netdev - Re: [PATCHv2 net-next 2/3] bonding: use correct return value

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Message-ID: <20241018142104.GP1697@kernel.org>
Date: Fri, 18 Oct 2024 15:21:04 +0100
From: Simon Horman <horms@...nel.org>
To: Toke Høiland-Jørgensen <toke@...hat.com>
Cc: Hangbin Liu <liuhangbin@...il.com>, 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>,
	Alexei Starovoitov <ast@...nel.org>,
	Daniel Borkmann <daniel@...earbox.net>,
	Jesper Dangaard Brouer <hawk@...nel.org>,
	John Fastabend <john.fastabend@...il.com>,
	Jiri Pirko <jiri@...nulli.us>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Lorenzo Bianconi <lorenzo@...nel.org>,
	Andrii Nakryiko <andriin@...com>, Jussi Maki <joamaki@...il.com>,
	Jay Vosburgh <jv@...sburgh.net>,
	Andy Gospodarek <andy@...yhouse.net>,
	Jonathan Corbet <corbet@....net>,
	Andrew Lunn <andrew+netdev@...n.ch>, linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org, bpf@...r.kernel.org,
	Nikolay Aleksandrov <razor@...ckwall.org>
Subject: Re: [PATCHv2 net-next 2/3] bonding: use correct return value

On Fri, Oct 18, 2024 at 01:29:30PM +0200, Toke Høiland-Jørgensen wrote:
> Simon Horman <horms@...nel.org> writes:
> 
> > On Fri, Oct 18, 2024 at 12:46:18AM +0000, Hangbin Liu wrote:
> >> On Thu, Oct 17, 2024 at 04:47:19PM +0200, Toke Høiland-Jørgensen wrote:
> >> > > diff --git a/drivers/net/bonding/bond_main.c b/drivers/net/bonding/bond_main.c
> >> > > index f0f76b6ac8be..6887a867fe8b 100644
> >> > > --- a/drivers/net/bonding/bond_main.c
> >> > > +++ b/drivers/net/bonding/bond_main.c
> >> > > @@ -5699,7 +5699,7 @@ static int bond_xdp_set(struct net_device *dev, struct bpf_prog *prog,
> >> > >  		if (dev_xdp_prog_count(slave_dev) > 0) {
> >> > >  			SLAVE_NL_ERR(dev, slave_dev, extack,
> >> > >  				     "Slave has XDP program loaded, please unload before enslaving");
> >> > > -			err = -EOPNOTSUPP;
> >> > > +			err = -EEXIST;
> >> > 
> >> > Hmm, this has been UAPI since kernel 5.15, so can we really change it
> >> > now? What's the purpose of changing it, anyway?
> >> 
> >> I just think it should return EXIST when the error is "Slave has XDP program
> >> loaded". No special reason. If all others think we should not change it, I
> >> can drop this patch.
> >
> > Hi Toke,
> >
> > Could you add some colour to what extent user's might rely on this error code?
> >
> > Basically I think that if they do then we shouldn't change this.
> 
> Well, that's the trouble with UAPI, we don't really know. In libxdp and
> xdp-tools we look at the return code to provide a nicer error message,
> like:
> 
> https://github.com/xdp-project/xdp-tools/blob/master/lib/libxdp/libxdp.c#L615
> 
> and as a signal to fall back to loading the programme without a dispatcher:
> 
> https://github.com/xdp-project/xdp-tools/blob/master/lib/libxdp/libxdp.c#L1824
> 
> Both of these cases would be unaffected (or even improved) by this
> patch, so in that sense I don't have a concrete objection, just a
> general "userspace may react to this". In other words, my concern is
> more of a general "we don't know, so this seems risky". If any of you
> have more information about how bonding XDP is generally used, that may
> help get a better idea of this?

Yes, that is the trouble with the UAPI. I was hoping you might be able to
provide the clarity you ask for above. But alas, things are as clear as
mud.

In lieu of more information I suggest caution and dropping this change for
now.

-- 
pw-bot: cr