netdev - Re: [PATCH net-next v2 3/3] genetlink: fit NLMSG

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Date: Tue, 19 Mar 2024 18:17:47 +0100
From: Eric Dumazet <edumazet@...gle.com>
To: Jakub Kicinski <kuba@...nel.org>
Cc: Stefano Brivio <sbrivio@...hat.com>, davem@...emloft.net, netdev@...r.kernel.org, 
	pabeni@...hat.com, jiri@...nulli.us, idosch@...sch.org, 
	johannes@...solutions.net, fw@...len.de, pablo@...filter.org, 
	Martin Pitt <mpitt@...hat.com>, Paul Holzinger <pholzing@...hat.com>, 
	David Gibson <david@...son.dropbear.id.au>
Subject: Re: [PATCH net-next v2 3/3] genetlink: fit NLMSG_DONE into same
 read() as families

On Tue, Mar 19, 2024 at 4:55 PM Jakub Kicinski <kuba@...nel.org> wrote:
>
> On Fri, 15 Mar 2024 12:48:08 +0100 Stefano Brivio wrote:
> > > Make sure ctrl_fill_info() returns sensible error codes and
> > > propagate them out to netlink core. Let netlink core decide
> > > when to return skb->len and when to treat the exit as an
> > > error. Netlink core does better job at it, if we always
> > > return skb->len the core doesn't know when we're done
> > > dumping and NLMSG_DONE ends up in a separate read().
> >
> > While this change is obviously correct, it breaks... well, broken
> > applications that _wrongly_ rely on the fact that NLMSG_DONE is
> > delivered in a separate datagram.
> >
> > This was the (embarrassing) case for passt(1), which I just fixed:
> >   https://archives.passt.top/passt-dev/20240315112432.382212-1-sbrivio@redhat.com/
> >
> > but the "separate" NLMSG_DONE is such an established behaviour,
> > I think, that this might raise a more general concern.
> >
> > From my perspective, I'm just happy that this change revealed the
> > issue, but I wanted to report this anyway in case somebody has
> > similar possible breakages in mind.
>
> Hi Stefano! I was worried this may happen :( I think we should revert
> offending commits, but I'd like to take it on case by case basis.
> I'd imagine majority of netlink is only exercised by iproute2 and
> libmnl-based tools. Does passt hang specifically on genetlink family
> dump? Your commit also mentions RTM_GETROUTE. This is not the only
> commit which removed DONE:
>
> $ git log --since='1 month ago' --grep=NLMSG_DONE --no-merges  --oneline
>
> 9cc4cc329d30 ipv6: use xa_array iterator to implement inet6_dump_addr()
> 87d381973e49 genetlink: fit NLMSG_DONE into same read() as families
> 4ce5dc9316de inet: switch inet_dump_fib() to RCU protection
> 6647b338fc5c netlink: fix netlink_diag_dump() return value

Lets not bring back more RTNL locking please for the handlers that
still require it.

The core can generate an NLMSG_DONE by itself, if we decide this needs
to be done.

I find this discussion a bit strange, because NLMSG_DONE being
piggybacked has been a long established behavior.

Jason patch was from 2017