netdev - Re: [PATCH net v3 0/2] ipv6: Fix listing and flushing of cached route exceptions

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Message-ID: <20190611201946.tokf7su5hlxyrlhs@kafai-mbp.dhcp.thefacebook.com>
Date:   Tue, 11 Jun 2019 20:19:48 +0000
From:   Martin Lau <kafai@...com>
To:     Stefano Brivio <sbrivio@...hat.com>
CC:     David Ahern <dsahern@...il.com>,
        David Miller <davem@...emloft.net>,
        Jianlin Shi <jishi@...hat.com>, Wei Wang <weiwan@...gle.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Matti Vaittinen <matti.vaittinen@...rohmeurope.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH net v3 0/2] ipv6: Fix listing and flushing of cached route
 exceptions

On Tue, Jun 11, 2019 at 12:47:58AM +0200, Stefano Brivio wrote:
> On Mon, 10 Jun 2019 23:53:15 +0200
> Stefano Brivio <sbrivio@...hat.com> wrote:
> 
> > On Mon, 10 Jun 2019 15:38:06 -0600
> > David Ahern <dsahern@...il.com> wrote:
> > 
> > > in dot releases of stable trees, I think it would be better to converge
> > > on consistent behavior between v4 and v6. By that I mean without the
> > > CLONED flag, no exceptions are returned (default FIB dump). With the
> > > CLONED flag only exceptions are returned.  
> > 
> > Again, this needs a change in iproute2, because RTM_F_CLONED is *not*
> > passed on 'flush'. And sure, let's *also* do that, but not everybody
> > runs recent versions of iproute2.
> 
> One thing that sounds a bit more acceptable to me is:
> 
> - dump (in IPv4 and IPv6):
>   - regular routes only, if !RTM_F_CLONED and NLM_F_MATCH
>   - exceptions only, if RTM_F_CLONED and NLM_F_MATCH
That seems reasonable since DavidAhern pointed out iproute2 already has
#define NLM_F_DUMP      (NLM_F_ROOT|NLM_F_MATCH)

>   - everything if !NLM_F_MATCH
I am not sure how may the kernel change looks like.  At least I don't
see the current ipv6/route.c or ipv6/ip6_fib.c is handling
nlmsg_flags.  I would defer to DavidAhern for comment.

> 
> - fix iproute2 so that RTM_F_CLONED is passed on 'flush cache', or
I would just pass RTM_F_CLONED with NLM_F_DUMP.

>   don't pass NLM_F_MATCH in that case
> 
> this way, the kernel respects the intended semantics of flags, and we
> fix a bug in iproute2 (that was always present).
> 
> I think it's not ideal, because the kernel unexpectedly changed the
> behaviour and we're not guaranteeing that older iproute2 works. The
> fact it was broken for two years is probably a partial excuse for this,
> though.
> 
> What do you think? I'll prepare a v4 for net-next if we all agree.
> 
> I'm not entirely sure which trees I should target. I guess this
> introduces a feature in the kernel, so net-next, and fixes a bug in
> iproute2, so iproute2.git?
> 
> -- 
> Stefano