[PATCH v7 10/10] netfilter: ipset: add comment how cidr bookkeeping is working

[Jozsef Kadlecsik <kadlec@netfilter.org>]

Sashiko thinks that cidr bookkeeping might be unsafe because the
concurrent RCU reader in mtype_test_cidrs() uses the data without
sequence locks or read-side barriers. However every right shift
(add new entry) and left shift (delete entry) is performed by
duplicating the entry just shifted. Therefore concurrent reader
will just duplicate a test with the same values as just before:
existing entries cannot be skipped.

Signed-off-by: Jozsef Kadlecsik <kadlec@netfilter.org>
---
 net/netfilter/ipset/ip_set_hash_gen.h | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/net/netfilter/ipset/ip_set_hash_gen.h b/net/netfilter/ipset/ip_set_hash_gen.h
index 9d1fcf6c8328..6838b46df9b8 100644
--- a/net/netfilter/ipset/ip_set_hash_gen.h
+++ b/net/netfilter/ipset/ip_set_hash_gen.h
@@ -342,6 +342,12 @@ mtype_add_cidr(struct ip_set *set, struct htype *h, u8 cidr, u8 n)
 		}
 	}
 	if (j != -1) {
+		/* We shift the cidr values to the right
+		 * by duplicating the entries one by one,
+		 * starting from the end.
+		 * It means the same test can be repeated twice
+		 * by a concurrent mtype_test_cidrs() reader.
+		 */
 		for (; i > j; i--)
 			h->nets[i].cidr[n] = h->nets[i - 1].cidr[n];
 	}
@@ -363,6 +369,11 @@ mtype_del_cidr(struct ip_set *set, struct htype *h, u8 cidr, u8 n)
 		h->nets[CIDR_POS(cidr)].nets[n]--;
 		if (h->nets[CIDR_POS(cidr)].nets[n] > 0)
 			goto unlock;
+		/* We shift the cidr values to the left
+		 * by duplicating the remaining entries one by one.
+		 * It means the same test can be repeated twice
+		 * by a concurrent mtype_test_cidrs() reader.
+		 */
 		for (j = i; j < net_end && h->nets[j].cidr[n]; j++)
 			h->nets[j].cidr[n] = h->nets[j + 1].cidr[n];
 		h->nets[j].cidr[n] = 0;
-- 
2.39.5