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Date:   Tue, 17 May 2022 15:33:26 -0700
From:   Jakub Kicinski <kuba@kernel.org>
To:     Jay Vosburgh <jay.vosburgh@canonical.com>
Cc:     Jonathan Toppins <jtoppins@redhat.com>, netdev@vger.kernel.org,
        Veaceslav Falico <vfalico@gmail.com>,
        Andy Gospodarek <andy@greyhouse.net>,
        "David S. Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Paolo Abeni <pabeni@redhat.com>, linux-kernel@vger.kernel.org
Subject: Re: [RFC net-next] bonding: netlink error message support for
 options
Message-ID: <20220517153326.1fbbe2cc@kernel.org>
In-Reply-To: <2125.1652821874@famine>
References: <5a6ba6f14b0fad6d4ba077a5230ee71cbf970934.1652819479.git.jtoppins@redhat.com>
        <2125.1652821874@famine>
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On Tue, 17 May 2022 14:11:14 -0700 Jay Vosburgh wrote:
> 	If I'm reading the code correctly, rtnl isn't held that long.
> Once the ->doit() returns, rtnl is dropped, but the copy happens later:
> 
> rtnetlink_rcv()
> 	netlink_rcv_skb(skb, &rtnetlink_rcv_msg)
> 		rtnetlink_rcv_msg()	[ as cb(skb, nlh, &extack) ]
> 			rtnl_lock()
> 			link->doit()	[ rtnl_setlink, rtnl_newlink, et al ]
> 			rtnl_unlock()
> 		netlink_ack()
> 
> inside netlink_ack():
> 
>         if (nlk_has_extack && extack) {
>                 if (extack->_msg) {
>                         WARN_ON(nla_put_string(skb, NLMSGERR_ATTR_MSG,
>                                                extack->_msg));
>                 }

Indeed.

> 	Even if the strings have to be constant (via NL_SET_ERR_MSG),
> adding extack messages is likely still an improvement.

At a quick glance it seems like the major use of the printf here is to
point at a particular option. If options are carried in individual
attributes pointing at the right attribute with NL_SET_ERR_MSG_ATTR()
should also be helpful. Maybe that's stating the obvious.