Re: [PATCH v2 3/6] netlink/compat: Append NLMSG_DONE/extack to frag_list

Johannes Berg
2020-08-26T07:20:43Z

On Wed, 2020-08-26 at 02:49 +0100, Dmitry Safonov wrote:
> Modules those use netlink may supply a 2nd skb, (via frag_list)
> that contains an alternative data set meant for applications
> using 32bit compatibility mode.

Do note, however, that until this day the facility here was only used by
(and originally intended for) wireless extensions, where it exclusively
applies to *event* messages. Hence, we really didn't worry about dump or
any other kind of netlink usage.

That said, it's really just a historic note explaining why it didn't
work for you out of the box :)

> In such a case, netlink_recvmsg will use this 2nd skb instead of the
> original one.
>
> Without this patch, such compat applications will retrieve
> all netlink dump data, but will then get an unexpected EOF.
>
> Cc: Johannes Berg <johannes@sipsolutions.net>
> Signed-off-by: Florian Westphal <fw@strlen.de>
> Signed-off-by: Dmitry Safonov <dima@arista.com>
> ---
> net/netlink/af_netlink.c | 48 ++++++++++++++++++++++++++++------------
> 1 file changed, 34 insertions(+), 14 deletions(-)
>
> diff --git a/net/netlink/af_netlink.c b/net/netlink/af_netlink.c
> index b5f30d7d30d0..b096f2b4a50d 100644
> --- a/net/netlink/af_netlink.c
> +++ b/net/netlink/af_netlink.c
> @@ -2186,13 +2186,36 @@ EXPORT_SYMBOL(__nlmsg_put);
> * It would be better to create kernel thread.
> */
>
> +static int netlink_dump_done(struct netlink_sock *nlk, struct sk_buff *skb,
> + struct netlink_callback *cb,
> + struct netlink_ext_ack *extack)
> +{
> + struct nlmsghdr *nlh;
> +
> + nlh = nlmsg_put_answer(skb, cb, NLMSG_DONE, sizeof(nlk->dump_done_errno),
> + NLM_F_MULTI | cb->answer_flags);
> + if (WARN_ON(!nlh))
> + return -ENOBUFS;
> +
> + nl_dump_check_consistent(cb, nlh);
> + memcpy(nlmsg_data(nlh), &nlk->dump_done_errno,
> + sizeof(nlk->dump_done_errno));

nit: indentation here looks odd.

Other than that, looks reasonable to me.

Reviewed-by: Johannes Berg <johannes@sipsolutions.net>

johannes