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

[Ujjal Roy <royujjal@gmail.com>]

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