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

[Stefano Brivio <sbrivio@redhat.com>]

On Tue,  3 Mar 2026 14:16:41 -0500
Jon Maloy <jmaloy@redhat.com> wrote:

> 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.
> 
> Signed-off-by: Jon Maloy <jmaloy@redhat.com>
> Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
> ---
>  netlink.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/netlink.c b/netlink.c
> index 82a2f0c..769cb23 100644
> --- a/netlink.c
> +++ b/netlink.c
> @@ -752,7 +752,7 @@ int nl_addr_set_ll_nodad(int s, unsigned int ifi)
>   * @ifi:	Interface index in outer network namespace
>   * @af:		Address family
>   * @addr:	Global address to fill
> - * @prefix_len:	Mask or prefix length, to fill (for IPv4)
> + * @prefix_len:	Mask or prefix length, to fill
>   * @addr_l:	Link-scoped address to fill (for IPv6)
>   *
>   * Return: 0 on success, negative error code on failure
> @@ -797,7 +797,7 @@ int nl_addr_get(int s, unsigned int ifi, sa_family_t af,
>  				   ifa->ifa_prefixlen > prefix_max) {
>  				memcpy(addr, RTA_DATA(rta), RTA_PAYLOAD(rta));
>  
> -				prefix_max = ifa->ifa_prefixlen;
> +				prefix_max = *prefix_len = ifa->ifa_prefixlen;

Sorry, I spotted this just now and not in the original series, because
I hadn't really looked at the context there.

The main reason why there are two separate blocks for IPv4 and IPv6,
here, is the *prefix_len assignment. Now that this becomes the same for
IPv6, that doesn't hold anymore, so, instead of:

			if (af == AF_INET && ifa->ifa_prefixlen > prefix_max) {
				memcpy(addr, RTA_DATA(rta), RTA_PAYLOAD(rta));

				prefix_max = *prefix_len = ifa->ifa_prefixlen;
			} else if (af == AF_INET6 && addr &&
				   ifa->ifa_scope < RT_SCOPE_LINK &&
				   ifa->ifa_prefixlen > prefix_max) {
				memcpy(addr, RTA_DATA(rta), RTA_PAYLOAD(rta));

				prefix_max = *prefix_len = ifa->ifa_prefixlen;
			}

you could simplify all that to:

			if (ifa->ifa_prefixlen > prefix_max &&
			    (af == AF_INET || ifa->ifa_scope < RT_SCOPE_LINK)) {
				memcpy(addr, RTA_DATA(rta), RTA_PAYLOAD(rta));

				prefix_max = *prefix_len = ifa->ifa_prefixlen;
			}

and save 5 lines out of 11.

I haven't checked the possible meanings of ifa_scope for IPv4 by the
way, maybe that part of the condition could go away as well.

-- 
Stefano