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SubjectRe: [PATCH] af_unix: Allow Unix sockets to raise SIGURG
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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;

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