Re: [PATCH net-next 2/3] ip: load balance tcp connections to single dst addr and port

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

Willem de Bruijn wrote:
> From: Willem de Bruijn <willemb@google.com>
> 
> Load balance new TCP connections across nexthops also when they
> connect to the same service at a single remote address and port.
> 
> This affects only port-based multipath hashing:
> fib_multipath_hash_policy 1 or 3.
> 
> Local connections must choose both a source address and port when
> connecting to a remote service, in ip_route_connect. This
> "chicken-and-egg problem" (commit 2d7192d6cbab ("ipv4: Sanitize and
> simplify ip_route_{connect,newports}()")) is resolved by first
> selecting a source address, by looking up a route using the zero
> wildcard source port and address.
> 
> As a result multiple connections to the same destination address and
> port have no entropy in fib_multipath_hash.
> 
> This is not a problem when forwarding, as skb-based hashing has a
> 4-tuple. Nor when establishing UDP connections, as autobind there
> selects a port before reaching ip_route_connect.
> 
> Load balance also TCP, by using a random port in fib_multipath_hash.
> Port assignment in inet_hash_connect is not atomic with
> ip_route_connect. Thus ports are unpredictable, effectively random.
> 
> Implementation details:
> 
> Do not actually pass a random fl4_sport, as that affects not only
> hashing, but routing more broadly, and can match a source port based
> policy route, which existing wildcard port 0 will not. Instead,
> define a new wildcard flowi flag that is used only for hashing.
> 
> Selecting a random source is equivalent to just selecting a random
> hash entirely. But for code clarity, follow the normal 4-tuple hash
> process and only update this field.
> 
> fib_multipath_hash can be reached with zero sport from other code
> paths, so explicitly pass this flowi flag, rather than trying to infer
> this case in the function itself.
> 
> Signed-off-by: Willem de Bruijn <willemb@google.com>
> ---
>  include/net/flow.h  |  1 +
>  include/net/route.h |  3 +++
>  net/ipv4/route.c    | 13 ++++++++++---
>  net/ipv6/route.c    | 13 ++++++++++---
>  net/ipv6/tcp_ipv6.c |  2 ++
>  5 files changed, 26 insertions(+), 6 deletions(-)
> 
> diff --git a/include/net/flow.h b/include/net/flow.h
> index 2a3f0c42f092..a1839c278d87 100644
> --- a/include/net/flow.h
> +++ b/include/net/flow.h
> @@ -39,6 +39,7 @@ struct flowi_common {
>  #define FLOWI_FLAG_ANYSRC		0x01
>  #define FLOWI_FLAG_KNOWN_NH		0x02
>  #define FLOWI_FLAG_L3MDEV_OIF		0x04
> +#define FLOWI_FLAG_ANY_SPORT		0x08
>  	__u32	flowic_secid;
>  	kuid_t  flowic_uid;
>  	__u32		flowic_multipath_hash;
> diff --git a/include/net/route.h b/include/net/route.h
> index c605fd5ec0c0..8e39aa822cf9 100644
> --- a/include/net/route.h
> +++ b/include/net/route.h
> @@ -326,6 +326,9 @@ static inline void ip_route_connect_init(struct flowi4 *fl4, __be32 dst,
>  	if (inet_test_bit(TRANSPARENT, sk))
>  		flow_flags |= FLOWI_FLAG_ANYSRC;
>  
> +	if (IS_ENABLED(CONFIG_IP_ROUTE_MULTIPATH) && !sport)
> +		flow_flags |= FLOWI_FLAG_ANY_SPORT;
> +
>  	flowi4_init_output(fl4, oif, READ_ONCE(sk->sk_mark), ip_sock_rt_tos(sk),
>  			   ip_sock_rt_scope(sk), protocol, flow_flags, dst,
>  			   src, dport, sport, sk->sk_uid);
> diff --git a/net/ipv4/route.c b/net/ipv4/route.c
> index e5e4c71be3af..685e8d3b4f5d 100644
> --- a/net/ipv4/route.c
> +++ b/net/ipv4/route.c
> @@ -2037,8 +2037,12 @@ static u32 fib_multipath_custom_hash_fl4(const struct net *net,
>  		hash_keys.addrs.v4addrs.dst = fl4->daddr;
>  	if (hash_fields & FIB_MULTIPATH_HASH_FIELD_IP_PROTO)
>  		hash_keys.basic.ip_proto = fl4->flowi4_proto;
> -	if (hash_fields & FIB_MULTIPATH_HASH_FIELD_SRC_PORT)
> -		hash_keys.ports.src = fl4->fl4_sport;
> +	if (hash_fields & FIB_MULTIPATH_HASH_FIELD_SRC_PORT) {
> +		if (fl4->flowi4_flags & FLOWI_FLAG_ANY_SPORT)
> +			hash_keys.ports.src = get_random_u16();
> +		else
> +			hash_keys.ports.src = fl4->fl4_sport;
> +	}
>  	if (hash_fields & FIB_MULTIPATH_HASH_FIELD_DST_PORT)
>  		hash_keys.ports.dst = fl4->fl4_dport;
>  
> @@ -2093,7 +2097,10 @@ int fib_multipath_hash(const struct net *net, const struct flowi4 *fl4,
>  			hash_keys.control.addr_type = FLOW_DISSECTOR_KEY_IPV4_ADDRS;
>  			hash_keys.addrs.v4addrs.src = fl4->saddr;
>  			hash_keys.addrs.v4addrs.dst = fl4->daddr;
> -			hash_keys.ports.src = fl4->fl4_sport;
> +			if (fl4->flowi4_flags & FLOWI_FLAG_ANY_SPORT)
> +				hash_keys.ports.src = get_random_u16();
> +			else
> +				hash_keys.ports.src = fl4->fl4_sport;
>  			hash_keys.ports.dst = fl4->fl4_dport;
>  			hash_keys.basic.ip_proto = fl4->flowi4_proto;
>  		}
> diff --git a/net/ipv6/route.c b/net/ipv6/route.c
> index 945857a8bfe3..39f07cdbbc64 100644
> --- a/net/ipv6/route.c
> +++ b/net/ipv6/route.c
> @@ -2492,8 +2492,12 @@ static u32 rt6_multipath_custom_hash_fl6(const struct net *net,
>  		hash_keys.basic.ip_proto = fl6->flowi6_proto;
>  	if (hash_fields & FIB_MULTIPATH_HASH_FIELD_FLOWLABEL)
>  		hash_keys.tags.flow_label = (__force u32)flowi6_get_flowlabel(fl6);
> -	if (hash_fields & FIB_MULTIPATH_HASH_FIELD_SRC_PORT)
> -		hash_keys.ports.src = fl6->fl6_sport;
> +	if (hash_fields & FIB_MULTIPATH_HASH_FIELD_SRC_PORT) {
> +		if (fl6->flowi6_flags & FLOWI_FLAG_ANY_SPORT)
> +			hash_keys.ports.src = get_random_u16();
> +		else
> +			hash_keys.ports.src = fl6->fl6_sport;
> +	}
>  	if (hash_fields & FIB_MULTIPATH_HASH_FIELD_DST_PORT)
>  		hash_keys.ports.dst = fl6->fl6_dport;
>  
> @@ -2547,7 +2551,10 @@ u32 rt6_multipath_hash(const struct net *net, const struct flowi6 *fl6,
>  			hash_keys.control.addr_type = FLOW_DISSECTOR_KEY_IPV6_ADDRS;
>  			hash_keys.addrs.v6addrs.src = fl6->saddr;
>  			hash_keys.addrs.v6addrs.dst = fl6->daddr;
> -			hash_keys.ports.src = fl6->fl6_sport;
> +			if (fl6->flowi6_flags & FLOWI_FLAG_ANY_SPORT)
> +				hash_keys.ports.src = get_random_u16();

I missed the __be16 endianness here and in the related cases

That'll teach me to forget running

    make C=2 CF=-D__CHECK_ENDIAN__ net/ipv4/route.o [...]


> +			else
> +				hash_keys.ports.src = fl6->fl6_sport;