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

Ujjal Roy
2026-03-26T15:07:55Z

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