *   0x80 | (4 << 4) | 9 = 0xC9
+ */
+static inline u8 igmpv3_exp_field_encode(unsigned long value)
+{
+	u8 mc_exp, mc_man;
+
+	/* RFC3376: QQIC/MRC < 128 is literal */
+	if (value < IGMPV3_EXP_MIN_THRESHOLD)
+		return (u8)value;
+
+	/* Saturate at max representable (mant = 0xF, exp = 7) -> 31744 */
+	if (value >= IGMPV3_EXP_MAX_THRESHOLD)
+		return 0xFF;
+
+	mc_exp  = (u8)(fls(value) - 8);
+	mc_man = (u8)((value >> (mc_exp + 3)) & 0x0F);
+
+	return 0x80 | (mc_exp << 4) | mc_man;
+}
+
+/* Calculate Maximum Response Code from Max Resp Time */
+static inline u8 igmpv3_mrc(unsigned long mrt)
+{
+	/* RFC3376, relevant sections:
+	 *  - 4.1.1. Maximum Response Code
+	 *  - 8.3. Query Response Interval
+	 */
+	return igmpv3_exp_field_encode(mrt);
+}
+
+/* Calculate Querier's Query Interval Code from Query Interval */
+static inline u8 igmpv3_qqic(unsigned long qi)
+{
+	/* RFC3376, relevant sections:
+	 *  - 4.1.7. QQIC (Querier's Query Interval Code)
+	 *  - 8.2. Query Interval
+	 *  - 8.12. Older Version Querier Present Timeout
+	 *    (the [Query Interval] in the last Query received)
+	 */
+	return igmpv3_exp_field_encode(qi);
+}
 
 /* V3 exponential field decoding */
 
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;
 
@@ -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;
-- 
2.43.0