Re: [PATCH net-next 1/2] ipv4: devinet: list global scope addresses before link scope addresses

[Ido Schimmel]

On Mon, Jul 20, 2026 at 12:16:23PM -0600, Tim Wong wrote:
> __inet_insert_ifa() inserts a new primary address by advancing an
> insertion pointer past every existing primary address whose scope
> is >= the new address's scope. Because IPv4 scope values are
> numerically smaller for wider scopes (RT_SCOPE_UNIVERSE < ... <
> RT_SCOPE_LINK < RT_SCOPE_HOST), the comparison
> 
>     ifa->ifa_scope <= ifa1->ifa_scope
> 
> is true for a global-scope new address against essentially every
> existing entry, so the new address is pushed all the way to the
> tail of the primary address list, ending up *after* any link-scope
> addresses that were configured earlier.
> 
> On an interface carrying a mix of global- and link-scope IPv4
> addresses (e.g. a routable address alongside an RFC 3927
> 169.254.0.0/16 address, or any address explicitly assigned link
> scope), this makes the resulting order in in_dev->ifa_list -- and
> therefore the order addresses are reported via netlink
> (RTM_GETADDR), ioctl (SIOCGIFCONF), and /proc/net -- depend on
> configuration order rather than scope. Userspace consumers that
> pick the first address returned for an interface (e.g. via
> getifaddrs()) can end up preferring a link-scope address over a
> global one.

It's a user space problem. The kernel provides all the needed
information for user space to make an educated choice.

There was already an attempt to change IPv6's intra-scope order to match
IPv4's and it broke user space:

https://lore.kernel.org/all/20260529112357.5079-1-fmancera@xxxxxxx/

Now you propose changing IPv4's inter-scope order to match IPv6's. It
will most likely break user space and kernel selftests. Please solve
this in user space.

> 
> IPv6 already avoids this: ipv6_add_addr() keeps idev->addr_list
> ordered so global-scope addresses precede link-local ones
> regardless of configuration order.

I don't understand the point about configuration order. Only the
intra-scope order is determined by configuration order, no?

> 
> Fix the comparison so the insertion pointer only advances past
> addresses that are at least as global as the new one:
> 
>     ifa->ifa_scope >= ifa1->ifa_scope
> 
> This groups global-scope primary addresses ahead of link-scope
> primary addresses in in_dev->ifa_list, preserving insertion order
> within each scope group, and brings IPv4 address enumeration order
> in line with existing IPv6 behavior.
> 
> Signed-off-by: kanman.wong <kanman.wong@xxxxxxxx>
> ---
>  net/ipv4/devinet.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/net/ipv4/devinet.c b/net/ipv4/devinet.c
> index a35b72662e43..056f2169c6a3 100644
> --- a/net/ipv4/devinet.c
> +++ b/net/ipv4/devinet.c
> @@ -510,7 +510,7 @@ static int __inet_insert_ifa(struct in_ifaddr
> *ifa, struct nlmsghdr *nlh,
> 
>   while (ifa1) {
>   if (!(ifa1->ifa_flags & IFA_F_SECONDARY) &&
> -     ifa->ifa_scope <= ifa1->ifa_scope)
> +     ifa->ifa_scope >= ifa1->ifa_scope)
>   last_primary = &ifa1->ifa_next;
>   if (ifa1->ifa_mask == ifa->ifa_mask &&
>       inet_ifa_match(ifa1->ifa_address, ifa)) {

The patch is whitespace-damaged.

> 
> base-commit: ce6b4d3216b63f902bb8e9695ee6c10c83415f65
> -- 
> 2.51.0