lib: clean up nexthop hashing mess

We were hashing 4 bytes of the address.  Even for IPv6 addresses.

Oops.

The reason this was done was to try to make it faster, but made a
complex maze out of everything.  Time for a refactor.

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
(cherry picked from commit 001fcfa1dd9f7dc2639b4f5c7a52ab59cc425452)

2 files changed, 80 insertions, 111 deletions

diff --git a/lib/nexthop.c b/lib/nexthop.c
index 3cfa72bce0..8b37290b18 100644
--- a/lib/nexthop.c
+++ b/lib/nexthop.c
@@ -723,68 +723,30 @@ unsigned int nexthop_level(const struct nexthop *nexthop)
 	return rv;
 }
 
-/* Only hash word-sized things, let cmp do the rest. */
-uint32_t nexthop_hash_quick(const struct nexthop *nexthop)
+uint32_t nexthop_hash(const struct nexthop *nexthop)
 {
 	uint32_t key = 0x45afe398;
-	int i;
 
-	key = jhash_3words(nexthop->type, nexthop->vrf_id,
-			   nexthop->nh_label_type, key);
-
-	if (nexthop->nh_label) {
-		int labels = nexthop->nh_label->num_labels;
+	/* type, vrf, ifindex, ip addresses - see nexthop.h */
+	key = _nexthop_hash_bytes(nexthop, key);
 
-		i = 0;
+	key = jhash_1word(nexthop->flags & NEXTHOP_FLAGS_HASHED, key);
 
-		while (labels >= 3) {
-			key = jhash_3words(nexthop->nh_label->label[i],
-					   nexthop->nh_label->label[i + 1],
-					   nexthop->nh_label->label[i + 2],
-					   key);
-			labels -= 3;
-			i += 3;
-		}
-
-		if (labels >= 2) {
-			key = jhash_2words(nexthop->nh_label->label[i],
-					   nexthop->nh_label->label[i + 1],
-					   key);
-			labels -= 2;
-			i += 2;
-		}
+	if (nexthop->nh_label) {
+		const struct mpls_label_stack *ls = nexthop->nh_label;
 
-		if (labels >= 1)
-			key = jhash_1word(nexthop->nh_label->label[i], key);
+		/* num_labels itself isn't useful to hash, if the number of
+		 * labels is different, the hash value will change just due to
+		 * that already.
+		 */
+		key = jhash(ls->label, sizeof(ls->label[0]) * ls->num_labels, key);
 	}
 
-	key = jhash_2words(nexthop->ifindex,
-			   CHECK_FLAG(nexthop->flags, NEXTHOP_FLAG_ONLINK),
-			   key);
-
 	/* Include backup nexthops, if present */
 	if (CHECK_FLAG(nexthop->flags, NEXTHOP_FLAG_HAS_BACKUP)) {
 		int backups = nexthop->backup_num;
 
-		i = 0;
-
-		while (backups >= 3) {
-			key = jhash_3words(nexthop->backup_idx[i],
-					   nexthop->backup_idx[i + 1],
-					   nexthop->backup_idx[i + 2], key);
-			backups -= 3;
-			i += 3;
-		}
-
-		while (backups >= 2) {
-			key = jhash_2words(nexthop->backup_idx[i],
-					   nexthop->backup_idx[i + 1], key);
-			backups -= 2;
-			i += 2;
-		}
-
-		if (backups >= 1)
-			key = jhash_1word(nexthop->backup_idx[i], key);
+		key = jhash(nexthop->backup_idx, sizeof(nexthop->backup_idx[0]) * backups, key);
 	}
 
 	if (nexthop->nh_srv6) {
@@ -819,31 +781,6 @@ uint32_t nexthop_hash_quick(const struct nexthop *nexthop)
 	return key;
 }
 
-
-#define GATE_SIZE 4 /* Number of uint32_t words in struct g_addr */
-
-/* For a more granular hash */
-uint32_t nexthop_hash(const struct nexthop *nexthop)
-{
-	uint32_t gate_src_rmap_raw[GATE_SIZE * 3] = {};
-	/* Get all the quick stuff */
-	uint32_t key = nexthop_hash_quick(nexthop);
-
-	assert(((sizeof(nexthop->gate) + sizeof(nexthop->src)
-		 + sizeof(nexthop->rmap_src))
-		/ 3)
-	       == (GATE_SIZE * sizeof(uint32_t)));
-
-	memcpy(gate_src_rmap_raw, &nexthop->gate, GATE_SIZE);
-	memcpy(gate_src_rmap_raw + GATE_SIZE, &nexthop->src, GATE_SIZE);
-	memcpy(gate_src_rmap_raw + (2 * GATE_SIZE), &nexthop->rmap_src,
-	       GATE_SIZE);
-
-	key = jhash2(gate_src_rmap_raw, (GATE_SIZE * 3), key);
-
-	return key;
-}
-
 void nexthop_copy_no_recurse(struct nexthop *copy,
 			     const struct nexthop *nexthop,
 			     struct nexthop *rparent)
diff --git a/lib/nexthop.h b/lib/nexthop.h
index 958d06aa51..718ce2ee42 100644
--- a/lib/nexthop.h
+++ b/lib/nexthop.h
@@ -8,6 +8,7 @@
 #ifndef _LIB_NEXTHOP_H
 #define _LIB_NEXTHOP_H
 
+#include "jhash.h"
 #include "prefix.h"
 #include "mpls.h"
 #include "vxlan.h"
@@ -56,15 +57,48 @@ struct nexthop {
 	struct nexthop *next;
 	struct nexthop *prev;
 
-	/*
-	 * What vrf is this nexthop associated with?
+
+	/* begin of hashed data - all fields from here onwards are given to
+	 * jhash() as one consecutive chunk.  DO NOT create "padding holes".
+	 * DO NOT insert pointers that need to be deep-hashed.
+	 *
+	 * static_assert() below needs to be updated when fields are added
 	 */
+	char _hash_begin[0];
+
+	/* see above */
+	enum nexthop_types_t type;
+
+	/* What vrf is this nexthop associated with? */
 	vrf_id_t vrf_id;
 
 	/* Interface index. */
 	ifindex_t ifindex;
 
-	enum nexthop_types_t type;
+	/* Type of label(s), if any */
+	enum lsp_types_t nh_label_type;
+
+	/* padding: keep 16 byte alignment here */
+
+	/* Nexthop address
+	 * make sure all 16 bytes for IPv6 are zeroed when putting in an IPv4
+	 * address since the entire thing is hashed as-is
+	 */
+	union {
+		union g_addr gate;
+		enum blackhole_type bh_type;
+	};
+	union g_addr src;
+	union g_addr rmap_src; /* Src is set via routemap */
+
+	/* end of hashed data - remaining fields in this struct are not
+	 * directly fed into jhash().  Most of them are actually part of the
+	 * hash but have special rules or handling attached.
+	 */
+	char _hash_end[0];
+
+	/* Weight of the nexthop ( for unequal cost ECMP ) */
+	uint8_t weight;
 
 	uint16_t flags;
 #define NEXTHOP_FLAG_ACTIVE     (1 << 0) /* This nexthop is alive. */
@@ -82,18 +116,15 @@ struct nexthop {
 #define NEXTHOP_FLAG_EVPN       (1 << 8) /* nexthop is EVPN */
 #define NEXTHOP_FLAG_LINKDOWN   (1 << 9) /* is not removed on link down */
 
+	/* which flags are part of nexthop_hash().  Should probably be split
+	 * off into a separate field...
+	 */
+#define NEXTHOP_FLAGS_HASHED NEXTHOP_FLAG_ONLINK
+
 #define NEXTHOP_IS_ACTIVE(flags)                                               \
 	(CHECK_FLAG(flags, NEXTHOP_FLAG_ACTIVE)                                \
 	 && !CHECK_FLAG(flags, NEXTHOP_FLAG_DUPLICATE))
 
-	/* Nexthop address */
-	union {
-		union g_addr gate;
-		enum blackhole_type bh_type;
-	};
-	union g_addr src;
-	union g_addr rmap_src; /* Src is set via routemap */
-
 	/* Nexthops obtained by recursive resolution.
 	 *
 	 * If the nexthop struct needs to be resolved recursively,
@@ -104,15 +135,9 @@ struct nexthop {
 	/* Recursive parent */
 	struct nexthop *rparent;
 
-	/* Type of label(s), if any */
-	enum lsp_types_t nh_label_type;
-
 	/* Label(s) associated with this nexthop. */
 	struct mpls_label_stack *nh_label;
 
-	/* Weight of the nexthop ( for unequal cost ECMP ) */
-	uint8_t weight;
-
 	/* Count and index of corresponding backup nexthop(s) in a backup list;
 	 * only meaningful if the HAS_BACKUP flag is set.
 	 */
@@ -138,6 +163,29 @@ struct nexthop {
 	struct nexthop_srv6 *nh_srv6;
 };
 
+/* all hashed fields (including padding, if it is necessary to add) need to
+ * be listed in the static_assert below
+ */
+
+#define S(field) sizeof(((struct nexthop *)NULL)->field)
+
+static_assert(
+	offsetof(struct nexthop, _hash_end) - offsetof(struct nexthop, _hash_begin) ==
+		S(type) + S(vrf_id) + S(ifindex) + S(nh_label_type) + S(gate) + S(src) + S(rmap_src),
+	"struct nexthop contains padding, this can break things. insert _pad fields at appropriate places");
+
+#undef S
+
+/* this is here to show exactly what is meant by the comments above about
+ * the hashing
+ */
+static inline uint32_t _nexthop_hash_bytes(const struct nexthop *nh, uint32_t seed)
+{
+	return jhash(&nh->_hash_begin,
+		     offsetof(struct nexthop, _hash_end) - offsetof(struct nexthop, _hash_begin),
+		     seed);
+}
+
 /* Utility to append one nexthop to another. */
 #define NEXTHOP_APPEND(to, new)           \
 	do {                              \
@@ -181,27 +229,11 @@ struct nexthop *nexthop_from_blackhole(enum blackhole_type bh_type,
 /*
  * Hash a nexthop. Suitable for use with hash tables.
  *
- * This function uses the following values when computing the hash:
- * - vrf_id
- * - ifindex
- * - type
- * - gate
- *
- * nexthop
- *    The nexthop to hash
- *
- * Returns:
- *    32-bit hash of nexthop
+ * Please double check the code on what is included in the hash, there was
+ * documentation here but it got outdated and the only thing worse than no
+ * doc is incorrect doc.
  */
 uint32_t nexthop_hash(const struct nexthop *nexthop);
-/*
- * Hash a nexthop only on word-sized attributes:
- * - vrf_id
- * - ifindex
- * - type
- * - (some) flags
- */
-uint32_t nexthop_hash_quick(const struct nexthop *nexthop);
 
 extern bool nexthop_same(const struct nexthop *nh1, const struct nexthop *nh2);
 extern bool nexthop_same_no_labels(const struct nexthop *nh1,