netdev - Re: [PATCH v3 1/5] netlink: extended ACK reporting

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Date:   Mon, 10 Apr 2017 09:35:27 -0600
From:   David Ahern <dsa@...ulusnetworks.com>
To:     Johannes Berg <johannes@...solutions.net>,
        linux-wireless@...r.kernel.org, netdev@...r.kernel.org
Cc:     pablo@...filter.org, Jamal Hadi Salim <jhs@...atatu.com>,
        Jiri Benc <jbenc@...hat.com>, jiri@...nulli.us
Subject: Re: [PATCH v3 1/5] netlink: extended ACK reporting

On 4/10/17 9:30 AM, Johannes Berg wrote:
> On Mon, 2017-04-10 at 09:26 -0600, David Ahern wrote:
>> On 4/8/17 2:24 PM, Johannes Berg wrote:
>>> @@ -2300,14 +2332,35 @@ void netlink_ack(struct sk_buff *in_skb,
>>> struct nlmsghdr *nlh, int err)
>>>  			  NLMSG_ERROR, payload, 0);
>>>  	errmsg = nlmsg_data(rep);
>>>  	errmsg->error = err;
>>> -	memcpy(&errmsg->msg, nlh, payload > sizeof(*errmsg) ? nlh-
>>>> nlmsg_len : sizeof(*nlh));
>>> +	memcpy(&errmsg->msg, nlh,
>>> +	       !(nlk->flags & NETLINK_F_CAP_ACK) ? nlh->nlmsg_len
>>> +						 : sizeof(*nlh));
>>> +
>>
>> generically this makes userspace parsing more problematic: the
>> parsing layer may not know if the socket option has been set to
>> precisely know the size of errmsg->msg and how much data needs to be
>> skipped to get to the new attributes.
> 
> Yes, I know. I'd hope that userspace can remember that per socket - I
> don't see a good other way to do this.
> 
> If we insert the TLVs in front of, or instead of (with a TLV containing
> it), the request message then at least libnl's debugging will need to
> be changed.
> 
> As it is, I can assume that libnl will not set the CAP setting, and
> everything works fine even if I don't change libnl, which makes things
> easier.
> 
> Do you have any better ideas?

NETLINK_F_CAP_ACK and NETLINK_F_EXT_ACK should be incompatible -- if one
is set the other can not be set. CAP_ACK means abbreviate the response
and EXT_ACK means give me more data.