Re: [PATCH v4] netlink: Return prefix length for IPv6 addresses in nl_addr_get()

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

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On Mon, Mar 09, 2026 at 03:52:33PM -0400, 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.
> 
> Signed-off-by: Jon Maloy <jmaloy@redhat.com>

Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

As discussed on the last call there are some good questions about
exactly what semantics we want here, but I think keeping the existing
ones for now is the right call.

> 
> ---
> 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.
> v4: - Reverted from the IPv4 link local exclusion change
> ---
>  netlink.c | 11 +++--------
>  1 file changed, 3 insertions(+), 8 deletions(-)
> 
> diff --git a/netlink.c b/netlink.c
> index 82a2f0c..ee9a6b1 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
> @@ -788,16 +788,11 @@ 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) {
> +			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;
> -			} 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 = ifa->ifa_prefixlen;
>  			}
>  
>  			if (addr_l &&
> -- 
> 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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