[PATCH 1/6] flow: Fix incorrect hash probe in flowside_lookup()

[David Gibson <david@gibson.dropbear.id.au>]

Our flow hash table uses linear probing in which we step backwards through
clusters of adjacent hash entries when we have near collisions.  Usually
that's implemented by flow_hash_probe().  However, due to some details we
need a second implementation in flowside_lookup().  An embarrassing
oversight in rebasing from earlier versions has mean that version is
incorrect, trying to step forward through clusters rather than backward.

In situations with the right sorts of has near-collisions this can lead to
us not associating an ACK from the tap device with the right flow, leaving
it in a not-quite-established state.  If the remote peer does a shutdown()
at the right time, this can lead to a storm of EPOLLRDHUP events causing
high CPU load.

Fixes: acca4235c ("flow, tcp: Generalise TCP hash table to general...")
Link: https://bugs.passt.top/show_bug.cgi?id=94

Suggested-by: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
---
 flow.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/flow.c b/flow.c
index 02631eb2..a00e01d1 100644
--- a/flow.c
+++ b/flow.c
@@ -697,7 +697,7 @@ static flow_sidx_t flowside_lookup(const struct ctx *c, uint8_t proto,
 	       !(FLOW_PROTO(&flow->f) == proto &&
 		 flow->f.pif[sidx.sidei] == pif &&
 		 flowside_eq(&flow->f.side[sidx.sidei], side)))
-		b = (b + 1) % FLOW_HASH_SIZE;
+		b = mod_sub(b, 1, FLOW_HASH_SIZE);
 
 	return flow_hashtab[b];
 }
-- 
2.46.0