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

[Jakub Kicinski]

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?

> 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;