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Date:   Fri, 29 Jan 2021 12:18:37 -0800
From:   Jakub Kicinski <kuba@kernel.org>
To:     Shoaib Rao <rao.shoaib@oracle.com>
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
Subject: Re: [PATCH] af_unix: Allow Unix sockets to raise SIGURG
Message-ID: <20210129121837.467280fb@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
In-Reply-To: <cef52fb0-43cb-9038-7e48-906b58b356b6@oracle.com>
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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: =20
> >> Data was discarded because the flag was not supported, this patch
> >> changes that but does not support any urgent data. =20
> > When you say it does not support any urgent data do you mean the
> > message len must be =3D=3D 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.=
 =20
>=20
> The code does not care about the size of data -- All it does is that if=20
> MSG_OOB is set it will deliver the signal to the peer process=20
> irrespective of the length of the data (which can be zero length). Let's=
=20
> look at the code of unix_stream_sendmsg() It does the following (sent is=
=20
> 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=20
I'm asking you to make sure that there is no data in the message.=20
That way when someone wants _actual_ OOB data on UNIX sockets they=20
can implement it without breaking backwards compatibility of the=20
kernel uAPI.

> while (sent < len) {
>  =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0 size =3D len - sent;
> <..>
>=20
> }
>=20
>  =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 if (msg->msg_flags & MSG_OOB)
>  =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0 sk_send_sigurg(other);
>=20
> Before the patch there was a check above the while loop that checked the=
=20
> flag and returned and error, that has been removed.