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

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Date:   Mon, 10 Apr 2017 09:26:05 -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,
        Johannes Berg <johannes.berg@...el.com>
Subject: Re: [PATCH v3 1/5] netlink: extended ACK reporting

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.