path: root/sys/kern/uipc_usrreq.c

/*	$OpenBSD: uipc_usrreq.c,v 1.13 2001/06/26 19:56:52 dugsong Exp $	*/
/*	$NetBSD: uipc_usrreq.c,v 1.18 1996/02/09 19:00:50 christos Exp $	*/

/*
 * Copyright (c) 1982, 1986, 1989, 1991, 1993
 *	The Regents of the University of California.  All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. All advertising materials mentioning features or use of this software
 *    must display the following acknowledgement:
 *	This product includes software developed by the University of
 *	California, Berkeley and its contributors.
 * 4. Neither the name of the University nor the names of its contributors
 *    may be used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 *
 *	@(#)uipc_usrreq.c	8.3 (Berkeley) 1/4/94
 */

#include <sys/param.h>
#include <sys/systm.h>
#include <sys/proc.h>
#include <sys/filedesc.h>
#include <sys/domain.h>
#include <sys/protosw.h>
#include <sys/socket.h>
#include <sys/socketvar.h>
#include <sys/unpcb.h>
#include <sys/un.h>
#include <sys/namei.h>
#include <sys/vnode.h>
#include <sys/file.h>
#include <sys/stat.h>
#include <sys/mbuf.h>

/*
 * Unix communications domain.
 *
 * TODO:
 *	SEQPACKET, RDM
 *	rethink name space problems
 *	need a proper out-of-band
 */
struct	sockaddr sun_noname = { sizeof(sun_noname), AF_UNIX };
ino_t	unp_ino;			/* prototype for fake inode numbers */

/*ARGSUSED*/
int
uipc_usrreq(so, req, m, nam, control)
	struct socket *so;
	int req;
	struct mbuf *m, *nam, *control;
{
	struct unpcb *unp = sotounpcb(so);
	register struct socket *so2;
	register int error = 0;
	struct proc *p = curproc;	/* XXX */

	if (req == PRU_CONTROL)
		return (EOPNOTSUPP);
	if (req != PRU_SEND && control && control->m_len) {
		error = EOPNOTSUPP;
		goto release;
	}
	if (unp == 0 && req != PRU_ATTACH) {
		error = EINVAL;
		goto release;
	}
	switch (req) {

	case PRU_ATTACH:
		if (unp) {
			error = EISCONN;
			break;
		}
		error = unp_attach(so);
		break;

	case PRU_DETACH:
		unp_detach(unp);
		break;

	case PRU_BIND:
		error = unp_bind(unp, nam, p);
		break;

	case PRU_LISTEN:
		if (unp->unp_vnode == 0)
			error = EINVAL;
		break;

	case PRU_CONNECT:
		error = unp_connect(so, nam, p);
		break;

	case PRU_CONNECT2:
		error = unp_connect2(so, (struct socket *)nam);
		break;

	case PRU_DISCONNECT:
		unp_disconnect(unp);
		break;

	case PRU_ACCEPT:
		/*
		 * Pass back name of connected socket,
		 * if it was bound and we are still connected
		 * (our peer may have closed already!).
		 */
		if (unp->unp_conn && unp->unp_conn->unp_addr) {
			nam->m_len = unp->unp_conn->unp_addr->m_len;
			bcopy(mtod(unp->unp_conn->unp_addr, caddr_t),
			    mtod(nam, caddr_t), (unsigned)nam->m_len);
		} else {
			nam->m_len = sizeof(sun_noname);
			*(mtod(nam, struct sockaddr *)) = sun_noname;
		}
		break;

	case PRU_SHUTDOWN:
		socantsendmore(so);
		unp_shutdown(unp);
		break;

	case PRU_RCVD:
		switch (so->so_type) {

		case SOCK_DGRAM:
			panic("uipc 1");
			/*NOTREACHED*/

		case SOCK_STREAM:
#define	rcv (&so->so_rcv)
#define snd (&so2->so_snd)
			if (unp->unp_conn == 0)
				break;
			so2 = unp->unp_conn->unp_socket;
			/*
			 * Adjust backpressure on sender
			 * and wakeup any waiting to write.
			 */
			snd->sb_mbmax += unp->unp_mbcnt - rcv->sb_mbcnt;
			unp->unp_mbcnt = rcv->sb_mbcnt;
			snd->sb_hiwat += unp->unp_cc - rcv->sb_cc;
			unp->unp_cc = rcv->sb_cc;
			sowwakeup(so2);
#undef snd
#undef rcv
			break;

		default:
			panic("uipc 2");
		}
		break;

	case PRU_SEND:
		if (control && (error = unp_internalize(control, p)))
			break;
		switch (so->so_type) {

		case SOCK_DGRAM: {
			struct sockaddr *from;

			if (nam) {
				if (unp->unp_conn) {
					error = EISCONN;
					break;
				}
				error = unp_connect(so, nam, p);
				if (error)
					break;
			} else {
				if (unp->unp_conn == 0) {
					error = ENOTCONN;
					break;
				}
			}
			so2 = unp->unp_conn->unp_socket;
			if (unp->unp_addr)
				from = mtod(unp->unp_addr, struct sockaddr *);
			else
				from = &sun_noname;
			if (sbappendaddr(&so2->so_rcv, from, m, control)) {
				sorwakeup(so2);
				m = 0;
				control = 0;
			} else
				error = ENOBUFS;
			if (nam)
				unp_disconnect(unp);
			break;
		}

		case SOCK_STREAM:
#define	rcv (&so2->so_rcv)
#define	snd (&so->so_snd)
			if (so->so_state & SS_CANTSENDMORE) {
				error = EPIPE;
				break;
			}
			if (unp->unp_conn == 0)
				panic("uipc 3");
			so2 = unp->unp_conn->unp_socket;
			/*
			 * Send to paired receive port, and then reduce
			 * send buffer hiwater marks to maintain backpressure.
			 * Wake up readers.
			 */
			if (control) {
				if (sbappendcontrol(rcv, m, control))
					control = 0;
			} else
				sbappend(rcv, m);
			snd->sb_mbmax -=
			    rcv->sb_mbcnt - unp->unp_conn->unp_mbcnt;
			unp->unp_conn->unp_mbcnt = rcv->sb_mbcnt;
			snd->sb_hiwat -= rcv->sb_cc - unp->unp_conn->unp_cc;
			unp->unp_conn->unp_cc = rcv->sb_cc;
			sorwakeup(so2);
			m = 0;
#undef snd
#undef rcv
			break;

		default:
			panic("uipc 4");
		}
		break;

	case PRU_ABORT:
		unp_drop(unp, ECONNABORTED);
		break;

	case PRU_SENSE:
		((struct stat *) m)->st_blksize = so->so_snd.sb_hiwat;
		if (so->so_type == SOCK_STREAM && unp->unp_conn != 0) {
			so2 = unp->unp_conn->unp_socket;
			((struct stat *) m)->st_blksize += so2->so_rcv.sb_cc;
		}
		((struct stat *) m)->st_dev = NODEV;
		if (unp->unp_ino == 0)
			unp->unp_ino = unp_ino++;
		((struct stat *) m)->st_atimespec =
		    ((struct stat *) m)->st_mtimespec =
		    ((struct stat *) m)->st_ctimespec = unp->unp_ctime;
		((struct stat *) m)->st_ino = unp->unp_ino;
		return (0);

	case PRU_RCVOOB:
		return (EOPNOTSUPP);

	case PRU_SENDOOB:
		error = EOPNOTSUPP;
		break;

	case PRU_SOCKADDR:
		if (unp->unp_addr) {
			nam->m_len = unp->unp_addr->m_len;
			bcopy(mtod(unp->unp_addr, caddr_t),
			    mtod(nam, caddr_t), (unsigned)nam->m_len);
		} else
			nam->m_len = 0;
		break;

	case PRU_PEERADDR:
		if (unp->unp_conn && unp->unp_conn->unp_addr) {
			nam->m_len = unp->unp_conn->unp_addr->m_len;
			bcopy(mtod(unp->unp_conn->unp_addr, caddr_t),
			    mtod(nam, caddr_t), (unsigned)nam->m_len);
		} else
			nam->m_len = 0;
		break;

	case PRU_PEEREID:
		if (unp->unp_flags & UNP_FEIDS) {
			nam->m_len = sizeof(struct unpcbid);
			bcopy((caddr_t)(&(unp->unp_connid)),
			    mtod(nam, caddr_t), (unsigned)nam->m_len);
		} else
			nam->m_len = 0;
		break;

	case PRU_SLOWTIMO:
		break;

	default:
		panic("piusrreq");
	}
release:
	if (control)
		m_freem(control);
	if (m)
		m_freem(m);
	return (error);
}

/*
 * Both send and receive buffers are allocated PIPSIZ bytes of buffering
 * for stream sockets, although the total for sender and receiver is
 * actually only PIPSIZ.
 * Datagram sockets really use the sendspace as the maximum datagram size,
 * and don't really want to reserve the sendspace.  Their recvspace should
 * be large enough for at least one max-size datagram plus address.
 */
#define	PIPSIZ	4096
u_long	unpst_sendspace = PIPSIZ;
u_long	unpst_recvspace = PIPSIZ;
u_long	unpdg_sendspace = 2*1024;	/* really max datagram size */
u_long	unpdg_recvspace = 4*1024;

int	unp_rights;			/* file descriptors in flight */

int
unp_attach(so)
	struct socket *so;
{
	register struct unpcb *unp;
	struct timeval tv;
	int error;
	
	if (so->so_snd.sb_hiwat == 0 || so->so_rcv.sb_hiwat == 0) {
		switch (so->so_type) {

		case SOCK_STREAM:
			error = soreserve(so, unpst_sendspace, unpst_recvspace);
			break;

		case SOCK_DGRAM:
			error = soreserve(so, unpdg_sendspace, unpdg_recvspace);
			break;

		default:
			panic("unp_attach");
		}
		if (error)
			return (error);
	}
	unp = malloc(sizeof(*unp), M_PCB, M_NOWAIT);
	if (unp == NULL)
		return (ENOBUFS);
	bzero((caddr_t)unp, sizeof(*unp));
	unp->unp_socket = so;
	so->so_pcb = unp;
	microtime(&tv);
	TIMEVAL_TO_TIMESPEC(&tv, &unp->unp_ctime);
	return (0);
}

void
unp_detach(unp)
	register struct unpcb *unp;
{
	
	if (unp->unp_vnode) {
		unp->unp_vnode->v_socket = 0;
		vrele(unp->unp_vnode);
		unp->unp_vnode = 0;
	}
	if (unp->unp_conn)
		unp_disconnect(unp);
	while (unp->unp_refs)
		unp_drop(unp->unp_refs, ECONNRESET);
	soisdisconnected(unp->unp_socket);
	unp->unp_socket->so_pcb = 0;
	m_freem(unp->unp_addr);
	if (unp_rights) {
		/*
		 * Normally the receive buffer is flushed later,
		 * in sofree, but if our receive buffer holds references
		 * to descriptors that are now garbage, we will dispose
		 * of those descriptor references after the garbage collector
		 * gets them (resulting in a "panic: closef: count < 0").
		 */
		sorflush(unp->unp_socket);
		free(unp, M_PCB);
		unp_gc();
	} else
		free(unp, M_PCB);
}

int
unp_bind(unp, nam, p)
	struct unpcb *unp;
	struct mbuf *nam;
	struct proc *p;
{
	struct sockaddr_un *soun = mtod(nam, struct sockaddr_un *);
	register struct vnode *vp;
	struct vattr vattr;
	int error, namelen;
	struct nameidata nd;
	char buf[MLEN];

	if (unp->unp_vnode != NULL)
		return (EINVAL);
	namelen = soun->sun_len - offsetof(struct sockaddr_un, sun_path);
	if (namelen <= 0 || namelen >= MLEN)
		return EINVAL;
	strncpy(buf, soun->sun_path, namelen);
	buf[namelen] = 0;       /* null-terminate the string */
	NDINIT(&nd, CREATE, NOFOLLOW | LOCKPARENT, UIO_SYSSPACE, buf, p);
/* SHOULD BE ABLE TO ADOPT EXISTING AND wakeup() ALA FIFO's */
	if ((error = namei(&nd)) != 0)
		return (error);
	vp = nd.ni_vp;
	if (vp != NULL) {
		VOP_ABORTOP(nd.ni_dvp, &nd.ni_cnd);
		if (nd.ni_dvp == vp)
			vrele(nd.ni_dvp);
		else
			vput(nd.ni_dvp);
		vrele(vp);
		return (EADDRINUSE);
	}
	VATTR_NULL(&vattr);
	vattr.va_type = VSOCK;
	vattr.va_mode = ACCESSPERMS &~ p->p_fd->fd_cmask;
	VOP_LEASE(nd.ni_dvp, p, p->p_ucred, LEASE_WRITE);
	error = VOP_CREATE(nd.ni_dvp, &nd.ni_vp, &nd.ni_cnd, &vattr);
	if (error)
		return (error);
	vp = nd.ni_vp;
	vp->v_socket = unp->unp_socket;
	unp->unp_vnode = vp;
	unp->unp_addr = m_copy(nam, 0, (int)M_COPYALL);
	VOP_UNLOCK(vp, 0, p);
	return (0);
}

int
unp_connect(so, nam, p)
	struct socket *so;
	struct mbuf *nam;
	struct proc *p;
{
	register struct sockaddr_un *soun = mtod(nam, struct sockaddr_un *);
	register struct vnode *vp;
	register struct socket *so2, *so3;
	struct unpcb *unp2, *unp3;
	int error;
	struct nameidata nd;

	NDINIT(&nd, LOOKUP, FOLLOW | LOCKLEAF, UIO_SYSSPACE, soun->sun_path, p);
	if (nam->m_data + nam->m_len == &nam->m_dat[MLEN]) {	/* XXX */
		if (*(mtod(nam, caddr_t) + nam->m_len - 1) != 0)
			return (EMSGSIZE);
	} else
		*(mtod(nam, caddr_t) + nam->m_len) = 0;
	if ((error = namei(&nd)) != 0)
		return (error);
	vp = nd.ni_vp;
	if (vp->v_type != VSOCK) {
		error = ENOTSOCK;
		goto bad;
	}
	if ((error = VOP_ACCESS(vp, VWRITE, p->p_ucred, p)) != 0)
		goto bad;
	so2 = vp->v_socket;
	if (so2 == 0) {
		error = ECONNREFUSED;
		goto bad;
	}
	if (so->so_type != so2->so_type) {
		error = EPROTOTYPE;
		goto bad;
	}
	if (so->so_proto->pr_flags & PR_CONNREQUIRED) {
		if ((so2->so_options & SO_ACCEPTCONN) == 0 ||
		    (so3 = sonewconn(so2, 0)) == 0) {
			error = ECONNREFUSED;
			goto bad;
		}
		unp2 = sotounpcb(so2);
		unp3 = sotounpcb(so3);
		if (unp2->unp_addr)
			unp3->unp_addr =
			    m_copy(unp2->unp_addr, 0, (int)M_COPYALL);
		unp3->unp_connid.unp_euid = p->p_ucred->cr_uid;
		unp3->unp_connid.unp_egid = p->p_ucred->cr_gid;
		unp3->unp_flags |= UNP_FEIDS;
		so2 = so3;
	}
	error = unp_connect2(so, so2);
bad:
	vput(vp);
	return (error);
}

int
unp_connect2(so, so2)
	register struct socket *so;
	register struct socket *so2;
{
	register struct unpcb *unp = sotounpcb(so);
	register struct unpcb *unp2;

	if (so2->so_type != so->so_type)
		return (EPROTOTYPE);
	unp2 = sotounpcb(so2);
	unp->unp_conn = unp2;
	switch (so->so_type) {

	case SOCK_DGRAM:
		unp->unp_nextref = unp2->unp_refs;
		unp2->unp_refs = unp;
		soisconnected(so);
		break;

	case SOCK_STREAM:
		unp2->unp_conn = unp;
		soisconnected(so);
		soisconnected(so2);
		break;

	default:
		panic("unp_connect2");
	}
	return (0);
}

void
unp_disconnect(unp)
	struct unpcb *unp;
{
	register struct unpcb *unp2 = unp->unp_conn;

	if (unp2 == 0)
		return;
	unp->unp_conn = 0;
	switch (unp->unp_socket->so_type) {

	case SOCK_DGRAM:
		if (unp2->unp_refs == unp)
			unp2->unp_refs = unp->unp_nextref;
		else {
			unp2 = unp2->unp_refs;
			for (;;) {
				if (unp2 == 0)
					panic("unp_disconnect");
				if (unp2->unp_nextref == unp)
					break;
				unp2 = unp2->unp_nextref;
			}
			unp2->unp_nextref = unp->unp_nextref;
		}
		unp->unp_nextref = 0;
		unp->unp_socket->so_state &= ~SS_ISCONNECTED;
		break;

	case SOCK_STREAM:
		soisdisconnected(unp->unp_socket);
		unp2->unp_conn = 0;
		soisdisconnected(unp2->unp_socket);
		break;
	}
}

#ifdef notdef
unp_abort(unp)
	struct unpcb *unp;
{

	unp_detach(unp);
}
#endif

void
unp_shutdown(unp)
	struct unpcb *unp;
{
	struct socket *so;

	if (unp->unp_socket->so_type == SOCK_STREAM && unp->unp_conn &&
	    (so = unp->unp_conn->unp_socket))
		socantrcvmore(so);
}

void
unp_drop(unp, errno)
	struct unpcb *unp;
	int errno;
{
	struct socket *so = unp->unp_socket;

	so->so_error = errno;
	unp_disconnect(unp);
	if (so->so_head) {
		so->so_pcb = 0;
		sofree(so);
		m_freem(unp->unp_addr);
		free(unp, M_PCB);
	}
}

#ifdef notdef
unp_drain()
{

}
#endif

int
unp_externalize(rights)
	struct mbuf *rights;
{
	struct proc *p = curproc;		/* XXX */
	register struct cmsghdr *cm = mtod(rights, struct cmsghdr *);
	struct file **rp = (struct file **)(cm + 1), *fp;
	int newfds = (cm->cmsg_len - sizeof(*cm)) / sizeof (int);
	int i, f, *ip;

	if (!fdavail(p, newfds)) {
		for (i = 0; i < newfds; i++) {
			fp = *rp;
			unp_discard(fp);
			*rp++ = 0;
		}
		return (EMSGSIZE);
	}
	ip = (int *)rp;
	for (i = 0; i < newfds; i++) {
		if (fdalloc(p, 0, &f))
			panic("unp_externalize");
		bcopy(rp, &fp, sizeof fp);
		rp++;
		p->p_fd->fd_ofiles[f] = fp;
		fp->f_msgcount--;
		unp_rights--;
		bcopy(&f, ip, sizeof f);
		ip++;
	}
	return (0);
}

int
unp_internalize(control, p)
	struct mbuf *control;
	struct proc *p;
{
	struct filedesc *fdp = p->p_fd;
	register struct cmsghdr *cm = mtod(control, struct cmsghdr *);
	struct file **rp, *fp;
	register int i;
	struct mbuf *n = NULL;
	int oldfds, *ip, fd;

	if (cm->cmsg_type != SCM_RIGHTS || cm->cmsg_level != SOL_SOCKET ||
	    cm->cmsg_len != control->m_len)
		return (EINVAL);
	oldfds = (cm->cmsg_len - sizeof (*cm)) / sizeof (int);
	ip = (int *)(cm + 1);
	for (i = 0; i < oldfds; i++) {
		fd = *ip++;
		if ((unsigned)fd >= fdp->fd_nfiles ||
		    fdp->fd_ofiles[fd] == NULL)
			return (EBADF);
		if (fdp->fd_ofiles[fd]->f_count == LONG_MAX-2 ||
		    fdp->fd_ofiles[fd]->f_msgcount == LONG_MAX-2)
			return (EDEADLK);
	}
	ip = (int *)(cm + 1);
	if (sizeof(int) != sizeof(struct file *)) {
		MGET(n, M_WAIT, MT_DATA);
		rp = (struct file **)mtod(n, caddr_t);
	} else
		rp = (struct file **)ip;
	for (i = 0; i < oldfds; i++) {
		bcopy(ip, &fd, sizeof fd);
		ip++;
		fp = fdp->fd_ofiles[fd];
		bcopy(&fp, rp, sizeof fp);
		rp++;
		fp->f_count++;
		fp->f_msgcount++;
		unp_rights++;
	}
	if (n) {
		m_adj(control, -(oldfds * sizeof(int)));
		n->m_len = oldfds * sizeof(struct file *);
		m_cat(control, n);
	}
	return (0);
}

int	unp_defer, unp_gcing;
extern	struct domain unixdomain;

void
unp_gc()
{
	register struct file *fp, *nextfp;
	register struct socket *so;
	struct file **extra_ref, **fpp;
	int nunref, i;

	if (unp_gcing)
		return;
	unp_gcing = 1;
	unp_defer = 0;
	for (fp = filehead.lh_first; fp != 0; fp = fp->f_list.le_next)
		fp->f_flag &= ~(FMARK|FDEFER);
	do {
		for (fp = filehead.lh_first; fp != 0; fp = fp->f_list.le_next) {
			if (fp->f_count == 0)
				continue;
			if (fp->f_flag & FDEFER) {
				fp->f_flag &= ~FDEFER;
				unp_defer--;
			} else {
				if (fp->f_flag & FMARK)
					continue;
				if (fp->f_count == fp->f_msgcount)
					continue;
				fp->f_flag |= FMARK;
			}
			if (fp->f_type != DTYPE_SOCKET ||
			    (so = (struct socket *)fp->f_data) == 0)
				continue;
			if (so->so_proto->pr_domain != &unixdomain ||
			    (so->so_proto->pr_flags&PR_RIGHTS) == 0)
				continue;
#ifdef notdef
			if (so->so_rcv.sb_flags & SB_LOCK) {
				/*
				 * This is problematical; it's not clear
				 * we need to wait for the sockbuf to be
				 * unlocked (on a uniprocessor, at least),
				 * and it's also not clear what to do
				 * if sbwait returns an error due to receipt
				 * of a signal.  If sbwait does return
				 * an error, we'll go into an infinite
				 * loop.  Delete all of this for now.
				 */
				(void) sbwait(&so->so_rcv);
				goto restart;
			}
#endif
			unp_scan(so->so_rcv.sb_mb, unp_mark);
		}
	} while (unp_defer);
	/*
	 * We grab an extra reference to each of the file table entries
	 * that are not otherwise accessible and then free the rights
	 * that are stored in messages on them.
	 *
	 * The bug in the original code is a little tricky, so I'll describe
	 * what's wrong with it here.
	 *
	 * It is incorrect to simply unp_discard each entry for f_msgcount
	 * times -- consider the case of sockets A and B that contain
	 * references to each other.  On a last close of some other socket,
	 * we trigger a gc since the number of outstanding rights (unp_rights)
	 * is non-zero.  If during the sweep phase the gc code un_discards,
	 * we end up doing a (full) closef on the descriptor.  A closef on A
	 * results in the following chain.  Closef calls soo_close, which
	 * calls soclose.   Soclose calls first (through the switch
	 * uipc_usrreq) unp_detach, which re-invokes unp_gc.  Unp_gc simply
	 * returns because the previous instance had set unp_gcing, and
	 * we return all the way back to soclose, which marks the socket
	 * with SS_NOFDREF, and then calls sofree.  Sofree calls sorflush
	 * to free up the rights that are queued in messages on the socket A,
	 * i.e., the reference on B.  The sorflush calls via the dom_dispose
	 * switch unp_dispose, which unp_scans with unp_discard.  This second
	 * instance of unp_discard just calls closef on B.
	 *
	 * Well, a similar chain occurs on B, resulting in a sorflush on B,
	 * which results in another closef on A.  Unfortunately, A is already
	 * being closed, and the descriptor has already been marked with
	 * SS_NOFDREF, and soclose panics at this point.
	 *
	 * Here, we first take an extra reference to each inaccessible
	 * descriptor.  Then, we call sorflush ourself, since we know
	 * it is a Unix domain socket anyhow.  After we destroy all the
	 * rights carried in messages, we do a last closef to get rid
	 * of our extra reference.  This is the last close, and the
	 * unp_detach etc will shut down the socket.
	 *
	 * 91/09/19, bsy@cs.cmu.edu
	 */
	extra_ref = malloc(nfiles * sizeof(struct file *), M_FILE, M_WAITOK);
	for (nunref = 0, fp = filehead.lh_first, fpp = extra_ref; fp != 0;
	    fp = nextfp) {
		nextfp = fp->f_list.le_next;
		if (fp->f_count == 0)
			continue;
		if (fp->f_count == fp->f_msgcount && !(fp->f_flag & FMARK)) {
			*fpp++ = fp;
			nunref++;
			fp->f_count++;
		}
	}
	for (i = nunref, fpp = extra_ref; --i >= 0; ++fpp)
	        if ((*fpp)->f_type == DTYPE_SOCKET && (*fpp)->f_data != NULL)
		        sorflush((struct socket *)(*fpp)->f_data);
	for (i = nunref, fpp = extra_ref; --i >= 0; ++fpp)
		(void) closef(*fpp, (struct proc *)0);
	free((caddr_t)extra_ref, M_FILE);
	unp_gcing = 0;
}

void
unp_dispose(m)
	struct mbuf *m;
{

	if (m)
		unp_scan(m, unp_discard);
}

void
unp_scan(m0, op)
	register struct mbuf *m0;
	void (*op) __P((struct file *));
{
	register struct mbuf *m;
	struct file **rp, *fp;
	register struct cmsghdr *cm;
	register int i;
	int qfds;

	while (m0) {
		for (m = m0; m; m = m->m_next)
			if (m->m_type == MT_CONTROL &&
			    m->m_len >= sizeof(*cm)) {
				cm = mtod(m, struct cmsghdr *);
				if (cm->cmsg_level != SOL_SOCKET ||
				    cm->cmsg_type != SCM_RIGHTS)
					continue;
				qfds = (cm->cmsg_len - sizeof *cm) / sizeof (int);
				rp = (struct file **)(cm + 1);
				for (i = 0; i < qfds; i++) {
					bcopy(rp, &fp, sizeof fp);
					rp++;
					(*op)(fp);
				}
				break;		/* XXX, but saves time */
			}
		m0 = m0->m_act;
	}
}

void
unp_mark(fp)
	struct file *fp;
{

	if (fp->f_flag & FMARK)
		return;
	unp_defer++;
	fp->f_flag |= (FMARK|FDEFER);
}

void
unp_discard(fp)
	struct file *fp;
{

	fp->f_msgcount--;
	unp_rights--;
	(void) closef(fp, (struct proc *)0);
}