Merge branch 'allow-configuration-of-multipath-hash-seed'

Petr Machata says:

====================
Allow configuration of multipath hash seed

Let me just quote the commit message of patch #2 here to inform the
motivation and some of the implementation:

    When calculating hashes for the purpose of multipath forwarding,
    both IPv4 and IPv6 code currently fall back on
    flow_hash_from_keys(). That uses a randomly-generated seed. That's a
    fine choice by default, but unfortunately some deployments may need
    a tighter control over the seed used.

    In this patchset, make the seed configurable by adding a new sysctl
    key, net.ipv4.fib_multipath_hash_seed to control the seed. This seed
    is used specifically for multipath forwarding and not for the other
    concerns that flow_hash_from_keys() is used for, such as queue
    selection. Expose the knob as sysctl because other such settings,
    such as headers to hash, are also handled that way.

    Despite being placed in the net.ipv4 namespace, the multipath seed
    sysctl is used for both IPv4 and IPv6, similarly to e.g. a number of
    TCP variables. Like those, the multipath hash seed is a per-netns
    variable.

    The seed used by flow_hash_from_keys() is a 128-bit quantity.
    However it seems that usually the seed is a much more modest value.
    32 bits seem typical (Cisco, Cumulus), some systems go even lower.
    For that reason, and to decouple the user interface from
    implementation details, go with a 32-bit quantity, which is then
    quadruplicated to form the siphash key.

One example of use of this interface is avoiding hash polarization,
where two ECMP routers, one behind the other, happen to make consistent
hashing decisions, and as a result, part of the ECMP space of the latter
router is never used. Another is a load balancer where several machines
forward traffic to one of a number of leaves, and the forwarding
decisions need to be made consistently. (This is a case of a desired
hash polarization, mentioned e.g. in chapter 6.3 of [0].)

There has already been a proposal to include a hash seed control
interface in the past[1].

- Patches #1-#2 contain the substance of the work
- Patch #3 is an mlxsw offload
- Patches #4 and #5 are a selftest

[0] https://www.usenix.org/system/files/conference/nsdi18/nsdi18-araujo.pdf
[1] https://lore.kernel.org/netdev/YIlVpYMCn%2F8WfE1P@rnd/
====================

Link: https://lore.kernel.org/r/20240607151357.421181-1-petrm@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

1 files changed, 66 insertions, 0 deletions

diff --git a/net/ipv4/sysctl_net_ipv4.c b/net/ipv4/sysctl_net_ipv4.c
index bb64c0ef092d..9140d20eb2d4 100644
--- a/net/ipv4/sysctl_net_ipv4.c
+++ b/net/ipv4/sysctl_net_ipv4.c
@@ -464,6 +464,61 @@ static int proc_fib_multipath_hash_fields(struct ctl_table *table, int write,
 
 	return ret;
 }
+
+static u32 proc_fib_multipath_hash_rand_seed __ro_after_init;
+
+static void proc_fib_multipath_hash_init_rand_seed(void)
+{
+	get_random_bytes(&proc_fib_multipath_hash_rand_seed,
+			 sizeof(proc_fib_multipath_hash_rand_seed));
+}
+
+static void proc_fib_multipath_hash_set_seed(struct net *net, u32 user_seed)
+{
+	struct sysctl_fib_multipath_hash_seed new = {
+		.user_seed = user_seed,
+		.mp_seed = (user_seed ? user_seed :
+			    proc_fib_multipath_hash_rand_seed),
+	};
+
+	WRITE_ONCE(net->ipv4.sysctl_fib_multipath_hash_seed, new);
+}
+
+static int proc_fib_multipath_hash_seed(struct ctl_table *table, int write,
+					void *buffer, size_t *lenp,
+					loff_t *ppos)
+{
+	struct sysctl_fib_multipath_hash_seed *mphs;
+	struct net *net = table->data;
+	struct ctl_table tmp;
+	u32 user_seed;
+	int ret;
+
+	mphs = &net->ipv4.sysctl_fib_multipath_hash_seed;
+	user_seed = mphs->user_seed;
+
+	tmp = *table;
+	tmp.data = &user_seed;
+
+	ret = proc_douintvec_minmax(&tmp, write, buffer, lenp, ppos);
+
+	if (write && ret == 0) {
+		proc_fib_multipath_hash_set_seed(net, user_seed);
+		call_netevent_notifiers(NETEVENT_IPV4_MPATH_HASH_UPDATE, net);
+	}
+
+	return ret;
+}
+#else
+
+static void proc_fib_multipath_hash_init_rand_seed(void)
+{
+}
+
+static void proc_fib_multipath_hash_set_seed(struct net *net, u32 user_seed)
+{
+}
+
 #endif
 
 static struct ctl_table ipv4_table[] = {
@@ -1072,6 +1127,13 @@ static struct ctl_table ipv4_net_table[] = {
 		.extra1		= SYSCTL_ONE,
 		.extra2		= &fib_multipath_hash_fields_all_mask,
 	},
+	{
+		.procname	= "fib_multipath_hash_seed",
+		.data		= &init_net,
+		.maxlen		= sizeof(u32),
+		.mode		= 0644,
+		.proc_handler	= proc_fib_multipath_hash_seed,
+	},
 #endif
 	{
 		.procname	= "ip_unprivileged_port_start",
@@ -1550,6 +1612,8 @@ static __net_init int ipv4_sysctl_init_net(struct net *net)
 	if (!net->ipv4.sysctl_local_reserved_ports)
 		goto err_ports;
 
+	proc_fib_multipath_hash_set_seed(net, 0);
+
 	return 0;
 
 err_ports:
@@ -1584,6 +1648,8 @@ static __init int sysctl_ipv4_init(void)
 	if (!hdr)
 		return -ENOMEM;
 
+	proc_fib_multipath_hash_init_rand_seed();
+
 	if (register_pernet_subsys(&ipv4_sysctl_ops)) {
 		unregister_net_sysctl_table(hdr);
 		return -ENOMEM;