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

[Shoaib Rao]

On 1/29/21 12:44 PM, Shoaib Rao wrote:
> On 1/29/21 12:18 PM, Jakub Kicinski wrote:
> > On Fri, 29 Jan 2021 12:10:21 -0800 Shoaib Rao wrote:
> > > On 1/29/21 12:02 PM, Jakub Kicinski wrote:
> > > > On Fri, 29 Jan 2021 11:48:15 -0800 Shoaib Rao wrote:
> > > > > Data was discarded because the flag was not supported, this patch
> > > > > changes that but does not support any urgent data.
> > > > When you say it does not support any urgent data do you mean the
> > > > message len must be == 0 because something is checking it, or that
> > > > the code does not support its handling?
> > > >
> > > > I'm perfectly fine with the former, just point me at the check, 
> > > > please.
> > > The code does not care about the size of data -- All it does is that if
> > > MSG_OOB is set it will deliver the signal to the peer process
> > > irrespective of the length of the data (which can be zero length). 
> > > Let's
> > > look at the code of unix_stream_sendmsg() It does the following 
> > > (sent is
> > > initialized to zero)
> > Okay. Let me try again. AFAICS your code makes it so that data sent
> > with MSG_OOB is treated like any other data. It just sends a signal.
> Correct.
> > So you're hijacking the MSG_OOB to send a signal, because OOB also
> > sends a signal.
> Correct.
> >   But there is nothing OOB about the data itself.
> Correct.
> >   So
> > I'm asking you to make sure that there is no data in the message.
> Yes I can do that.
> > That way when someone wants _actual_ OOB data on UNIX sockets they
> > can implement it without breaking backwards compatibility of the
> > kernel uAPI.
>
> I see what you are trying to achieve. However it may not work.
>
> Let's assume that __actual__ OOB data has been implemented. An 
> application sends a zero length message with MSG_OOB, after that it 
> sends some data (not suppose to be OOB data). How is the receiver 
> going to differentiate if the data an OOB or not.
>
> We could use a different flag (MSG_SIGURG) or implement the _actual_ 
> OOB data semantics (If anyone is interested in it). MSG_SIGURG could 
> be a generic flag that just sends SIGURG irrespective of the length of 
> the data.
>
> Shoaib

There is a relevant issue that I want to point out, Is it acceptable to 
send SIGURG without the receiver having any means to know what the 
urgent condition is?

Shoaib

> > > while (sent < len) {
> > >                   size = len - sent;
> > > <..>
> > >
> > > }
> > >
> > >           if (msg->msg_flags & MSG_OOB)
> > >                   sk_send_sigurg(other);
> > >
> > > Before the patch there was a check above the while loop that checked 
> > > the
> > > flag and returned and error, that has been removed.