linux-kernel - Exporting obscene amounts of data in rtnl_link_ops->fill

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Message-ID: <CAHmME9pps6BJwubBNgp294JKGkGXHUyb1kMdTGCEwok__aQjmw@mail.gmail.com>
Date:	Thu, 20 Aug 2015 02:47:50 +0200
From:	"Jason A. Donenfeld" <Jason@...c4.com>
To:	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Exporting obscene amounts of data in rtnl_link_ops->fill_info()

Hi guys,

I have a new link driver that registers a rtnl_link_ops. For many
things, the rtnl interfaces are perfectly suited: I can use netlink in
userspace to check out packet counts, adjust interface parameters, and
all sorts of things. There is even the "fill_info" function exporting
interface-specific types of data to userspace through the standard
netlink interfaces. I'm glad this is here, because it's exactly what I
want.

Problem: sometimes I want to export a *lot* of data to userspace. When
this happens, even if I make the netlink socket receive buffer really
huge, this code path is still reached in rtnetlink.c:

                        err = rtnl_fill_ifinfo(skb, dev, RTM_NEWLINK,
                                               NETLINK_CB(cb->skb).portid,
                                               cb->nlh->nlmsg_seq, 0,
                                               NLM_F_MULTI,
                                               ext_filter_mask);
                        /* If we ran out of room on the first message,
                         * we're in trouble
                         */
                        WARN_ON((err == -EMSGSIZE) && (skb->len == 0));

That is -- it tries to fill the skb (for sending it back to
userspace), but doesn't have enough room, so it returns -EMSGSIZE.
That seems like reasonable behavior, but it doesn't really help me
obtain my goal. I'd like to send quite a bit of data back to userspace
for a network interface, and I'd like to do it using the standard
netlink APIs. Is this possible?

Thanks,
Jason
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