Re: [PATCH v3] netlink: Return prefix length for IPv6 addresses in nl_addr_get()

On Mon, Mar 09, 2026 at 02:04:40PM -0400, Jon Maloy wrote:

On 2026-03-09 05:56, Stefano Brivio wrote:

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The patch looks good to me now, but I have two questions:

On Sat, 7 Mar 2026 13:41:57 -0500 Jon Maloy wrote:

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nl_addr_get() was not setting the prefix_len output parameter for IPv6 addresses, only for IPv4. This meant callers always got 0 for IPv6, forcing them to use a hardcoded default (64).

Fix by assigning *prefix_len even in the IPv6 case.

We also add another functional change. We now check for if an AF_INET address is link local, in which case we have to skip it.

The reason why the original code skipped IPv6 link-local addresses and not IPv4 link-local ones is that copying a IPv6 link-local address clearly makes no sense and breaks things.

For IPv4 I wasn't quite sure, and it seemed to work just like other addresses, so I never took care of excluding them.

I tend to think it's correct to exclude them, also for consistency with IPv6, but I'm not quite sure if we risk breaking something. I have some vague recollection of link-local addresses being used in some cloud (probably Google Computing Platform), at least for some Podman tests. I'll try to find some pointers to it.

Did you already look into the matter, though?

Honestly I though it was just an oversight.

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By the way, this makes things inconsistent with nl_addr_dup() (used by the vast majority of users), where IPv4 link-local addresses are copied just like all the other ones.

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Although it is conventional to set the scope of such addresses to RT_SCOPE_LINK,

You mean that users manually do that? I think it's kind of rare actually. Does the kernel do that? Or configuration agents such as NetworkManager? I wonder a bit what you consider as "user" here.

It is normally set by NetworkManager,

Or, presumably by ip(8).

but I don't think that means we can trust it at 100%.

Actually, I think this means we should trust it more. I'd see this as an explicit indication that regardless of any normal conventions, it's being treated as a link-local address in the configuration of this specific host, so we should too. That's moot for the current patch, based on our earlier discussions, but I think it's a relevant point for what we do in future. -- David Gibson (he or they) | I'll have my music baroque, and my code david AT gibson.dropbear.id.au | minimalist, thank you, not the other way | around. http://www.ozlabs.org/~dgibson