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Re: kern/54435: reading TCP urgent data with MSG_OOB doesn't clear poll(2) status

The following reply was made to PR kern/54435; it has been noted by GNATS.

From: Taylor R Campbell <>
Subject: Re: kern/54435: reading TCP urgent data with MSG_OOB doesn't clear poll(2) status
Date: Sat, 3 Aug 2019 19:37:34 +0000

 This is a multi-part message in MIME format.
 poll on a socket returns POLLPRI|POLLRDBAND as long as so->so_oobmark
 is nonzero or so->so_state has SS_RCVATMARK is set.
 - so_oobmark is the pointer to how many bytes away the urgent data are.
 - SS_RCVATMARK is set when the urgent data are clamouring at the gate.
 These are updated by normal reads, but not by the recv(MSG_OOB) call
 that actually returns the urgent data.
 Candidate fix: set so_oobmark = 0, clear SS_RCVATMARK when we do
 Content-Type: text/plain; charset="ISO-8859-1"; name="oobfix"
 Content-Transfer-Encoding: quoted-printable
 Content-Disposition: attachment; filename="oobfix.patch"
 From 8f07765d4d7f455359d1a4d131b4b9ad3973b224 Mon Sep 17 00:00:00 2001
 From: Taylor R Campbell <>
 Date: Sat, 3 Aug 2019 19:31:42 +0000
 Subject: [PATCH] Clear the oobmark when we read oob data.
 Candidate fix for spurious POLLPRI/POLLRDBAND availability after
 recv(MSG_OOB) just consumed it.
  sys/kern/uipc_socket.c | 3 +++
  1 file changed, 3 insertions(+)
 diff --git a/sys/kern/uipc_socket.c b/sys/kern/uipc_socket.c
 index c89dc0f4d576..f19d86321a21 100644
 --- a/sys/kern/uipc_socket.c
 +++ b/sys/kern/uipc_socket.c
 @@ -1179,6 +1179,9 @@ soreceive(struct socket *so, struct mbuf **paddr, str=
 uct uio *uio,
  			    MIN(uio->uio_resid, m->m_len), uio);
  			m =3D m_free(m);
  		} while (uio->uio_resid > 0 && error =3D=3D 0 && m);
 +		/* We consumed the oob data, no more oobmark.  */
 +		so->so_oobmark =3D 0;
 +		so->so_state &=3D ~SS_RCVATMARK;
  		if (m !=3D NULL)

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