netdev - Re: [PATCH net-next] sctp: support MSG

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Date: Wed, 13 Dec 2023 16:49:50 +0100
From: Eric Dumazet <edumazet@...gle.com>
To: Xin Long <lucien.xin@...il.com>
Cc: "David S . Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, 
	Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org, eric.dumazet@...il.com, 
	Marcelo Ricardo Leitner <marcelo.leitner@...il.com>, Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH net-next] sctp: support MSG_ERRQUEUE flag in recvmsg()

On Wed, Dec 13, 2023 at 4:19 PM Xin Long <lucien.xin@...il.com> wrote:
>
> On Tue, Dec 12, 2023 at 9:55 AM Eric Dumazet <edumazet@...gle.com> wrote:
> >
> > For some reason sctp_poll() generates EPOLLERR if sk->sk_error_queue
> > is not empty but recvmsg() can not drain the error queue yet.
> I'm checking the code but can't see how SCTP may enqueue skbs into
> sk->sk_error_queue. Have you ever seen it happen in any of your cases?
>
> >
> > This is needed to better support timestamping.
> I think SCTP doesn't support timestamping, there's no functions like
> tcp_tx_timestamp()/tcp_gso_tstamp() to enable it.
>
> Or do you mean SO_TXTIME socket option, and then tc-etf may
> enqueue a skb into sk->sk_error_queue if it's enabled?
>

I think this is the goal yes.

Also this prevents developers from using MSG_ERRQUEUE and having
the kernel read the receive queue.

mptcp had a similar issue in the past, Paolo fixed it.

> >
> > I had to export inet_recv_error(), since sctp
> > can be compiled as a module.
> >
> > Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> > Cc: Xin Long <lucien.xin@...il.com>
> > Cc: Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
> > Cc: Willem de Bruijn <willemb@...gle.com>
> > ---
> >  net/ipv4/af_inet.c | 1 +
> >  net/sctp/socket.c  | 3 +++
> >  2 files changed, 4 insertions(+)
> >
> > diff --git a/net/ipv4/af_inet.c b/net/ipv4/af_inet.c
> > index fbeacf04dbf3744e5888360e0b74bf6f70ff214f..835f4f9d98d25559fb8965a7531c6863448a55c2 100644
> > --- a/net/ipv4/af_inet.c
> > +++ b/net/ipv4/af_inet.c
> > @@ -1633,6 +1633,7 @@ int inet_recv_error(struct sock *sk, struct msghdr *msg, int len, int *addr_len)
> >  #endif
> >         return -EINVAL;
> >  }
> > +EXPORT_SYMBOL(inet_recv_error);
> >
> >  int inet_gro_complete(struct sk_buff *skb, int nhoff)
> >  {
> > diff --git a/net/sctp/socket.c b/net/sctp/socket.c
> > index 7f89e43154c091f6f7a3c995c1ba8abb62a8e767..5fb02bbb4b349ef9ab9c2790cccb30fb4c4e897c 100644
> > --- a/net/sctp/socket.c
> > +++ b/net/sctp/socket.c
> > @@ -2099,6 +2099,9 @@ static int sctp_recvmsg(struct sock *sk, struct msghdr *msg, size_t len,
> >         pr_debug("%s: sk:%p, msghdr:%p, len:%zd, flags:0x%x, addr_len:%p)\n",
> >                  __func__, sk, msg, len, flags, addr_len);
> >
> > +       if (unlikely(flags & MSG_ERRQUEUE))
> > +               return inet_recv_error(sk, msg, len, addr_len);
> > +
> >         lock_sock(sk);
> >
> >         if (sctp_style(sk, TCP) && !sctp_sstate(sk, ESTABLISHED) &&
> > --
> > 2.43.0.472.g3155946c3a-goog
> >