Re: [PATCH net] netlink: specs: rt-link: fix ipv6 devconf doc

[Fernando Fernandez Mancera]

On 8/24/26 12:38 PM, Asbjørn Sloth Tønnesen wrote:
> devconf is even odder than described in the blamed commit, where bpth
> IPv4 and IPv6 are described in an identical way:
> "u32 indexed by ipv?-devconf - 1 on output, on input it's a nest".
>
> There are two issues with that line for IPv6:
>
> 1) The subtraction is an IPv4 specific quirk, to avoid having an unused
>     u32 at index 0, thus saving 4 bytes per net device being dumped:
>
>     - In include/uapi/linux/ip.h the IPV4_DEVCONF_* enum begins with
>       IPV4_DEVCONF_FORWARDING = 1, so the enum starts at 1, which is
>       the reason for the subtraction in the IPv4 variant.
>
>     - In include/uapi/linux/ipv6.h the DEVCONF_* enum begins with
>       DEVCONF_FORWARDING = 0, so subtraction would underflow.
>
>     - ipv6_store_devconf() in net/ipv6/addrconf.c also doesn't do the
>       subtraction in the output path.
>
> 2) Setting IPv6 devconf through Netlink is not yet supported, as
>     IFLA_INET6_CONF is not handled in inet6_set_link_af().
>
> Fixes: 720447bd0b24 ("netlink: specs: rt-link: remove implicit structs from devconf")

Does this really require a fixes tag + stable Cc'ed? In any case, LGTM.

> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Asbjørn Sloth Tønnesen <ast@xxxxxxxxxxx>
> ---
>
> Fernando, please remember to reintroduce the ", on input it's a nest"
> in your WIP/planned patchset that introduces support for setting IPv6
> devconf via IFLA_INET6_CONF.
> https://lore.kernel.org/bd560c89-c71f-4ef5-a0fd-5ea096a0f767@xxxxxxx/

Thanks!

I will make sure to reintroduce it, once the netdev is open again I will 
send a patch implementing it.

Reviewed-by: Fernando Fernandez Mancera <fmancera@xxxxxxx>

>   Documentation/netlink/specs/rt-link.yaml | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/Documentation/netlink/specs/rt-link.yaml b/Documentation/netlink/specs/rt-link.yaml
> index b80c2ac3ac31..00c915031efd 100644
> --- a/Documentation/netlink/specs/rt-link.yaml
> +++ b/Documentation/netlink/specs/rt-link.yaml
> @@ -2218,7 +2218,7 @@ attribute-sets:
>           name: conf
>           type: binary
>           sub-type: u32
> -        doc: u32 indexed by ipv6-devconf - 1 on output, on input it's a nest
> +        doc: u32 indexed by ipv6-devconf on output, input is not yet implemented
>         -
>           name: stats
>           type: binary
>
> base-commit: 7cbfb180945ce529608e4d4e24a6d483699fab1e