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

[David Gibson <david@gibson.dropbear.id.au>]

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On Sat, Mar 07, 2026 at 01:41:57PM -0500, Jon Maloy 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.
> 
> 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. Although it
> is conventional to set the scope of such addresses to RT_SCOPE_LINK,
> this is not stated in any RFC, and we cannot trust it to have been set
> correctly by the user, just as we cannot trust it to have been set
> correctly for any other AF_INET address. We therefore add this check
> explicitly on the address itself.
> 
> Signed-off-by: Jon Maloy <jmaloy@redhat.com>
> 
> ---
> v2: - Simplified according to feedback from S. Brivio
>     - Added test for AF_INET link local property
> v3: - Corrected claim about convention for IPv4 link local address
>       scope in commit log.
> ---
>  netlink.c | 17 +++++++----------
>  1 file changed, 7 insertions(+), 10 deletions(-)
> 
> diff --git a/netlink.c b/netlink.c
> index 82a2f0c..f0e8fa7 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
> @@ -777,6 +777,7 @@ int nl_addr_get(int s, unsigned int ifi, sa_family_t af,
>  	nl_foreach_oftype(nh, status, s, buf, seq, RTM_NEWADDR) {
>  		struct ifaddrmsg *ifa = (struct ifaddrmsg *)NLMSG_DATA(nh);
>  		struct rtattr *rta;
> +		bool link_local;
>  		size_t na;
>  
>  		if (ifa->ifa_index != ifi || ifa->ifa_flags & IFA_F_DEPRECATED)
> @@ -788,18 +789,14 @@ int nl_addr_get(int s, unsigned int ifi, sa_family_t af,
>  			    (af == AF_INET6 && rta->rta_type != IFA_ADDRESS))
>  				continue;
>  
> -			if (af == AF_INET && ifa->ifa_prefixlen > prefix_max) {
> -				memcpy(addr, RTA_DATA(rta), RTA_PAYLOAD(rta));
> +			link_local = (af == AF_INET6) ?
> +				ifa->ifa_scope >= RT_SCOPE_LINK :
> +				IN4_IS_ADDR_LINKLOCAL(RTA_DATA(rta));


Is ifa->ifa_scope not populatedcorrectly for IPv4 link local addresses?
>  
> -				prefix_max = *prefix_len = ifa->ifa_prefixlen;
> -			} else if (af == AF_INET6 && addr &&
> -				   ifa->ifa_scope < RT_SCOPE_LINK &&
> -				   ifa->ifa_prefixlen > prefix_max) {
> +			if (ifa->ifa_prefixlen > prefix_max && !link_local) {
>  				memcpy(addr, RTA_DATA(rta), RTA_PAYLOAD(rta));
> -
> -				prefix_max = ifa->ifa_prefixlen;
> +				prefix_max = *prefix_len = ifa->ifa_prefixlen;
>  			}
> -
>  			if (addr_l &&
>  			    af == AF_INET6 && ifa->ifa_scope == RT_SCOPE_LINK &&
>  			    ifa->ifa_prefixlen > prefix_max_ll) {
> -- 
> 2.52.0
> 

-- 
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

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