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Subject: Re: [PATCH] af_unix: Allow Unix sockets to raise SIGURG
To:     Matthew Wilcox <willy@infradead.org>
Cc:     Jakub Kicinski <kuba@kernel.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
References: <20210122150638.210444-1-willy@infradead.org>
 <20210125153650.18c84b1a@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
 <23fc3de2-7541-04c9-a56f-4006a7dc773f@oracle.com>
 <20210129191939.GB308988@casper.infradead.org>
From:   Shoaib Rao <rao.shoaib@oracle.com>
Message-ID: <6cef3799-088c-b017-94cb-18971ac74727@oracle.com>
Date:   Fri, 29 Jan 2021 11:54:30 -0800
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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@oracle.com>
>>>>
>>>> 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

>