Merge branch 'net-bridge-mcast-support-exponential-field-encoding'

Ujjal Roy says:

====================
net: bridge: mcast: support exponential field encoding

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).

* selftests: net: bridge: add MRC and QQIC field encoding tests
  Updates bridge selftests to validate both linear and non-linear
  (exponential) encoding for MRC and QQIC fields, ensuring
  protocol compliance across IGMPv3 and MLDv2.

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.
====================

Link: https://patch.msgid.link/20260502131907.987-1-royujjal@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

2 files changed, 335 insertions, 17 deletions

diff --git a/include/linux/igmp.h b/include/linux/igmp.h
index 073b30a9b850..3a2d35a9f307 100644
--- a/include/linux/igmp.h
+++ b/include/linux/igmp.h
@@ -92,15 +92,166 @@ 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)
+ *
+ * For both MRC and QQIC, values >= 128 use the same floating-point
+ * encoding 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 min threshold */
+#define IGMPV3_EXP_MIN_THRESHOLD	128
+/* IGMPv3 FP max threshold (mant = 0xF, exp = 7) -> 31744 */
+#define IGMPV3_EXP_MAX_THRESHOLD	31744
+
+/* V3 exponential field encoding */
+
+/* IGMPv3 MRC/QQIC 8-bit exponential field encode
+ *
+ * RFC3376, 4.1.1 & 4.1.7. defines only the decoding formula:
+ *     MRT/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 igmpv3_exp_field_encode(unsigned long value)
+{
+	u8 mc_exp, mc_man;
+
+	/* MRC/QQIC < 128 is literal */
+	if (value < IGMPV3_EXP_MIN_THRESHOLD)
+		return value;
+
+	/* Saturate at max representable (mant = 0xF, exp = 7) -> 31744 */
+	if (value >= IGMPV3_EXP_MAX_THRESHOLD)
+		return 0xFF;
+
+	mc_exp  = fls(value) - 8;
+	mc_man = (value >> (mc_exp + 3)) & 0x0F;
+
+	return 0x80 | (mc_exp << 4) | mc_man;
+}
+
+/* Calculate Maximum Response Code from Max Resp Time
+ *
+ * RFC3376, relevant sections:
+ *  - 4.1.1. Maximum Response Code
+ *  - 8.3. Query Response Interval
+ *
+ * MRC represents the encoded form of Max Resp Time (MRT); once
+ * decoded, the resulting value is in units of 0.1 seconds (100 ms).
+ */
+static inline u8 igmpv3_mrc(unsigned long mrt)
+{
+	return igmpv3_exp_field_encode(mrt);
+}
+
+/* Calculate Querier's Query Interval Code from Querier's Query Interval
+ *
+ * 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)
+ *
+ * QQIC represents the encoded form of Querier's Query Interval (QQI);
+ * once decoded, the resulting value is in units of seconds.
+ */
+static inline u8 igmpv3_qqic(unsigned long qi)
+{
+	return igmpv3_exp_field_encode(qi);
+}
+
 /* 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 MRC/QQIC 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)
+{
+	if (code < IGMPV3_EXP_MIN_THRESHOLD) {
+		return 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
+ *
+ * RFC3376, relevant sections:
+ *  - 4.1.1. Maximum Response Code
+ *  - 8.3. Query Response Interval
+ *
+ * After decode, MRC represents the Maximum Response Time (MRT) in
+ * units of 0.1 seconds (100 ms).
+ */
+static inline unsigned long igmpv3_mrt(const struct igmpv3_query *ih3)
+{
+	return igmpv3_exp_field_decode(ih3->code);
+}
+
+/* Calculate Querier's Query Interval from Querier's Query Interval Code
+ *
+ * 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)
+ *
+ * After decode, QQIC represents the Querier's Query Interval in units
+ * of seconds.
+ */
+static inline unsigned long igmpv3_qqi(const struct igmpv3_query *ih3)
+{
+	return igmpv3_exp_field_decode(ih3->qqic);
+}
 
 static inline int ip_mc_may_pull(struct sk_buff *skb, unsigned int len)
 {
diff --git a/include/net/mld.h b/include/net/mld.h
index c07359808493..23255cc47ced 100644
--- a/include/net/mld.h
+++ b/include/net/mld.h
@@ -89,29 +89,196 @@ 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)
+/* MLDv2 QQIC floating-point exponential field min threshold */
+#define MLD_QQIC_MIN_THRESHOLD	128
+/* MLDv2 QQIC FP max threshold (mant = 0xF, exp = 7) -> 31744 */
+#define MLD_QQIC_MAX_THRESHOLD	31744
+/* MLDv2 MRC floating-point exponential field min threshold */
+#define MLD_MRC_MIN_THRESHOLD	32768UL
+/* MLDv2 MRC FP max threshold (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
 
-static inline unsigned long mldv2_mrc(const struct mld2_query *mlh2)
+/* V2 exponential field encoding */
+
+/*
+ * Calculate Maximum Response Code from Maximum Response Delay
+ *
+ * MRC represents the 16-bit encoded form of Maximum Response Delay (MRD);
+ * once decoded, the resulting value is in milliseconds.
+ *
+ * RFC3810, 5.1.3. 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;
+
+	/* MRC < 32768 is literal */
+	if (mrd < MLD_MRC_MIN_THRESHOLD)
+		return mrd;
+
+	/* Saturate at max representable (mant = 0xFFF, exp = 7) -> 8387584 */
+	if (mrd >= MLD_MRC_MAX_THRESHOLD)
+		return 0xFFFF;
+
+	mc_exp = fls(mrd) - 16;
+	mc_man = (mrd >> (mc_exp + 3)) & 0x0FFF;
+
+	return 0x8000 | (mc_exp << 12) | mc_man;
+}
+
+/*
+ * Calculate Querier's Query Interval Code from Querier's Query Interval
+ *
+ * QQIC represents the 8-bit encoded form of Querier's Query Interval (QQI);
+ * once decoded, the resulting value is in seconds.
+ *
+ * RFC3810, 5.1.9. 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;
+
+	/* QQIC < 128 is literal */
+	if (value < MLD_QQIC_MIN_THRESHOLD)
+		return value;
+
+	/* Saturate at max representable (mant = 0xF, exp = 7) -> 31744 */
+	if (value >= MLD_QQIC_MAX_THRESHOLD)
+		return 0xFF;
+
+	mc_exp  = fls(value) - 8;
+	mc_man = (value >> (mc_exp + 3)) & 0x0F;
+
+	return 0x80 | (mc_exp << 4) | mc_man;
+}
+
+/* V2 exponential field decoding */
+
+/* Calculate Maximum Response Delay from Maximum Response Code
+ *
+ * RFC3810, relevant sections:
+ *  - 5.1.3. Maximum Response Code 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)
+ *  - 9.3. Query Response Interval
+ *
+ * After decode, MRC represents the Maximum Response Delay (MRD) in
+ * units of milliseconds.
+ */
+static inline unsigned long mldv2_mrd(const struct mld2_query *mlh2)
 {
-	/* RFC3810, 5.1.3. Maximum Response Code */
-	unsigned long ret, mc_mrc = ntohs(mlh2->mld2q_mrc);
+	unsigned long mc_mrc = ntohs(mlh2->mld2q_mrc);
 
-	if (mc_mrc < MLD_EXP_MIN_LIMIT) {
-		ret = mc_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, relevant sections:
+ *  - 5.1.9. QQIC (Querier's Query Interval Code) defines the decoding formula:
+ *      0 1 2 3 4 5 6 7
+ *     +-+-+-+-+-+-+-+-+
+ *     |1| exp | mant  |
+ *     +-+-+-+-+-+-+-+-+
+ *    QQI = (mant | 0x10) << (exp + 3)
+ *  - 9.2. Query Interval
+ *  - 9.12. Older Version Querier Present Timeout
+ *    (the [Query Interval] in the last Query received)
+ *
+ * After decode, QQIC represents the Querier's Query Interval in units
+ * of seconds.
+ */
+static inline unsigned long mldv2_qqi(const struct mld2_query *mlh2)
+{
+	unsigned long qqic = 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