ipv4: Refactor nhc evaluation in fib_table_lookup

FIB lookups can return an entry that references an external nexthop.
While walking the nexthop struct we do not want to make multiple calls
into the nexthop code which can result in 2 different structs getting
accessed - one returning the number of paths the rest of the loop
seeing a different nh_grp struct. If the nexthop group shrunk, the
result is an attempt to access a fib_nh_common that does not exist for
the new nh_grp struct but did for the old one.

To fix that move the device evaluation code to a helper that can be
used for inline fib_nh path as well as external nexthops.

Update the existing check for fi->nh in fib_table_lookup to call a
new helper, nexthop_get_nhc_lookup, which walks the external nexthop
with a single rcu dereference.

Fixes: 430a049190de ("nexthop: Add support for nexthop groups")
Signed-off-by: David Ahern <dsahern@gmail.com>
Acked-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

2 files changed, 35 insertions, 0 deletions

diff --git a/include/net/ip_fib.h b/include/net/ip_fib.h
index b219a8fe0950..771ce068bc96 100644
--- a/include/net/ip_fib.h
+++ b/include/net/ip_fib.h
@@ -479,6 +479,8 @@ void fib_nh_common_release(struct fib_nh_common *nhc);
 void fib_alias_hw_flags_set(struct net *net, const struct fib_rt_info *fri);
 void fib_trie_init(void);
 struct fib_table *fib_trie_table(u32 id, struct fib_table *alias);
+bool fib_lookup_good_nhc(const struct fib_nh_common *nhc, int fib_flags,
+			 const struct flowi4 *flp);
 
 static inline void fib_combine_itag(u32 *itag, const struct fib_result *res)
 {
diff --git a/include/net/nexthop.h b/include/net/nexthop.h
index f09e8d7d9886..9414ae46fc1c 100644
--- a/include/net/nexthop.h
+++ b/include/net/nexthop.h
@@ -233,6 +233,39 @@ struct fib_nh_common *nexthop_fib_nhc(struct nexthop *nh, int nhsel)
 	return &nhi->fib_nhc;
 }
 
+/* called from fib_table_lookup with rcu_lock */
+static inline
+struct fib_nh_common *nexthop_get_nhc_lookup(const struct nexthop *nh,
+					     int fib_flags,
+					     const struct flowi4 *flp,
+					     int *nhsel)
+{
+	struct nh_info *nhi;
+
+	if (nh->is_group) {
+		struct nh_group *nhg = rcu_dereference(nh->nh_grp);
+		int i;
+
+		for (i = 0; i < nhg->num_nh; i++) {
+			struct nexthop *nhe = nhg->nh_entries[i].nh;
+
+			nhi = rcu_dereference(nhe->nh_info);
+			if (fib_lookup_good_nhc(&nhi->fib_nhc, fib_flags, flp)) {
+				*nhsel = i;
+				return &nhi->fib_nhc;
+			}
+		}
+	} else {
+		nhi = rcu_dereference(nh->nh_info);
+		if (fib_lookup_good_nhc(&nhi->fib_nhc, fib_flags, flp)) {
+			*nhsel = 0;
+			return &nhi->fib_nhc;
+		}
+	}
+
+	return NULL;
+}
+
 static inline unsigned int fib_info_num_path(const struct fib_info *fi)
 {
 	if (unlikely(fi->nh))