Re: [PATCH 11/16 net-next v3] ipv4: disable IPv4-only sysctls when CONFIG_IPV4=n

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- [Medium] The patch moves the net.ipv4.fib_multipath_hash_seed entry of
  ipv4_net_table (net/ipv4/sysctl_net_ipv4.c, lines 1644-1650) inside…

--

ipv4: disable IPv4-only sysctls when CONFIG_IPV4=n

This commit moves the IPv4-specific entries of ipv4_net_table into a new
IS_ENABLED(CONFIG_IPV4) block at the end of the table. Those sysctls are
then not registered on kernels built without IPv4. Handlers and min/max
variables used only by those entries are marked __maybe_unused.

> diff --git a/net/ipv4/sysctl_net_ipv4.c b/net/ipv4/sysctl_net_ipv4.c
> index 0e7fef5973db6..6096e9e4d82d8 100644
> --- a/net/ipv4/sysctl_net_ipv4.c
> +++ b/net/ipv4/sysctl_net_ipv4.c

[ ... ]

> @@ -1656,6 +1469,196 @@ static const struct ctl_table ipv4_net_table[] = {
>  		.extra1		= SYSCTL_ONE_THOUSAND,
>  		.extra2		= &tcp_rto_max_max,
>  	},
> +#if IS_ENABLED(CONFIG_IPV4)

[ ... ]

> +#ifdef CONFIG_IP_ROUTE_MULTIPATH

[ ... ]

> +	{
> +		.procname	= "fib_multipath_hash_seed",
> +		.data		= &init_net,
> +		.maxlen		= sizeof(u32),
> +		.mode		= 0644,
> +		.proc_handler	= proc_fib_multipath_hash_seed,
> +	},
> +#endif

[Severity: Medium]
Is fib_multipath_hash_seed really IPv4-only? The commit message says it
hides "the sysctls that are related to IPv4 only". However, the IPv6
datapath also uses this seed.

The shared helper in include/net/ip_fib.h is built whenever
CONFIG_IP_ROUTE_MULTIPATH is set:

include/net/ip_fib.h:fib_multipath_hash_from_keys() {
    ...
	mp_seed = READ_ONCE(net->ipv4.sysctl_fib_multipath_hash_seed.mp_seed);
	fib_multipath_hash_construct_key(&hash_key, mp_seed);
    ...
}

IPv6 ECMP reaches it through rt6_multipath_hash() and its custom hash
helpers in net/ipv6/route.c:

rt6_multipath_hash()->fib_multipath_hash_from_keys()

IP_ROUTE_MULTIPATH depends only on IP_ADVANCED_ROUTER, not on IPV4. Also,
ipv4_sysctl_init_net() still calls proc_fib_multipath_hash_set_seed(net, 0)
for every netns.

Take a kernel with CONFIG_IPV4=n, CONFIG_IPV6=y and
CONFIG_IP_ROUTE_MULTIPATH=y. That becomes possible once "ipv4: make
CONFIG_IPV4 boolean" later in this series makes IPV4 selectable. Won't
IPv6 multipath hashing then keep using a random seed that userspace can
no longer set?

There is no net.ipv6 copy of this seed. net/ipv6/sysctl_net_ipv6.c only
has its own fib_multipath_hash_policy. The NETEVENT_IPV4_MPATH_HASH_UPDATE
notification sent from proc_fib_multipath_hash_seed() for offload drivers
would also never be sent.

Documentation/networking/ip-sysctl.rst describes this sysctl as:

  The seed value used when calculating hash for multipath routes. Applies
  to both IPv4 and IPv6 datapath. Only present for kernels built with
  CONFIG_IP_ROUTE_MULTIPATH enabled.

proc_fib_multipath_hash_seed() is marked __maybe_unused in this patch.
That suggests the IPV4=n with IP_ROUTE_MULTIPATH=y case was expected.

Should this entry stay outside the CONFIG_IPV4 block, guarded only by
CONFIG_IP_ROUTE_MULTIPATH?

> +	{
> +		.procname	= "fib_notify_on_flag_change",

[ ... ]

-- 
Sashiko AI review · https://netdev-ai.bots.linux.dev/sashiko/#/patchset/20260930135334.4739-1-fmancera%40suse.de