linux-kernel - Re: [PATCH v5 10/13] xen/pvcalls: implement recvmsg

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Date:   Thu, 19 Oct 2017 18:38:49 -0700 (PDT)
From:   Stefano Stabellini <sstabellini@...nel.org>
To:     Boris Ostrovsky <boris.ostrovsky@...cle.com>
cc:     Stefano Stabellini <sstabellini@...nel.org>,
        xen-devel@...ts.xen.org, linux-kernel@...r.kernel.org,
        jgross@...e.com, Stefano Stabellini <stefano@...reto.com>
Subject: Re: [PATCH v5 10/13] xen/pvcalls: implement recvmsg

On Tue, 17 Oct 2017, Boris Ostrovsky wrote:
> > +
> > +int pvcalls_front_recvmsg(struct socket *sock, struct msghdr *msg, size_t len,
> > +		     int flags)
> > +{
> > +	struct pvcalls_bedata *bedata;
> > +	int ret;
> > +	struct sock_mapping *map;
> > +
> > +	if (flags & (MSG_CMSG_CLOEXEC|MSG_ERRQUEUE|MSG_OOB|MSG_TRUNC))
> > +		return -EOPNOTSUPP;
> > +
> > +	pvcalls_enter();
> > +	if (!pvcalls_front_dev) {
> > +		pvcalls_exit();
> > +		return -ENOTCONN;
> > +	}
> > +	bedata = dev_get_drvdata(&pvcalls_front_dev->dev);
> > +
> > +	map = (struct sock_mapping *) sock->sk->sk_send_head;
> > +	if (!map) {
> > +		pvcalls_exit();
> > +		return -ENOTSOCK;
> > +	}
> > +
> > +	mutex_lock(&map->active.in_mutex);
> > +	if (len > XEN_FLEX_RING_SIZE(PVCALLS_RING_ORDER))
> > +		len = XEN_FLEX_RING_SIZE(PVCALLS_RING_ORDER);
> > +
> > +	while (!(flags & MSG_DONTWAIT) && !pvcalls_front_read_todo(map)) {
> > +		wait_event_interruptible(map->active.inflight_conn_req,
> > +					 pvcalls_front_read_todo(map));
> > +	}
> > +	ret = __read_ring(map->active.ring, &map->active.data,
> > +			  &msg->msg_iter, len, flags);
> > +
> > +	if (ret > 0)
> > +		notify_remote_via_irq(map->active.irq);
> > +	if (ret == 0)
> > +		ret = -EAGAIN;
> 
> Why not 0? The manpage says:
> 
>        EAGAIN or EWOULDBLOCK
>               The  socket  is  marked nonblocking and the receive
> operation would block, or a receive timeout
>               had been set and the timeout expired before data was
> received.  POSIX.1 allows either error  to
>               be  returned  for  this case, and does not require these
> constants to have the same value, so a
>               portable application should check for both possibilities.
> 
> 
> I don't think either of these conditions is true here.
> 
> (Again, should have noticed this earlier, sorry)

In case the socket is MSG_DONTWAIT, then we should return -EAGAIN here.
However, it is true that if the socket is not MSG_DONTWAIT, then
returning 0 would make more sense.

So I'll do:

if (ret == 0)
    ret = (flags & MSG_DONTWAIT) ? -EAGAIN : 0;