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The current implementation of vn_lock() is racy.  Modification of
the vnode operations vector for active vnodes is unsafe because it
is not known whether deadfs or the original file system will be
called.

- Pass down LK_RETRY to the lock operation (hint for deadfs only).

- Change deadfs lock operation to return ENOENT if LK_RETRY is unset.

- Change all other lock operations to check for dead vnode once
  the vnode is locked and unlock and return ENOENT in this case.

With these changes in place vnode lock operations will never succeed
after vclean() has marked the vnode as VI_XLOCK and before vclean()
has changed the operations vector.

Adresses PR kern/37706 (Forced unmount of file systems is unsafe)

Discussed on tech-kern.

Welcome to 6.99.33

/*	$NetBSD: ulfs_ihash.c,v 1.4 2014/02/27 16:51:39 hannken Exp $	*/
/*  from NetBSD: ufs_ihash.c,v 1.31 2011/06/12 03:36:02 rmind 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. 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.
 *
 *	@(#)ufs_ihash.c	8.7 (Berkeley) 5/17/95
 */

#include <sys/cdefs.h>
__KERNEL_RCSID(0, "$NetBSD: ulfs_ihash.c,v 1.4 2014/02/27 16:51:39 hannken Exp $");

#include <sys/param.h>
#include <sys/systm.h>
#include <sys/vnode.h>
#include <sys/proc.h>
#include <sys/mutex.h>

#include <ufs/lfs/ulfs_inode.h>
#include <ufs/lfs/ulfs_extern.h>

/*
 * Structures associated with inode cacheing.
 */
static LIST_HEAD(ihashhead, inode) *ihashtbl;
static u_long	ihash;		/* size of hash table - 1 */
#define INOHASH(device, inum)	(((device) + (inum)) & ihash)

kmutex_t	ulfs_ihash_lock;
kmutex_t	ulfs_hashlock;

/*
 * Initialize inode hash table.
 */
void
ulfs_ihashinit(void)
{

	mutex_init(&ulfs_hashlock, MUTEX_DEFAULT, IPL_NONE);
	mutex_init(&ulfs_ihash_lock, MUTEX_DEFAULT, IPL_NONE);
	ihashtbl = hashinit(desiredvnodes, HASH_LIST, true, &ihash);
}

/*
 * Reinitialize inode hash table.
 */

void
ulfs_ihashreinit(void)
{
	struct inode *ip;
	struct ihashhead *oldhash, *hash;
	u_long oldmask, mask, val;
	int i;

	hash = hashinit(desiredvnodes, HASH_LIST, true, &mask);
	mutex_enter(&ulfs_ihash_lock);
	oldhash = ihashtbl;
	oldmask = ihash;
	ihashtbl = hash;
	ihash = mask;
	for (i = 0; i <= oldmask; i++) {
		while ((ip = LIST_FIRST(&oldhash[i])) != NULL) {
			LIST_REMOVE(ip, i_hash);
			val = INOHASH(ip->i_dev, ip->i_number);
			LIST_INSERT_HEAD(&hash[val], ip, i_hash);
		}
	}
	mutex_exit(&ulfs_ihash_lock);
	hashdone(oldhash, HASH_LIST, oldmask);
}

/*
 * Free inode hash table.
 */
void
ulfs_ihashdone(void)
{

	hashdone(ihashtbl, HASH_LIST, ihash);
	mutex_destroy(&ulfs_hashlock);
	mutex_destroy(&ulfs_ihash_lock);
}

/*
 * Use the device/inum pair to find the incore inode, and return a pointer
 * to it. If it is in core, return it, even if it is locked.
 */
struct vnode *
ulfs_ihashlookup(dev_t dev, ino_t inum)
{
	struct inode *ip;
	struct ihashhead *ipp;

	KASSERT(mutex_owned(&ulfs_ihash_lock));

	ipp = &ihashtbl[INOHASH(dev, inum)];
	LIST_FOREACH(ip, ipp, i_hash) {
		if (inum == ip->i_number && dev == ip->i_dev)
			break;
	}
	if (ip)
		return (ITOV(ip));
	return (NULLVP);
}

/*
 * Use the device/inum pair to find the incore inode, and return a pointer
 * to it. If it is in core, but locked, wait for it.
 */
struct vnode *
ulfs_ihashget(dev_t dev, ino_t inum, int flags)
{
	struct ihashhead *ipp;
	struct inode *ip;
	struct vnode *vp;

 loop:
	mutex_enter(&ulfs_ihash_lock);
	ipp = &ihashtbl[INOHASH(dev, inum)];
	LIST_FOREACH(ip, ipp, i_hash) {
		if (inum == ip->i_number && dev == ip->i_dev) {
			vp = ITOV(ip);
			if (flags == 0) {
				mutex_exit(&ulfs_ihash_lock);
			} else {
				mutex_enter(vp->v_interlock);
				mutex_exit(&ulfs_ihash_lock);
				if (vget(vp, flags))
					goto loop;
			}
			return (vp);
		}
	}
	mutex_exit(&ulfs_ihash_lock);
	return (NULL);
}

/*
 * Insert the inode into the hash table, and return it locked.
 */
void
ulfs_ihashins(struct inode *ip)
{
	struct ihashhead *ipp;
	int error __diagused;

	KASSERT(mutex_owned(&ulfs_hashlock));

	/* lock the inode, then put it on the appropriate hash list */
	error = VOP_LOCK(ITOV(ip), LK_EXCLUSIVE);
	KASSERT(error == 0);

	mutex_enter(&ulfs_ihash_lock);
	ipp = &ihashtbl[INOHASH(ip->i_dev, ip->i_number)];
	LIST_INSERT_HEAD(ipp, ip, i_hash);
	mutex_exit(&ulfs_ihash_lock);
}

/*
 * Remove the inode from the hash table.
 */
void
ulfs_ihashrem(struct inode *ip)
{
	mutex_enter(&ulfs_ihash_lock);
	LIST_REMOVE(ip, i_hash);
	mutex_exit(&ulfs_ihash_lock);
}