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

[Shoaib Rao]

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

> > 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.