[PATCH 0/4] net: bridge: mcast: add multicast exponential field encoding

[PATCH 0/4] net: bridge: mcast: add multicast exponential field encoding

[Ujjal Roy]

Description:
This series addresses a mismatch in how multicast query
intervals and response codes are handled across IPv4 (IGMPv3)
and IPv6 (MLDv2). While decoding logic currently exists,
the corresponding encoding logic is missing during query
packet generation. This leads to incorrect intervals being
transmitted when values exceed their linear thresholds.

The patches introduce a unified floating-point encoding
approach based on RFC3376 and RFC3810, ensuring that large
intervals are correctly represented in QQIC and MRC fields
using the exponent-mantissa format.

Key Changes:
* ipv4: igmp: get rid of IGMPV3_{QQIC,MRC} and simplify calculation
  Removes legacy macros in favor of a cleaner, unified
  calculation for retrieving intervals from encoded fields,
  improving code maintainability.

* ipv6: mld: rename mldv2_mrc() and add mldv2_qqi()
  Standardizes MLDv2 terminology by renaming mldv2_mrc()
  to mldv2_mrd() (Maximum Response Delay) and introducing
  a new API mldv2_qqi for QQI calculation, improving code
  readability.

* ipv4: igmp: encode multicast exponential fields
  Introduces 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).

* ipv6: mld: encode multicast exponential fields
  Applies similar encoding logic for MLDv2. This ensures
  QQIC (8-bit) and MRC (16-bit) fields are properly encoded
  when transmitting query packets with intervals that exceed
  their respective linear thresholds (128 for QQI; 32768
  for MRD).

Impact:
These changes ensure that multicast queriers and listeners
stay synchronized on timing intervals, preventing protocol
timeouts or premature group membership expiration caused
by incorrectly formatted packet headers.

Ujjal Roy (4):
  ipv4: igmp: get rid of IGMPV3_{QQIC,MRC} and simplify calculation
  ipv6: mld: rename mldv2_mrc() and add mldv2_qqi()
  ipv4: igmp: encode multicast exponential fields
  ipv6: mld: encode multicast exponential fields

 include/linux/igmp.h      | 158 +++++++++++++++++++++++++++++++--
 include/net/mld.h         | 179 ++++++++++++++++++++++++++++++++++++--
 net/bridge/br_multicast.c |  22 +++--
 net/ipv4/igmp.c           |   6 +-
 net/ipv6/mcast.c          |  19 +---
 5 files changed, 336 insertions(+), 48 deletions(-)

-- 
2.43.0

[PATCH 1/4] ipv4: igmp: get rid of IGMPV3_{QQIC,MRC} and simplify calculation

[Ujjal Roy]

Get rid of the IGMPV3_MRC macro and use the igmpv3_mrt() API to
calculate the Max Resp Time from the Maximum Response Code.

Similarly, for IGMPV3_QQIC, use the igmpv3_qqi() API to calculate
the Querier's Query Interval from the QQIC field.

Signed-off-by: Ujjal Roy <royujjal@gmail.com>
---
 include/linux/igmp.h      | 78 +++++++++++++++++++++++++++++++++++----
 net/bridge/br_multicast.c |  2 +-
 net/ipv4/igmp.c           |  6 +--
 3 files changed, 74 insertions(+), 12 deletions(-)

diff --git a/include/linux/igmp.h b/include/linux/igmp.h
index 073b30a9b850..3c12c0a63492 100644
--- a/include/linux/igmp.h
+++ b/include/linux/igmp.h
@@ -92,15 +92,77 @@ struct ip_mc_list {
 	struct rcu_head		rcu;
 };
 
+/* RFC3376, relevant sections:
+ *  - 4.1.1. Maximum Response Code
+ *  - 4.1.7. QQIC (Querier's Query Interval Code)
+ *
+ * If Max Resp Code >= 128, Max Resp Code represents a floating-point
+ * value as follows:
+ * If QQIC >= 128, QQIC represents a floating-point value as follows:
+ *
+ *  0 1 2 3 4 5 6 7
+ * +-+-+-+-+-+-+-+-+
+ * |1| exp | mant  |
+ * +-+-+-+-+-+-+-+-+
+ */
+#define IGMPV3_FP_EXP(value)		(((value) >> 4) & 0x07)
+#define IGMPV3_FP_MAN(value)		((value) & 0x0f)
+
+/* IGMPV3 floating-point exponential field threshold */
+#define IGMPV3_EXP_MIN_THRESHOLD	128
+
 /* V3 exponential field decoding */
-#define IGMPV3_MASK(value, nb) ((nb)>=32 ? (value) : ((1<<(nb))-1) & (value))
-#define IGMPV3_EXP(thresh, nbmant, nbexp, value) \
-	((value) < (thresh) ? (value) : \
-        ((IGMPV3_MASK(value, nbmant) | (1<<(nbmant))) << \
-         (IGMPV3_MASK((value) >> (nbmant), nbexp) + (nbexp))))
-
-#define IGMPV3_QQIC(value) IGMPV3_EXP(0x80, 4, 3, value)
-#define IGMPV3_MRC(value) IGMPV3_EXP(0x80, 4, 3, value)
+
+/*
+ * IGMPv3 QQI/MRT 8-bit exponential field decode.
+ *
+ * RFC3376, 4.1.1 & 4.1.7. defines the decoding formula:
+ *      0 1 2 3 4 5 6 7
+ *     +-+-+-+-+-+-+-+-+
+ *     |1| exp | mant  |
+ *     +-+-+-+-+-+-+-+-+
+ * Max Resp Time = (mant | 0x10) << (exp + 3)
+ * QQI = (mant | 0x10) << (exp + 3)
+ */
+static inline unsigned long igmpv3_exp_field_decode(const u8 code)
+{
+	/* RFC3376, relevant sections:
+	 *  - 4.1.1. Maximum Response Code
+	 *  - 4.1.7. QQIC (Querier's Query Interval Code)
+	 */
+	if (code < IGMPV3_EXP_MIN_THRESHOLD) {
+		return (unsigned long)code;
+	} else {
+		unsigned long mc_man, mc_exp;
+
+		mc_exp = IGMPV3_FP_EXP(code);
+		mc_man = IGMPV3_FP_MAN(code);
+
+		return ((mc_man | 0x10) << (mc_exp + 3));
+	}
+}
+
+/* Calculate Max Resp Time from Maximum Response Code */
+static inline unsigned long igmpv3_mrt(const struct igmpv3_query *ih3)
+{
+	/* RFC3376, relevant sections:
+	 *  - 4.1.1. Maximum Response Code
+	 *  - 8.3. Query Response Interval
+	 */
+	return igmpv3_exp_field_decode(ih3->code);
+}
+
+/* Calculate Querier's Query Interval from Querier's Query Interval Code */
+static inline unsigned long igmpv3_qqi(const struct igmpv3_query *ih3)
+{
+	/* 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_decode(ih3->qqic);
+}
 
 static inline int ip_mc_may_pull(struct sk_buff *skb, unsigned int len)
 {
diff --git a/net/bridge/br_multicast.c b/net/bridge/br_multicast.c
index 881d866d687a..9fec76e887bc 100644
--- a/net/bridge/br_multicast.c
+++ b/net/bridge/br_multicast.c
@@ -3518,7 +3518,7 @@ static void br_ip4_multicast_query(struct net_bridge_mcast *brmctx,
 			goto out;
 
 		max_delay = ih3->code ?
-			    IGMPV3_MRC(ih3->code) * (HZ / IGMP_TIMER_SCALE) : 1;
+			    igmpv3_mrt(ih3) * (HZ / IGMP_TIMER_SCALE) : 1;
 	} else {
 		goto out;
 	}
diff --git a/net/ipv4/igmp.c b/net/ipv4/igmp.c
index a674fb44ec25..8c6102737096 100644
--- a/net/ipv4/igmp.c
+++ b/net/ipv4/igmp.c
@@ -991,7 +991,7 @@ static bool igmp_heard_query(struct in_device *in_dev, struct sk_buff *skb,
 		 * different encoding. We use the v3 encoding as more likely
 		 * to be intended in a v3 query.
 		 */
-		max_delay = IGMPV3_MRC(ih3->code)*(HZ/IGMP_TIMER_SCALE);
+		max_delay = igmpv3_mrt(ih3)*(HZ/IGMP_TIMER_SCALE);
 		if (!max_delay)
 			max_delay = 1;	/* can't mod w/ 0 */
 	} else { /* v3 */
@@ -1006,7 +1006,7 @@ static bool igmp_heard_query(struct in_device *in_dev, struct sk_buff *skb,
 			ih3 = igmpv3_query_hdr(skb);
 		}
 
-		max_delay = IGMPV3_MRC(ih3->code)*(HZ/IGMP_TIMER_SCALE);
+		max_delay = igmpv3_mrt(ih3)*(HZ/IGMP_TIMER_SCALE);
 		if (!max_delay)
 			max_delay = 1;	/* can't mod w/ 0 */
 		WRITE_ONCE(in_dev->mr_maxdelay, max_delay);
@@ -1016,7 +1016,7 @@ static bool igmp_heard_query(struct in_device *in_dev, struct sk_buff *skb,
 		 * configured value.
 		 */
 		in_dev->mr_qrv = ih3->qrv ?: READ_ONCE(net->ipv4.sysctl_igmp_qrv);
-		in_dev->mr_qi = IGMPV3_QQIC(ih3->qqic)*HZ ?: IGMP_QUERY_INTERVAL;
+		in_dev->mr_qi = igmpv3_qqi(ih3)*HZ ?: IGMP_QUERY_INTERVAL;
 
 		/* RFC3376, 8.3. Query Response Interval:
 		 * The number of seconds represented by the [Query Response
-- 
2.43.0

[PATCH 2/4] ipv6: mld: rename mldv2_mrc() and add mldv2_qqi()

[Ujjal Roy]

Rename mldv2_mrc() to mldv2_mrd() as it used to calculate
the Maximum Response Delay from the Maximum Response Code.

Introduce a new API mldv2_qqi() to define the existing
calculation logic of QQI from QQIC. This also organizes
the existing mld_update_qi() API.

Added by e3f5b1704 ("net: ipv6: mld: get rid of MLDV2_MRC and simplify calculation").

Signed-off-by: Ujjal Roy <royujjal@gmail.com>
---
 include/net/mld.h         | 64 +++++++++++++++++++++++++++++++++------
 net/bridge/br_multicast.c |  2 +-
 net/ipv6/mcast.c          | 19 ++----------
 3 files changed, 58 insertions(+), 27 deletions(-)

diff --git a/include/net/mld.h b/include/net/mld.h
index c07359808493..6373eb76efe5 100644
--- a/include/net/mld.h
+++ b/include/net/mld.h
@@ -89,29 +89,73 @@ struct mld2_query {
 #define MLDV2_QQIC_EXP(value)	(((value) >> 4) & 0x07)
 #define MLDV2_QQIC_MAN(value)	((value) & 0x0f)
 
-#define MLD_EXP_MIN_LIMIT	32768UL
-#define MLDV1_MRD_MAX_COMPAT	(MLD_EXP_MIN_LIMIT - 1)
+#define MLD_QQIC_MIN_THRESHOLD	128
+#define MLD_MRC_MIN_THRESHOLD	32768UL
+#define MLDV1_MRD_MAX_COMPAT	(MLD_MRC_MIN_THRESHOLD - 1)
 
 #define MLD_MAX_QUEUE		8
 #define MLD_MAX_SKBS		32
 
-static inline unsigned long mldv2_mrc(const struct mld2_query *mlh2)
-{
-	/* RFC3810, 5.1.3. Maximum Response Code */
-	unsigned long ret, mc_mrc = ntohs(mlh2->mld2q_mrc);
+/* V2 exponential field decoding */
 
-	if (mc_mrc < MLD_EXP_MIN_LIMIT) {
-		ret = mc_mrc;
+/* Calculate Maximum Response Delay from Maximum Response Code
+ *
+ * RFC3810, 5.1.3. defines the decoding formula:
+ *      0 1 2 3 4 5 6 7 8 9 A B C D E F
+ *     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ *     |1| exp |          mant         |
+ *     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ * Maximum Response Delay = (mant | 0x1000) << (exp+3)
+ */
+static inline unsigned long mldv2_mrd(const struct mld2_query *mlh2)
+{
+	/* RFC3810, relevant sections:
+	 *  - 5.1.3. Maximum Response Code
+	 *  - 9.3. Query Response Interval
+	 */
+	unsigned long mc_mrc = ntohs(mlh2->mld2q_mrc);
+
+	if (mc_mrc < MLD_MRC_MIN_THRESHOLD) {
+		return mc_mrc;
 	} else {
 		unsigned long mc_man, mc_exp;
 
 		mc_exp = MLDV2_MRC_EXP(mc_mrc);
 		mc_man = MLDV2_MRC_MAN(mc_mrc);
 
-		ret = (mc_man | 0x1000) << (mc_exp + 3);
+		return ((mc_man | 0x1000) << (mc_exp + 3));
 	}
+}
 
-	return ret;
+/* Calculate Querier's Query Interval from Querier's Query Interval Code
+ *
+ * RFC3810, 5.1.9. defines the decoding formula:
+ *      0 1 2 3 4 5 6 7
+ *     +-+-+-+-+-+-+-+-+
+ *     |1| exp | mant  |
+ *     +-+-+-+-+-+-+-+-+
+ * QQI = (mant | 0x10) << (exp + 3)
+ */
+static inline unsigned long mldv2_qqi(const struct mld2_query *mlh2)
+{
+	/* RFC3810, relevant sections:
+	 *  - 5.1.9. QQIC (Querier's Query Interval Code)
+	 *  - 9.2. Query Interval
+	 *  - 9.12. Older Version Querier Present Timeout
+	 *    (the [Query Interval] in the last Query received)
+	 */
+	unsigned long qqic = ntohs(mlh2->mld2q_qqic);
+
+	if (qqic < MLD_QQIC_MIN_THRESHOLD) {
+		return qqic;
+	} else {
+		unsigned long mc_man, mc_exp;
+
+		mc_exp = MLDV2_QQIC_EXP(qqic);
+		mc_man = MLDV2_QQIC_MAN(qqic);
+
+		return ((mc_man | 0x10) << (mc_exp + 3));
+	}
 }
 
 #endif
diff --git a/net/bridge/br_multicast.c b/net/bridge/br_multicast.c
index 9fec76e887bc..1438c023db62 100644
--- a/net/bridge/br_multicast.c
+++ b/net/bridge/br_multicast.c
@@ -3606,7 +3606,7 @@ static int br_ip6_multicast_query(struct net_bridge_mcast *brmctx,
 		    mld2q->mld2q_suppress)
 			goto out;
 
-		max_delay = max(msecs_to_jiffies(mldv2_mrc(mld2q)), 1UL);
+		max_delay = max(msecs_to_jiffies(mldv2_mrd(mld2q)), 1UL);
 	}
 
 	is_general_query = group && ipv6_addr_any(group);
diff --git a/net/ipv6/mcast.c b/net/ipv6/mcast.c
index 3330adcf26db..6ddc18ac59b9 100644
--- a/net/ipv6/mcast.c
+++ b/net/ipv6/mcast.c
@@ -1315,20 +1315,7 @@ static void mld_update_qi(struct inet6_dev *idev,
 	 *  - 9.12. Older Version Querier Present Timeout
 	 *    (the [Query Interval] in the last Query received)
 	 */
-	unsigned long mc_qqi;
-
-	if (mlh2->mld2q_qqic < 128) {
-		mc_qqi = mlh2->mld2q_qqic;
-	} else {
-		unsigned long mc_man, mc_exp;
-
-		mc_exp = MLDV2_QQIC_EXP(mlh2->mld2q_qqic);
-		mc_man = MLDV2_QQIC_MAN(mlh2->mld2q_qqic);
-
-		mc_qqi = (mc_man | 0x10) << (mc_exp + 3);
-	}
-
-	idev->mc_qi = mc_qqi * HZ;
+	idev->mc_qi = mldv2_qqi(mlh2) * HZ;
 }
 
 static void mld_update_qri(struct inet6_dev *idev,
@@ -1338,7 +1325,7 @@ static void mld_update_qri(struct inet6_dev *idev,
 	 *  - 5.1.3. Maximum Response Code
 	 *  - 9.3. Query Response Interval
 	 */
-	idev->mc_qri = msecs_to_jiffies(mldv2_mrc(mlh2));
+	idev->mc_qri = msecs_to_jiffies(mldv2_mrd(mlh2));
 }
 
 static int mld_process_v1(struct inet6_dev *idev, struct mld_msg *mld,
@@ -1390,7 +1377,7 @@ static int mld_process_v1(struct inet6_dev *idev, struct mld_msg *mld,
 static void mld_process_v2(struct inet6_dev *idev, struct mld2_query *mld,
 			   unsigned long *max_delay)
 {
-	*max_delay = max(msecs_to_jiffies(mldv2_mrc(mld)), 1UL);
+	*max_delay = max(msecs_to_jiffies(mldv2_mrd(mld)), 1UL);
 
 	mld_update_qrv(idev, mld);
 	mld_update_qi(idev, mld);
-- 
2.43.0

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

[Ujjal Roy]

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

[PATCH 4/4] ipv6: mld: encode multicast exponential fields

[Ujjal Roy]

In MLD, QQIC and MRC fields are not currently encoded when
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.

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 (8-bit) and MRC (16-bit) fields are
properly encoded when transmitting query packets with
intervals that exceed their respective linear thresholds
(128 for QQI; 32768 for MRD).

RFC3810: If QQIC >= 128, the QQIC field represents a
floating-point value as follows:
     0 1 2 3 4 5 6 7
    +-+-+-+-+-+-+-+-+
    |1| exp | mant  |
    +-+-+-+-+-+-+-+-+

RFC3810: If Maximum Response Code >= 32768, the Maximum
Response Code field represents a floating-point value as
follows:
     0 1 2 3 4 5 6 7 8 9 A B C D E F
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |1| exp |          mant         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Signed-off-by: Ujjal Roy <royujjal@gmail.com>
---
 include/net/mld.h         | 117 ++++++++++++++++++++++++++++++++++++++
 net/bridge/br_multicast.c |   4 +-
 2 files changed, 119 insertions(+), 2 deletions(-)

diff --git a/include/net/mld.h b/include/net/mld.h
index 6373eb76efe5..0962312d29b9 100644
--- a/include/net/mld.h
+++ b/include/net/mld.h
@@ -90,12 +90,129 @@ struct mld2_query {
 #define MLDV2_QQIC_MAN(value)	((value) & 0x0f)
 
 #define MLD_QQIC_MIN_THRESHOLD	128
+/* Max representable (mant = 0xF, exp = 7) -> 31744 */
+#define MLD_QQIC_MAX_THRESHOLD	31744
 #define MLD_MRC_MIN_THRESHOLD	32768UL
+/* Max representable (mant = 0xFFF, exp = 7) -> 8387584 */
+#define MLD_MRC_MAX_THRESHOLD	8387584
 #define MLDV1_MRD_MAX_COMPAT	(MLD_MRC_MIN_THRESHOLD - 1)
 
 #define MLD_MAX_QUEUE		8
 #define MLD_MAX_SKBS		32
 
+/* V2 exponential field encoding */
+
+/*
+ * Calculate Maximum Response Code from Maximum Response Delay
+ *
+ * MLDv2 Maximum Response Code 16-bit encoding (RFC3810).
+ *
+ * RFC3810 defines only the decoding formula:
+ * Maximum Response Delay = (mant | 0x1000) << (exp + 3)
+ *
+ * but does NOT define the encoding procedure. To derive exponent:
+ *
+ * For the 16-bit MRC, the "hidden bit" (0x1000) is left shifted by 12
+ * to sit above the 12-bit mantissa. The RFC then shifts this entire
+ * block left by (exp + 3) to reconstruct the value.
+ * So, 'hidden bit' is the MSB which is shifted by (12 + exp + 3).
+ *
+ * - Total left shift of the hidden bit = 12 + (exp + 3) = exp + 15.
+ * - This is the MSB at the 0-based bit position: (exp + 15).
+ * - Since fls() is 1-based, fls(value) - 1 = exp + 15.
+ *
+ * Therefore:
+ *     exp  = fls(value) - 16
+ *     mant = (value >> (exp + 3)) & 0x0FFF
+ *
+ * Final encoding formula:
+ *     0x8000 | (exp << 12) | mant
+ *
+ * Example (value = 1311744):
+ *  0               1               2               3
+ *  0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
+ * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ * |0 0 0 0 0 0 0 0 0 0 0 1 0 1 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0| 1311744
+ * |                      ^-^--------mant---------^ ^...(exp+3)...^| exp=5
+ * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ *
+ * Encoded:
+ *   0x8000 | (5 << 12) | 0x404 = 0xD404
+ */
+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);
+}
+
+/*
+ * Calculate Querier's Query Interval Code from Query Interval
+ *
+ * MLDv2 QQIC 8-bit floating-point encoding (RFC3810).
+ *
+ * RFC3810 defines only the decoding formula:
+ * QQI = (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 mldv2_qqic(unsigned long value)
+{
+	u8 mc_man, mc_exp;
+
+	/* RFC3810: QQIC < 128 is literal */
+	if (value < MLD_QQIC_MIN_THRESHOLD)
+		return (u8)value;
+
+	/* Saturate at max representable (mant = 0xF, exp = 7) -> 31744 */
+	if (value >= MLD_QQIC_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);
+}
+
 /* V2 exponential field decoding */
 
 /* Calculate Maximum Response Delay from Maximum Response Code
diff --git a/net/bridge/br_multicast.c b/net/bridge/br_multicast.c
index 1de6242413e0..c59dab6fff4e 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)));
 		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

Re: [PATCH 0/4] net: bridge: mcast: add multicast exponential field encoding

[Nikolay Aleksandrov]

On 26/03/2026 17:07, Ujjal Roy wrote:
> Description:
> This series addresses a mismatch in how multicast query
> intervals and response codes are handled across IPv4 (IGMPv3)
> and IPv6 (MLDv2). While decoding logic currently exists,
> the corresponding encoding logic is missing during query
> packet generation. This leads to incorrect intervals being
> transmitted when values exceed their linear thresholds.
> 
> The patches introduce a unified floating-point encoding
> approach based on RFC3376 and RFC3810, ensuring that large
> intervals are correctly represented in QQIC and MRC fields
> using the exponent-mantissa format.
> 
> Key Changes:
> * ipv4: igmp: get rid of IGMPV3_{QQIC,MRC} and simplify calculation
>    Removes legacy macros in favor of a cleaner, unified
>    calculation for retrieving intervals from encoded fields,
>    improving code maintainability.
> 
> * ipv6: mld: rename mldv2_mrc() and add mldv2_qqi()
>    Standardizes MLDv2 terminology by renaming mldv2_mrc()
>    to mldv2_mrd() (Maximum Response Delay) and introducing
>    a new API mldv2_qqi for QQI calculation, improving code
>    readability.
> 
> * ipv4: igmp: encode multicast exponential fields
>    Introduces 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).
> 
> * ipv6: mld: encode multicast exponential fields
>    Applies similar encoding logic for MLDv2. This ensures
>    QQIC (8-bit) and MRC (16-bit) fields are properly encoded
>    when transmitting query packets with intervals that exceed
>    their respective linear thresholds (128 for QQI; 32768
>    for MRD).
> 
> Impact:
> These changes ensure that multicast queriers and listeners
> stay synchronized on timing intervals, preventing protocol
> timeouts or premature group membership expiration caused
> by incorrectly formatted packet headers.
> 

Can you add selftests which cover these cases?

Re: [PATCH 0/4] net: bridge: mcast: add multicast exponential field encoding

[Nikolay Aleksandrov]

On 27/03/2026 08:41, Nikolay Aleksandrov wrote:
> On 26/03/2026 17:07, Ujjal Roy wrote:
>> Description:
>> This series addresses a mismatch in how multicast query
>> intervals and response codes are handled across IPv4 (IGMPv3)
>> and IPv6 (MLDv2). While decoding logic currently exists,
>> the corresponding encoding logic is missing during query
>> packet generation. This leads to incorrect intervals being
>> transmitted when values exceed their linear thresholds.
>>
>> The patches introduce a unified floating-point encoding
>> approach based on RFC3376 and RFC3810, ensuring that large
>> intervals are correctly represented in QQIC and MRC fields
>> using the exponent-mantissa format.
>>
>> Key Changes:
>> * ipv4: igmp: get rid of IGMPV3_{QQIC,MRC} and simplify calculation
>>    Removes legacy macros in favor of a cleaner, unified
>>    calculation for retrieving intervals from encoded fields,
>>    improving code maintainability.
>>
>> * ipv6: mld: rename mldv2_mrc() and add mldv2_qqi()
>>    Standardizes MLDv2 terminology by renaming mldv2_mrc()
>>    to mldv2_mrd() (Maximum Response Delay) and introducing
>>    a new API mldv2_qqi for QQI calculation, improving code
>>    readability.
>>
>> * ipv4: igmp: encode multicast exponential fields
>>    Introduces 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).
>>
>> * ipv6: mld: encode multicast exponential fields
>>    Applies similar encoding logic for MLDv2. This ensures
>>    QQIC (8-bit) and MRC (16-bit) fields are properly encoded
>>    when transmitting query packets with intervals that exceed
>>    their respective linear thresholds (128 for QQI; 32768
>>    for MRD).
>>
>> Impact:
>> These changes ensure that multicast queriers and listeners
>> stay synchronized on timing intervals, preventing protocol
>> timeouts or premature group membership expiration caused
>> by incorrectly formatted packet headers.
>>
> 
> Can you add selftests which cover these cases?
> 
> 

Forgot to mention - for improvements and new features please target net-next.

Cheers,
  Nik

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 <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;