Messages in this thread | | | Subject | Re: [PATCH] af_unix: Allow Unix sockets to raise SIGURG | From | Shoaib Rao <> | Date | Fri, 29 Jan 2021 11:54:30 -0800 |
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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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