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    SubjectRe: [PATCH] af_unix: Allow Unix sockets to raise SIGURG
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    On 1/29/21 11:19 AM, Matthew Wilcox wrote:
    > On Fri, Jan 29, 2021 at 09:56:48AM -0800, Shoaib Rao wrote:
    >> 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.
    > I don't think that's the question Jakub is asking. As I understand it,
    > if you send a MSG_OOB on a TCP socket and the receiver calls recvmsg(),
    > it will see MSG_OOB set, even if SO_OOBINLINE is set.
    No it wont. Application just gets a signal.
    > That wouldn't
    > happen with Unix sockets. I'm OK with that difference in behaviour,
    > because MSG_OOB on Unix sockets _is not_ for sending out of band data.
    > It's just for sending an urgent signal.
    That is what I just explained in my email
    >
    > As you say, MSG_OOB does not require data to be sent for unix sockets
    > (unlike TCP which always requires at least one byte), but one can
    > choose to send data as part of a message which has MSG_OOB set. It
    > won't be tagged in any special way.
    >
    > To Jakub's other question, we could require SO_OOBINLINE to be set.
    > That'd provide another layer of insurance against applications being
    > surprised by a SIGURG they weren't expecting. I don't know that it's
    > really worth it though.

    SO_OOBINLINE has a meaning, that the urgent byte is part of the stream and using SO_OOBLINE to allow signalling would be wrong/confusing. We could add a socket option on the receiver to indicate if it supports or wants the signal.

    >
    > One thing I wasn't clear about, and maybe you know, if we send a MSG_OOB,
    > does this patch cause this part of the tcp(7) manpage to be true for
    > unix sockets too?
    >
    > When out-of-band data is present, select(2) indicates the file descrip‐
    > tor as having an exceptional condition and poll (2) indicates a POLLPRI
    > event.

    No because there is no data involved. Poll is associated with data not
    signals.

    Shoaib

    >

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