From: Stewart Smith trawets@amazon.com
[ Upstream commit d11b0df7ddf1831f3e170972f43186dad520bfcc ]
For both IPv4 and IPv6 incoming TCP connections are tracked in a hash table with a hash over the source & destination addresses and ports. However, the IPv6 hash is insufficient and can lead to a high rate of collisions.
The IPv6 hash used an XOR to fit everything into the 96 bits for the fast jenkins hash, meaning it is possible for an external entity to ensure the hash collides, thus falling back to a linear search in the bucket, which is slow.
We take the approach of hash the full length of IPv6 address in __ipv6_addr_jhash() so that all users can benefit from a more secure version.
While this may look like it adds overhead, the reality of modern CPUs means that this is unmeasurable in real world scenarios.
In simulating with llvm-mca, the increase in cycles for the hashing code was ~16 cycles on Skylake (from a base of ~155), and an extra ~9 on Nehalem (base of ~173).
In commit dd6d2910c5e0 ("netfilter: conntrack: switch to siphash") netfilter switched from a jenkins hash to a siphash, but even the faster hsiphash is a more significant overhead (~20-30%) in some preliminary testing. So, in this patch, we keep to the more conservative approach to ensure we don't add much overhead per SYN.
In testing, this results in a consistently even spread across the connection buckets. In both testing and real-world scenarios, we have not found any measurable performance impact.
Fixes: 08dcdbf6a7b9 ("ipv6: use a stronger hash for tcp") Signed-off-by: Stewart Smith trawets@amazon.com Signed-off-by: Samuel Mendoza-Jonas samjonas@amazon.com Suggested-by: Eric Dumazet edumazet@google.com Signed-off-by: Kuniyuki Iwashima kuniyu@amazon.com Reviewed-by: Eric Dumazet edumazet@google.com Link: https://lore.kernel.org/r/20230721222410.17914-1-kuniyu@amazon.com Signed-off-by: Jakub Kicinski kuba@kernel.org Signed-off-by: Sasha Levin sashal@kernel.org --- include/net/ipv6.h | 8 ++------ 1 file changed, 2 insertions(+), 6 deletions(-)
diff --git a/include/net/ipv6.h b/include/net/ipv6.h index d383c895592a9..e4ceef687c1c2 100644 --- a/include/net/ipv6.h +++ b/include/net/ipv6.h @@ -718,12 +718,8 @@ static inline u32 ipv6_addr_hash(const struct in6_addr *a) /* more secured version of ipv6_addr_hash() */ static inline u32 __ipv6_addr_jhash(const struct in6_addr *a, const u32 initval) { - u32 v = (__force u32)a->s6_addr32[0] ^ (__force u32)a->s6_addr32[1]; - - return jhash_3words(v, - (__force u32)a->s6_addr32[2], - (__force u32)a->s6_addr32[3], - initval); + return jhash2((__force const u32 *)a->s6_addr32, + ARRAY_SIZE(a->s6_addr32), initval); }
static inline bool ipv6_addr_loopback(const struct in6_addr *a)