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

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Message-ID: <87o73hy7hh.fsf@toke.dk>
Date: Fri, 18 Oct 2024 13:29:30 +0200
From: Toke Høiland-Jørgensen <toke@...hat.com>
To: Simon Horman <horms@...nel.org>, Hangbin Liu <liuhangbin@...il.com>
Cc: 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

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?

-Toke