Re: [PATCH] af_unix: Allow Unix sockets to raise SIGURG

[Shoaib Rao]

On 1/25/21 3:36 PM, Jakub Kicinski wrote:
> On Fri, 22 Jan 2021 15:06:37 +0000 Matthew Wilcox (Oracle) wrote:
> > From: Rao Shoaib <rao.shoaib@xxxxxxxxxx>
> >
> > TCP sockets allow SIGURG to be sent to the process holding the other
> > end of the socket.  Extend Unix sockets to have the same ability.
> >
> > The API is the same in that the sender uses sendmsg() with MSG_OOB to
> > raise SIGURG.  Unix sockets behave in the same way as TCP sockets with
> > SO_OOBINLINE set.
> Noob question, if we only want to support the inline mode, why don't we
> require SO_OOBINLINE to have been called on @other? Wouldn't that
> provide more consistent behavior across address families?
>
> With the current implementation the receiver will also not see MSG_OOB
> set in msg->msg_flags, right?

SO_OOBINLINE does not control the delivery of signal, It controls how OOB Byte is delivered. It may not be obvious but this change does not deliver any Byte, just a signal. So, as long as sendmsg flag contains MSG_OOB, signal will be delivered just like it happens for TCP.

Shoaib

> > SIGURG is ignored by default, so applications which do not know about this
> > feature will be unaffected.  In addition to installing a SIGURG handler,
> > the receiving application must call F_SETOWN or F_SETOWN_EX to indicate
> > which process or thread should receive the signal.
> >
> > Signed-off-by: Rao Shoaib <rao.shoaib@xxxxxxxxxx>
> > Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
> > ---
> >   net/unix/af_unix.c | 5 +++--
> >   1 file changed, 3 insertions(+), 2 deletions(-)
> >
> > diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
> > index 41c3303c3357..849dff688c2c 100644
> > --- a/net/unix/af_unix.c
> > +++ b/net/unix/af_unix.c
> > @@ -1837,8 +1837,6 @@ static int unix_stream_sendmsg(struct socket *sock, struct msghdr *msg,
> >   		return err;
> >   
> >   	err = -EOPNOTSUPP;
> > -	if (msg->msg_flags&MSG_OOB)
> > -		goto out_err;
> >   
> >   	if (msg->msg_namelen) {
> >   		err = sk->sk_state == TCP_ESTABLISHED ? -EISCONN : -EOPNOTSUPP;
> > @@ -1903,6 +1901,9 @@ static int unix_stream_sendmsg(struct socket *sock, struct msghdr *msg,
> >   		sent += size;
> >   	}
> >   
> > +	if (msg->msg_flags & MSG_OOB)
> > +		sk_send_sigurg(other);
> > +
> >   	scm_destroy(&scm);
> >   
> >   	return sent;