netlink: specs: rt-link: update ipv6 devconf doc

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devconf is even odder than described in commit 720447bd0b
("netlink: specs: rt-link: remove implicit structs from devconf"),
where both 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().

Reviewed-by: Fernando Fernandez Mancera <fmancera@suse.de>
Signed-off-by: Asbjørn Sloth Tønnesen <ast@fiberby.net>
Link: https://patch.msgid.link/20260831093458.472180-2-ast@fiberby.net
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

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Asbjørn Sloth Tønnesen 2026-08-31 09:34:53 +00:00 • committed by Paolo Abeni

parent 85ca3292d7

commit 5e63ade190

1 changed files with 1 additions and 1 deletions

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Documentation/netlink/specs/rt-link.yaml

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