netdev - Re: [PATCH v2 3/6] netlink/compat: Append NLMSG_DONE/extack to frag

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Date:   Wed, 26 Aug 2020 09:19:51 +0200
From:   Johannes Berg <johannes@...solutions.net>
To:     Dmitry Safonov <dima@...sta.com>, linux-kernel@...r.kernel.org
Cc:     Dmitry Safonov <0x7f454c46@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Florian Westphal <fw@...len.de>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Jakub Kicinski <kuba@...nel.org>,
        Steffen Klassert <steffen.klassert@...unet.com>,
        Stephen Suryaputra <ssuryaextr@...il.com>,
        netdev@...r.kernel.org
Subject: Re: [PATCH v2 3/6] netlink/compat: Append NLMSG_DONE/extack to
 frag_list

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@...solutions.net>
> Signed-off-by: Florian Westphal <fw@...len.de>
> Signed-off-by: Dmitry Safonov <dima@...sta.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@...solutions.net>

johannes