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Subject: Re: [PATCH] af_unix: Allow Unix sockets to raise SIGURG
From:   Shoaib Rao <rao.shoaib@oracle.com>
To:     Jakub Kicinski <kuba@kernel.org>
Cc:     "Matthew Wilcox (Oracle)" <willy@infradead.org>,
        linux-kernel@vger.kernel.org, linux-api@vger.kernel.org,
        netdev@vger.kernel.org, "David S. Miller" <davem@davemloft.net>,
        andy.rudoff@intel.com
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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.