Subject: [7/7] tcp: drop the hash_32() part from the index calculation

From: Willy Tarreau <[email-masked]>
Date: Mon, 2 May 2022 10:46:14 +0200

commit e8161345ddbb66e449abde10d2fdce93f867eba9 upstream.

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 <[email-masked]>
Reviewed-by: Eric Dumazet <[email-masked]>
Signed-off-by: Willy Tarreau <[email-masked]>
Signed-off-by: Jakub Kicinski <[email-masked]>
Signed-off-by: Ben Hutchings <[email-masked]>
---
 net/ipv4/inet_hashtables.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/ipv4/inet_hashtables.c b/net/ipv4/inet_hashtables.c
index 25766b9bdeff..19369fc9bcda 100644
--- a/net/ipv4/inet_hashtables.c
+++ b/net/ipv4/inet_hashtables.c
@@ -640,7 +640,7 @@ 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;