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

[Shoaib Rao]

On 1/29/21 11:54 AM, Shoaib Rao wrote:
> 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@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?
> > > 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

SO_OOBINLINE implies there is urgent data inline -- This patch will send 
a signal even if there is no data.

Shoaib

> >