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

[Jakub Kicinski <kuba@kernel.org>]

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.
So you're hijacking the MSG_OOB to send a signal, because OOB also
sends a signal. But there is nothing OOB about the data itself. So 
I'm asking you to make sure that there is no data in the message. 
That way when someone wants _actual_ OOB data on UNIX sockets they 
can implement it without breaking backwards compatibility of the 
kernel uAPI.

> 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.