netdev - Re: [PATCH RFC net-next 1/6] sock: MSG_PEEK support for sk_error

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Date:   Wed, 17 Jan 2018 18:50:56 -0500
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Sowmini Varadhan <sowmini.varadhan@...cle.com>
Cc:     Network Development <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>, rds-devel@....oracle.com,
        santosh.shilimkar@...cle.com
Subject: Re: [PATCH RFC net-next 1/6] sock: MSG_PEEK support for sk_error_queue

On Wed, Jan 17, 2018 at 7:19 AM, Sowmini Varadhan
<sowmini.varadhan@...cle.com> wrote:
> Allow the application the ability to use MSG_PEEK with sk_error_queue
> so that it can peek and re-read message in cases where MSG_TRUNC
> may be encountered.
>
> Signed-off-by: Sowmini Varadhan <sowmini.varadhan@...cle.com>

>  int sock_recv_errqueue(struct sock *sk, struct msghdr *msg, int len,
> -                      int level, int type)
> +                      int level, int type, int flags)
>  {
>         struct sock_exterr_skb *serr;
>         struct sk_buff *skb;
> @@ -2916,7 +2916,10 @@ int sock_recv_errqueue(struct sock *sk, struct msghdr *msg, int len,
>         err = copied;
>
>  out_free_skb:
> -       kfree_skb(skb);
> +       if (likely(!(flags & MSG_PEEK)))
> +               kfree_skb(skb);
> +       else
> +               skb_queue_head(&sk->sk_error_queue, skb);

This can cause reordering with parallel readers. Can we avoid the need
for peeking? It also caused a slew of subtle bugs previously.

How about just define a max number of cookies and require the caller
to always read with sufficient room to hold them?