netdev - Re: [PATCH net] net: ipv6: support reporting otherwise unknown prefix flags in RTM

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Date: Wed, 6 Dec 2023 13:19:25 +0900
From: Lorenzo Colitti <lorenzo@...gle.com>
To: David Ahern <dsahern@...nel.org>
Cc: Maciej Żenczykowski <maze@...gle.com>, 
	Maciej Żenczykowski <zenczykowski@...il.com>, 
	Linux Network Development Mailing List <netdev@...r.kernel.org>, "David S . Miller" <davem@...emloft.net>, 
	Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, 
	Shirley Ma <mashirle@...ibm.com>
Subject: Re: [PATCH net] net: ipv6: support reporting otherwise unknown prefix
 flags in RTM_NEWPREFIX

On Wed, Dec 6, 2023 at 12:39 PM David Ahern <dsahern@...nel.org> wrote:
> > Lorenzo points out that we effectively clear all unknown
> > flags from PIO when copying them to userspace in the netlink
> > RTM_NEWPREFIX notification.
>
> The existing flags have been there since before git (2005) and no new
> ones have been added. So, what is the problem with existing code?
> conflicts with an out-of-tree patch?

One thing that's incorrect today is that RTM_NEWPREFIX doesn't pass to
userspace the R flag added in RFC 6275 (2011).

Also, the 6man working group is in the process of defining another flag, P:

 https://datatracker.ietf.org/doc/html/draft-ietf-6man-pio-pflag

Of course, it's possible to change the kernel now to understand the R
flag, and then, if/when the P flag becomes an RFC, modify the kernel
again to  support that. But it seems better simply to copy the whole
field, so we don't have to change it again when other flags are added
in the future.