tcp: drop the hash_32() part from the index calculation

In commit 190cc82489f4 ("tcp: change source port randomizarion at
connect() time"), the table_perturb[] array was introduced and an
index was taken from the port_offset via hash_32(). But it turns
out that hash_32() performs a multiplication while the input here
comes from the output of SipHash in secure_seq, that is well
distributed enough to avoid the need for yet another hash.

Suggested-by: Amit Klein <aksecurity@gmail.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/net/ipv4/inet_hashtables.c b/net/ipv4/inet_hashtables.c
index cc5f663..a5d57fa 100644 (file)
--- a/net/ipv4/inet_hashtables.c
+++ b/net/ipv4/inet_hashtables.c
@@ -778,7 +778,7 @@ int __inet_hash_connect(struct inet_timewait_death_row *death_row,
 
        net_get_random_once(table_perturb,
                            INET_TABLE_PERTURB_SIZE * sizeof(*table_perturb));
-       index = hash_32(port_offset, INET_TABLE_PERTURB_SHIFT);
+       index = port_offset & (INET_TABLE_PERTURB_SIZE - 1);
 
        offset = READ_ONCE(table_perturb[index]) + (port_offset >> 32);
        offset %= remaining;