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Subject: Re: [PATCH] af_unix: Allow Unix sockets to raise SIGURG
To:     Jakub Kicinski <kuba@kernel.org>,
        "Matthew Wilcox (Oracle)" <willy@infradead.org>
Cc:     linux-kernel@vger.kernel.org, linux-api@vger.kernel.org,
        netdev@vger.kernel.org, "David S. Miller" <davem@davemloft.net>,
        andy.rudoff@intel.com
References: <20210122150638.210444-1-willy@infradead.org>
 <20210125153650.18c84b1a@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
From:   Shoaib Rao <rao.shoaib@oracle.com>
Message-ID: <23fc3de2-7541-04c9-a56f-4006a7dc773f@oracle.com>
Date:   Fri, 29 Jan 2021 09:56:48 -0800
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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@oracle.com>
>>
>> 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@oracle.com>
>> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
>> ---
>>   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;