Ignore machine-check MSRs

[freebsd-src/fkvm-freebsd.git] / sys / kern / vfs_lookup.c

/*-
 * Copyright (c) 1982, 1986, 1989, 1993
 *      The Regents of the University of California.  All rights reserved.
 * (c) UNIX System Laboratories, Inc.
 * All or some portions of this file are derived from material licensed
 * to the University of California by American Telephone and Telegraph
 * Co. or Unix System Laboratories, Inc. and are reproduced herein with
 * the permission of UNIX System Laboratories, Inc.
 *
 * 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.
 * 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.
 *
 *      @(#)vfs_lookup.c        8.4 (Berkeley) 2/16/94
 */

#include <sys/cdefs.h>
__FBSDID("$FreeBSD$");

#include "opt_ktrace.h"
#include "opt_mac.h"
#include "opt_vfs.h"

#include <sys/param.h>
#include <sys/systm.h>
#include <sys/kernel.h>
#include <sys/fcntl.h>
#include <sys/lock.h>
#include <sys/mutex.h>
#include <sys/namei.h>
#include <sys/vnode.h>
#include <sys/mount.h>
#include <sys/filedesc.h>
#include <sys/proc.h>
#include <sys/syscallsubr.h>
#include <sys/sysctl.h>
#ifdef KTRACE
#include <sys/ktrace.h>
#endif

#include <security/audit/audit.h>
#include <security/mac/mac_framework.h>

#include <vm/uma.h>

#define NAMEI_DIAGNOSTIC 1
#undef NAMEI_DIAGNOSTIC

/*
 * Allocation zone for namei
 */
uma_zone_t namei_zone;
/*
 * Placeholder vnode for mp traversal
 */
static struct vnode *vp_crossmp;

static void
nameiinit(void *dummy __unused)
{
        int error;

        namei_zone = uma_zcreate("NAMEI", MAXPATHLEN, NULL, NULL, NULL, NULL,
            UMA_ALIGN_PTR, 0);
        error = getnewvnode("crossmp", NULL, &dead_vnodeops, &vp_crossmp);
        if (error != 0)
                panic("nameiinit: getnewvnode");
        VN_LOCK_ASHARE(vp_crossmp);
}
SYSINIT(vfs, SI_SUB_VFS, SI_ORDER_SECOND, nameiinit, NULL);

#ifdef LOOKUP_SHARED
static int lookup_shared = 1;
#else
static int lookup_shared = 0;
#endif
SYSCTL_INT(_vfs, OID_AUTO, lookup_shared, CTLFLAG_RW, &lookup_shared, 0,
    "Enables/Disables shared locks for path name translation");

/*
 * Convert a pathname into a pointer to a locked vnode.
 *
 * The FOLLOW flag is set when symbolic links are to be followed
 * when they occur at the end of the name translation process.
 * Symbolic links are always followed for all other pathname
 * components other than the last.
 *
 * The segflg defines whether the name is to be copied from user
 * space or kernel space.
 *
 * Overall outline of namei:
 *
 *      copy in name
 *      get starting directory
 *      while (!done && !error) {
 *              call lookup to search path.
 *              if symbolic link, massage name in buffer and continue
 *      }
 */
int
namei(struct nameidata *ndp)
{
        struct filedesc *fdp;   /* pointer to file descriptor state */
        char *cp;               /* pointer into pathname argument */
        struct vnode *dp;       /* the directory we are searching */
        struct iovec aiov;              /* uio for reading symbolic links */
        struct uio auio;
        int error, linklen;
        struct componentname *cnp = &ndp->ni_cnd;
        struct thread *td = cnp->cn_thread;
        struct proc *p = td->td_proc;
        int vfslocked;

        KASSERT((cnp->cn_flags & MPSAFE) != 0 || mtx_owned(&Giant) != 0,
            ("NOT MPSAFE and Giant not held"));
        ndp->ni_cnd.cn_cred = ndp->ni_cnd.cn_thread->td_ucred;
        KASSERT(cnp->cn_cred && p, ("namei: bad cred/proc"));
        KASSERT((cnp->cn_nameiop & (~OPMASK)) == 0,
            ("namei: nameiop contaminated with flags"));
        KASSERT((cnp->cn_flags & OPMASK) == 0,
            ("namei: flags contaminated with nameiops"));
        if (!lookup_shared)
                cnp->cn_flags &= ~LOCKSHARED;
        fdp = p->p_fd;

        /*
         * Get a buffer for the name to be translated, and copy the
         * name into the buffer.
         */
        if ((cnp->cn_flags & HASBUF) == 0)
                cnp->cn_pnbuf = uma_zalloc(namei_zone, M_WAITOK);
        if (ndp->ni_segflg == UIO_SYSSPACE)
                error = copystr(ndp->ni_dirp, cnp->cn_pnbuf,
                            MAXPATHLEN, (size_t *)&ndp->ni_pathlen);
        else
                error = copyinstr(ndp->ni_dirp, cnp->cn_pnbuf,
                            MAXPATHLEN, (size_t *)&ndp->ni_pathlen);

        /* If we are auditing the kernel pathname, save the user pathname. */
        if (cnp->cn_flags & AUDITVNODE1)
                AUDIT_ARG(upath, td, cnp->cn_pnbuf, ARG_UPATH1);
        if (cnp->cn_flags & AUDITVNODE2)
                AUDIT_ARG(upath, td, cnp->cn_pnbuf, ARG_UPATH2);

        /*
         * Don't allow empty pathnames.
         */
        if (!error && *cnp->cn_pnbuf == '\0')
                error = ENOENT;

        if (error) {
                uma_zfree(namei_zone, cnp->cn_pnbuf);
#ifdef DIAGNOSTIC
                cnp->cn_pnbuf = NULL;
                cnp->cn_nameptr = NULL;
#endif
                ndp->ni_vp = NULL;
                return (error);
        }
        ndp->ni_loopcnt = 0;
#ifdef KTRACE
        if (KTRPOINT(td, KTR_NAMEI)) {
                KASSERT(cnp->cn_thread == curthread,
                    ("namei not using curthread"));
                ktrnamei(cnp->cn_pnbuf);
        }
#endif

        /*
         * Get starting point for the translation.
         */
        FILEDESC_SLOCK(fdp);
        ndp->ni_rootdir = fdp->fd_rdir;
        ndp->ni_topdir = fdp->fd_jdir;

        if (cnp->cn_pnbuf[0] != '/' && ndp->ni_dirfd != AT_FDCWD) {
                error = fgetvp(td, ndp->ni_dirfd, &dp);
                FILEDESC_SUNLOCK(fdp);
                if (error == 0 && dp->v_type != VDIR) {
                        vfslocked = VFS_LOCK_GIANT(dp->v_mount);
                        vrele(dp);
                        VFS_UNLOCK_GIANT(vfslocked);
                        error = ENOTDIR;
                }
                if (error) {
                        uma_zfree(namei_zone, cnp->cn_pnbuf);
#ifdef DIAGNOSTIC
                        cnp->cn_pnbuf = NULL;
                        cnp->cn_nameptr = NULL;
#endif
                        return (error);
                }
        } else {
                dp = fdp->fd_cdir;
                VREF(dp);
                FILEDESC_SUNLOCK(fdp);
        }
        vfslocked = VFS_LOCK_GIANT(dp->v_mount);
        for (;;) {
                /*
                 * Check if root directory should replace current directory.
                 * Done at start of translation and after symbolic link.
                 */
                cnp->cn_nameptr = cnp->cn_pnbuf;
                if (*(cnp->cn_nameptr) == '/') {
                        vrele(dp);
                        VFS_UNLOCK_GIANT(vfslocked);
                        while (*(cnp->cn_nameptr) == '/') {
                                cnp->cn_nameptr++;
                                ndp->ni_pathlen--;
                        }
                        dp = ndp->ni_rootdir;
                        vfslocked = VFS_LOCK_GIANT(dp->v_mount);
                        VREF(dp);
                }
                if (vfslocked)
                        ndp->ni_cnd.cn_flags |= GIANTHELD;
                ndp->ni_startdir = dp;
                error = lookup(ndp);
                if (error) {
                        uma_zfree(namei_zone, cnp->cn_pnbuf);
#ifdef DIAGNOSTIC
                        cnp->cn_pnbuf = NULL;
                        cnp->cn_nameptr = NULL;
#endif
                        return (error);
                }
                vfslocked = (ndp->ni_cnd.cn_flags & GIANTHELD) != 0;
                ndp->ni_cnd.cn_flags &= ~GIANTHELD;
                /*
                 * Check for symbolic link
                 */
                if ((cnp->cn_flags & ISSYMLINK) == 0) {
                        if ((cnp->cn_flags & (SAVENAME | SAVESTART)) == 0) {
                                uma_zfree(namei_zone, cnp->cn_pnbuf);
#ifdef DIAGNOSTIC
                                cnp->cn_pnbuf = NULL;
                                cnp->cn_nameptr = NULL;
#endif
                        } else
                                cnp->cn_flags |= HASBUF;

                        if ((cnp->cn_flags & MPSAFE) == 0) {
                                VFS_UNLOCK_GIANT(vfslocked);
                        } else if (vfslocked)
                                ndp->ni_cnd.cn_flags |= GIANTHELD;
                        return (0);
                }
                if (ndp->ni_loopcnt++ >= MAXSYMLINKS) {
                        error = ELOOP;
                        break;
                }
#ifdef MAC
                if ((cnp->cn_flags & NOMACCHECK) == 0) {
                        error = mac_vnode_check_readlink(td->td_ucred,
                            ndp->ni_vp);
                        if (error)
                                break;
                }
#endif
                if (ndp->ni_pathlen > 1)
                        cp = uma_zalloc(namei_zone, M_WAITOK);
                else
                        cp = cnp->cn_pnbuf;
                aiov.iov_base = cp;
                aiov.iov_len = MAXPATHLEN;
                auio.uio_iov = &aiov;
                auio.uio_iovcnt = 1;
                auio.uio_offset = 0;
                auio.uio_rw = UIO_READ;
                auio.uio_segflg = UIO_SYSSPACE;
                auio.uio_td = (struct thread *)0;
                auio.uio_resid = MAXPATHLEN;
                error = VOP_READLINK(ndp->ni_vp, &auio, cnp->cn_cred);
                if (error) {
                        if (ndp->ni_pathlen > 1)
                                uma_zfree(namei_zone, cp);
                        break;
                }
                linklen = MAXPATHLEN - auio.uio_resid;
                if (linklen == 0) {
                        if (ndp->ni_pathlen > 1)
                                uma_zfree(namei_zone, cp);
                        error = ENOENT;
                        break;
                }
                if (linklen + ndp->ni_pathlen >= MAXPATHLEN) {
                        if (ndp->ni_pathlen > 1)
                                uma_zfree(namei_zone, cp);
                        error = ENAMETOOLONG;
                        break;
                }
                if (ndp->ni_pathlen > 1) {
                        bcopy(ndp->ni_next, cp + linklen, ndp->ni_pathlen);
                        uma_zfree(namei_zone, cnp->cn_pnbuf);
                        cnp->cn_pnbuf = cp;
                } else
                        cnp->cn_pnbuf[linklen] = '\0';
                ndp->ni_pathlen += linklen;
                vput(ndp->ni_vp);
                dp = ndp->ni_dvp;
        }
        uma_zfree(namei_zone, cnp->cn_pnbuf);
#ifdef DIAGNOSTIC
        cnp->cn_pnbuf = NULL;
        cnp->cn_nameptr = NULL;
#endif
        vput(ndp->ni_vp);
        ndp->ni_vp = NULL;
        vrele(ndp->ni_dvp);
        VFS_UNLOCK_GIANT(vfslocked);
        return (error);
}

static int
compute_cn_lkflags(struct mount *mp, int lkflags)
{
        if (mp == NULL || 
            ((lkflags & LK_SHARED) && !(mp->mnt_kern_flag & MNTK_LOOKUP_SHARED))) {
                lkflags &= ~LK_SHARED;
                lkflags |= LK_EXCLUSIVE;
        }
        return lkflags;
}

/*
 * Search a pathname.
 * This is a very central and rather complicated routine.
 *
 * The pathname is pointed to by ni_ptr and is of length ni_pathlen.
 * The starting directory is taken from ni_startdir. The pathname is
 * descended until done, or a symbolic link is encountered. The variable
 * ni_more is clear if the path is completed; it is set to one if a
 * symbolic link needing interpretation is encountered.
 *
 * The flag argument is LOOKUP, CREATE, RENAME, or DELETE depending on
 * whether the name is to be looked up, created, renamed, or deleted.
 * When CREATE, RENAME, or DELETE is specified, information usable in
 * creating, renaming, or deleting a directory entry may be calculated.
 * If flag has LOCKPARENT or'ed into it, the parent directory is returned
 * locked. If flag has WANTPARENT or'ed into it, the parent directory is
 * returned unlocked. Otherwise the parent directory is not returned. If
 * the target of the pathname exists and LOCKLEAF is or'ed into the flag
 * the target is returned locked, otherwise it is returned unlocked.
 * When creating or renaming and LOCKPARENT is specified, the target may not
 * be ".".  When deleting and LOCKPARENT is specified, the target may be ".".
 *
 * Overall outline of lookup:
 *
 * dirloop:
 *      identify next component of name at ndp->ni_ptr
 *      handle degenerate case where name is null string
 *      if .. and crossing mount points and on mounted filesys, find parent
 *      call VOP_LOOKUP routine for next component name
 *          directory vnode returned in ni_dvp, unlocked unless LOCKPARENT set
 *          component vnode returned in ni_vp (if it exists), locked.
 *      if result vnode is mounted on and crossing mount points,
 *          find mounted on vnode
 *      if more components of name, do next level at dirloop
 *      return the answer in ni_vp, locked if LOCKLEAF set
 *          if LOCKPARENT set, return locked parent in ni_dvp
 *          if WANTPARENT set, return unlocked parent in ni_dvp
 */
int
lookup(struct nameidata *ndp)
{
        char *cp;               /* pointer into pathname argument */
        struct vnode *dp = 0;   /* the directory we are searching */
        struct vnode *tdp;              /* saved dp */
        struct mount *mp;               /* mount table entry */
        int docache;                    /* == 0 do not cache last component */
        int wantparent;                 /* 1 => wantparent or lockparent flag */
        int rdonly;                     /* lookup read-only flag bit */
        int trailing_slash;
        int error = 0;
        int dpunlocked = 0;             /* dp has already been unlocked */
        struct componentname *cnp = &ndp->ni_cnd;
        struct thread *td = cnp->cn_thread;
        int vfslocked;                  /* VFS Giant state for child */
        int dvfslocked;                 /* VFS Giant state for parent */
        int tvfslocked;
        int lkflags_save;
        
        /*
         * Setup: break out flag bits into variables.
         */
        dvfslocked = (ndp->ni_cnd.cn_flags & GIANTHELD) != 0;
        vfslocked = 0;
        ndp->ni_cnd.cn_flags &= ~GIANTHELD;
        wantparent = cnp->cn_flags & (LOCKPARENT | WANTPARENT);
        KASSERT(cnp->cn_nameiop == LOOKUP || wantparent,
            ("CREATE, DELETE, RENAME require LOCKPARENT or WANTPARENT."));
        docache = (cnp->cn_flags & NOCACHE) ^ NOCACHE;
        if (cnp->cn_nameiop == DELETE ||
            (wantparent && cnp->cn_nameiop != CREATE &&
             cnp->cn_nameiop != LOOKUP))
                docache = 0;
        rdonly = cnp->cn_flags & RDONLY;
        cnp->cn_flags &= ~ISSYMLINK;
        ndp->ni_dvp = NULL;
        /*
         * We use shared locks until we hit the parent of the last cn then
         * we adjust based on the requesting flags.
         */
        if (lookup_shared)
                cnp->cn_lkflags = LK_SHARED;
        else
                cnp->cn_lkflags = LK_EXCLUSIVE;
        dp = ndp->ni_startdir;
        ndp->ni_startdir = NULLVP;
        vn_lock(dp,
            compute_cn_lkflags(dp->v_mount, cnp->cn_lkflags | LK_RETRY));

dirloop:
        /*
         * Search a new directory.
         *
         * The last component of the filename is left accessible via
         * cnp->cn_nameptr for callers that need the name. Callers needing
         * the name set the SAVENAME flag. When done, they assume
         * responsibility for freeing the pathname buffer.
         */
        cnp->cn_consume = 0;
        for (cp = cnp->cn_nameptr; *cp != 0 && *cp != '/'; cp++)
                continue;
        cnp->cn_namelen = cp - cnp->cn_nameptr;
        if (cnp->cn_namelen > NAME_MAX) {
                error = ENAMETOOLONG;
                goto bad;
        }
#ifdef NAMEI_DIAGNOSTIC
        { char c = *cp;
        *cp = '\0';
        printf("{%s}: ", cnp->cn_nameptr);
        *cp = c; }
#endif
        ndp->ni_pathlen -= cnp->cn_namelen;
        ndp->ni_next = cp;

        /*
         * Replace multiple slashes by a single slash and trailing slashes
         * by a null.  This must be done before VOP_LOOKUP() because some
         * fs's don't know about trailing slashes.  Remember if there were
         * trailing slashes to handle symlinks, existing non-directories
         * and non-existing files that won't be directories specially later.
         */
        trailing_slash = 0;
        while (*cp == '/' && (cp[1] == '/' || cp[1] == '\0')) {
                cp++;
                ndp->ni_pathlen--;
                if (*cp == '\0') {
                        trailing_slash = 1;
                        *ndp->ni_next = '\0';   /* XXX for direnter() ... */
                }
        }
        ndp->ni_next = cp;

        cnp->cn_flags |= MAKEENTRY;
        if (*cp == '\0' && docache == 0)
                cnp->cn_flags &= ~MAKEENTRY;
        if (cnp->cn_namelen == 2 &&
            cnp->cn_nameptr[1] == '.' && cnp->cn_nameptr[0] == '.')
                cnp->cn_flags |= ISDOTDOT;
        else
                cnp->cn_flags &= ~ISDOTDOT;
        if (*ndp->ni_next == 0)
                cnp->cn_flags |= ISLASTCN;
        else
                cnp->cn_flags &= ~ISLASTCN;


        /*
         * Check for degenerate name (e.g. / or "")
         * which is a way of talking about a directory,
         * e.g. like "/." or ".".
         */
        if (cnp->cn_nameptr[0] == '\0') {
                if (dp->v_type != VDIR) {
                        error = ENOTDIR;
                        goto bad;
                }
                if (cnp->cn_nameiop != LOOKUP) {
                        error = EISDIR;
                        goto bad;
                }
                if (wantparent) {
                        ndp->ni_dvp = dp;
                        VREF(dp);
                }
                ndp->ni_vp = dp;

                if (cnp->cn_flags & AUDITVNODE1)
                        AUDIT_ARG(vnode, dp, ARG_VNODE1);
                else if (cnp->cn_flags & AUDITVNODE2)
                        AUDIT_ARG(vnode, dp, ARG_VNODE2);

                if (!(cnp->cn_flags & (LOCKPARENT | LOCKLEAF)))
                        VOP_UNLOCK(dp, 0);
                /* XXX This should probably move to the top of function. */
                if (cnp->cn_flags & SAVESTART)
                        panic("lookup: SAVESTART");
                goto success;
        }

        /*
         * Handle "..": four special cases.
         * 1. Return an error if this is the last component of
         *    the name and the operation is DELETE or RENAME.
         * 2. If at root directory (e.g. after chroot)
         *    or at absolute root directory
         *    then ignore it so can't get out.
         * 3. If this vnode is the root of a mounted
         *    filesystem, then replace it with the
         *    vnode which was mounted on so we take the
         *    .. in the other filesystem.
         * 4. If the vnode is the top directory of
         *    the jail or chroot, don't let them out.
         */
        if (cnp->cn_flags & ISDOTDOT) {
                if ((cnp->cn_flags & ISLASTCN) != 0 &&
                    (cnp->cn_nameiop == DELETE || cnp->cn_nameiop == RENAME)) {
                        error = EINVAL;
                        goto bad;
                }
                for (;;) {
                        if (dp == ndp->ni_rootdir || 
                            dp == ndp->ni_topdir || 
                            dp == rootvnode ||
                            ((dp->v_vflag & VV_ROOT) != 0 &&
                             (cnp->cn_flags & NOCROSSMOUNT) != 0)) {
                                ndp->ni_dvp = dp;
                                ndp->ni_vp = dp;
                                vfslocked = VFS_LOCK_GIANT(dp->v_mount);
                                VREF(dp);
                                goto nextname;
                        }
                        if ((dp->v_vflag & VV_ROOT) == 0)
                                break;
                        if (dp->v_iflag & VI_DOOMED) {  /* forced unmount */
                                error = EBADF;
                                goto bad;
                        }
                        tdp = dp;
                        dp = dp->v_mount->mnt_vnodecovered;
                        tvfslocked = dvfslocked;
                        dvfslocked = VFS_LOCK_GIANT(dp->v_mount);
                        VREF(dp);
                        vput(tdp);
                        VFS_UNLOCK_GIANT(tvfslocked);
                        vn_lock(dp,
                            compute_cn_lkflags(dp->v_mount, cnp->cn_lkflags |
                            LK_RETRY));
                }
        }

        /*
         * We now have a segment name to search for, and a directory to search.
         */
unionlookup:
#ifdef MAC
        if ((cnp->cn_flags & NOMACCHECK) == 0) {
                error = mac_vnode_check_lookup(td->td_ucred, dp, cnp);
                if (error)
                        goto bad;
        }
#endif
        ndp->ni_dvp = dp;
        ndp->ni_vp = NULL;
        ASSERT_VOP_LOCKED(dp, "lookup");
        VNASSERT(vfslocked == 0, dp, ("lookup: vfslocked %d", vfslocked));
        /*
         * If we have a shared lock we may need to upgrade the lock for the
         * last operation.
         */
        if (dp != vp_crossmp &&
            VOP_ISLOCKED(dp) == LK_SHARED &&
            (cnp->cn_flags & ISLASTCN) && (cnp->cn_flags & LOCKPARENT))
                vn_lock(dp, LK_UPGRADE|LK_RETRY);
        /*
         * If we're looking up the last component and we need an exclusive
         * lock, adjust our lkflags.
         */
        if ((cnp->cn_flags & (ISLASTCN|LOCKSHARED|LOCKLEAF)) ==
            (ISLASTCN|LOCKLEAF))
                cnp->cn_lkflags = LK_EXCLUSIVE;
#ifdef NAMEI_DIAGNOSTIC
        vprint("lookup in", dp);
#endif
        lkflags_save = cnp->cn_lkflags;
        cnp->cn_lkflags = compute_cn_lkflags(dp->v_mount, cnp->cn_lkflags);
        if ((error = VOP_LOOKUP(dp, &ndp->ni_vp, cnp)) != 0) {
                cnp->cn_lkflags = lkflags_save;
                KASSERT(ndp->ni_vp == NULL, ("leaf should be empty"));
#ifdef NAMEI_DIAGNOSTIC
                printf("not found\n");
#endif
                if ((error == ENOENT) &&
                    (dp->v_vflag & VV_ROOT) && (dp->v_mount != NULL) &&
                    (dp->v_mount->mnt_flag & MNT_UNION)) {
                        tdp = dp;
                        dp = dp->v_mount->mnt_vnodecovered;
                        tvfslocked = dvfslocked;
                        dvfslocked = VFS_LOCK_GIANT(dp->v_mount);
                        VREF(dp);
                        vput(tdp);
                        VFS_UNLOCK_GIANT(tvfslocked);
                        vn_lock(dp,
                            compute_cn_lkflags(dp->v_mount, cnp->cn_lkflags |
                            LK_RETRY));
                        goto unionlookup;
                }

                if (error != EJUSTRETURN)
                        goto bad;
                /*
                 * If creating and at end of pathname, then can consider
                 * allowing file to be created.
                 */
                if (rdonly) {
                        error = EROFS;
                        goto bad;
                }
                if (*cp == '\0' && trailing_slash &&
                     !(cnp->cn_flags & WILLBEDIR)) {
                        error = ENOENT;
                        goto bad;
                }
                if ((cnp->cn_flags & LOCKPARENT) == 0)
                        VOP_UNLOCK(dp, 0);
                /*
                 * This is a temporary assert to make sure I know what the
                 * behavior here was.
                 */
                KASSERT((cnp->cn_flags & (WANTPARENT|LOCKPARENT)) != 0,
                   ("lookup: Unhandled case."));
                /*
                 * We return with ni_vp NULL to indicate that the entry
                 * doesn't currently exist, leaving a pointer to the
                 * (possibly locked) directory vnode in ndp->ni_dvp.
                 */
                if (cnp->cn_flags & SAVESTART) {
                        ndp->ni_startdir = ndp->ni_dvp;
                        VREF(ndp->ni_startdir);
                }
                goto success;
        } else
                cnp->cn_lkflags = lkflags_save;
#ifdef NAMEI_DIAGNOSTIC
        printf("found\n");
#endif
        /*
         * Take into account any additional components consumed by
         * the underlying filesystem.
         */
        if (cnp->cn_consume > 0) {
                cnp->cn_nameptr += cnp->cn_consume;
                ndp->ni_next += cnp->cn_consume;
                ndp->ni_pathlen -= cnp->cn_consume;
                cnp->cn_consume = 0;
        }

        dp = ndp->ni_vp;
        vfslocked = VFS_LOCK_GIANT(dp->v_mount);

        /*
         * Check to see if the vnode has been mounted on;
         * if so find the root of the mounted filesystem.
         */
        while (dp->v_type == VDIR && (mp = dp->v_mountedhere) &&
               (cnp->cn_flags & NOCROSSMOUNT) == 0) {
                if (vfs_busy(mp, 0, 0))
                        continue;
                vput(dp);
                VFS_UNLOCK_GIANT(vfslocked);
                vfslocked = VFS_LOCK_GIANT(mp);
                if (dp != ndp->ni_dvp)
                        vput(ndp->ni_dvp);
                else
                        vrele(ndp->ni_dvp);
                VFS_UNLOCK_GIANT(dvfslocked);
                dvfslocked = 0;
                vref(vp_crossmp);
                ndp->ni_dvp = vp_crossmp;
                error = VFS_ROOT(mp, compute_cn_lkflags(mp, cnp->cn_lkflags), &tdp, td);
                vfs_unbusy(mp);
                if (vn_lock(vp_crossmp, LK_SHARED | LK_NOWAIT))
                        panic("vp_crossmp exclusively locked or reclaimed");
                if (error) {
                        dpunlocked = 1;
                        goto bad2;
                }
                ndp->ni_vp = dp = tdp;
        }

        /*
         * Check for symbolic link
         */
        if ((dp->v_type == VLNK) &&
            ((cnp->cn_flags & FOLLOW) || trailing_slash ||
             *ndp->ni_next == '/')) {
                cnp->cn_flags |= ISSYMLINK;
                if (dp->v_iflag & VI_DOOMED) {
                        /* We can't know whether the directory was mounted with
                         * NOSYMFOLLOW, so we can't follow safely. */
                        error = EBADF;
                        goto bad2;
                }
                if (dp->v_mount->mnt_flag & MNT_NOSYMFOLLOW) {
                        error = EACCES;
                        goto bad2;
                }
                /*
                 * Symlink code always expects an unlocked dvp.
                 */
                if (ndp->ni_dvp != ndp->ni_vp)
                        VOP_UNLOCK(ndp->ni_dvp, 0);
                goto success;
        }

        /*
         * Check for bogus trailing slashes.
         */
        if (trailing_slash && dp->v_type != VDIR) {
                error = ENOTDIR;
                goto bad2;
        }

nextname:
        /*
         * Not a symbolic link.  If more pathname,
         * continue at next component, else return.
         */
        KASSERT((cnp->cn_flags & ISLASTCN) || *ndp->ni_next == '/',
            ("lookup: invalid path state."));
        if (*ndp->ni_next == '/') {
                cnp->cn_nameptr = ndp->ni_next;
                while (*cnp->cn_nameptr == '/') {
                        cnp->cn_nameptr++;
                        ndp->ni_pathlen--;
                }
                if (ndp->ni_dvp != dp)
                        vput(ndp->ni_dvp);
                else
                        vrele(ndp->ni_dvp);
                VFS_UNLOCK_GIANT(dvfslocked);
                dvfslocked = vfslocked; /* dp becomes dvp in dirloop */
                vfslocked = 0;
                goto dirloop;
        }
        /*
         * Disallow directory write attempts on read-only filesystems.
         */
        if (rdonly &&
            (cnp->cn_nameiop == DELETE || cnp->cn_nameiop == RENAME)) {
                error = EROFS;
                goto bad2;
        }
        if (cnp->cn_flags & SAVESTART) {
                ndp->ni_startdir = ndp->ni_dvp;
                VREF(ndp->ni_startdir);
        }
        if (!wantparent) {
                if (ndp->ni_dvp != dp)
                        vput(ndp->ni_dvp);
                else
                        vrele(ndp->ni_dvp);
                VFS_UNLOCK_GIANT(dvfslocked);
                dvfslocked = 0;
        } else if ((cnp->cn_flags & LOCKPARENT) == 0 && ndp->ni_dvp != dp)
                VOP_UNLOCK(ndp->ni_dvp, 0);

        if (cnp->cn_flags & AUDITVNODE1)
                AUDIT_ARG(vnode, dp, ARG_VNODE1);
        else if (cnp->cn_flags & AUDITVNODE2)
                AUDIT_ARG(vnode, dp, ARG_VNODE2);

        if ((cnp->cn_flags & LOCKLEAF) == 0)
                VOP_UNLOCK(dp, 0);
success:
        /*
         * Because of lookup_shared we may have the vnode shared locked, but
         * the caller may want it to be exclusively locked.
         */
        if ((cnp->cn_flags & (ISLASTCN | LOCKSHARED | LOCKLEAF)) ==
            (ISLASTCN | LOCKLEAF) && VOP_ISLOCKED(dp) != LK_EXCLUSIVE) {
                vn_lock(dp, LK_UPGRADE | LK_RETRY);
        }
        if (vfslocked && dvfslocked)
                VFS_UNLOCK_GIANT(dvfslocked);   /* Only need one */
        if (vfslocked || dvfslocked)
                ndp->ni_cnd.cn_flags |= GIANTHELD;
        return (0);

bad2:
        if (dp != ndp->ni_dvp)
                vput(ndp->ni_dvp);
        else
                vrele(ndp->ni_dvp);
bad:
        if (!dpunlocked)
                vput(dp);
        VFS_UNLOCK_GIANT(vfslocked);
        VFS_UNLOCK_GIANT(dvfslocked);
        ndp->ni_cnd.cn_flags &= ~GIANTHELD;
        ndp->ni_vp = NULL;
        return (error);
}

/*
 * relookup - lookup a path name component
 *    Used by lookup to re-acquire things.
 */
int
relookup(struct vnode *dvp, struct vnode **vpp, struct componentname *cnp)
{
        struct vnode *dp = 0;           /* the directory we are searching */
        int wantparent;                 /* 1 => wantparent or lockparent flag */
        int rdonly;                     /* lookup read-only flag bit */
        int error = 0;

        KASSERT(cnp->cn_flags & ISLASTCN,
            ("relookup: Not given last component."));
        /*
         * Setup: break out flag bits into variables.
         */
        wantparent = cnp->cn_flags & (LOCKPARENT|WANTPARENT);
        KASSERT(wantparent, ("relookup: parent not wanted."));
        rdonly = cnp->cn_flags & RDONLY;
        cnp->cn_flags &= ~ISSYMLINK;
        dp = dvp;
        cnp->cn_lkflags = LK_EXCLUSIVE;
        vn_lock(dp, LK_EXCLUSIVE | LK_RETRY);

        /*
         * Search a new directory.
         *
         * The last component of the filename is left accessible via
         * cnp->cn_nameptr for callers that need the name. Callers needing
         * the name set the SAVENAME flag. When done, they assume
         * responsibility for freeing the pathname buffer.
         */
#ifdef NAMEI_DIAGNOSTIC
        printf("{%s}: ", cnp->cn_nameptr);
#endif

        /*
         * Check for degenerate name (e.g. / or "")
         * which is a way of talking about a directory,
         * e.g. like "/." or ".".
         */
        if (cnp->cn_nameptr[0] == '\0') {
                if (cnp->cn_nameiop != LOOKUP || wantparent) {
                        error = EISDIR;
                        goto bad;
                }
                if (dp->v_type != VDIR) {
                        error = ENOTDIR;
                        goto bad;
                }
                if (!(cnp->cn_flags & LOCKLEAF))
                        VOP_UNLOCK(dp, 0);
                *vpp = dp;
                /* XXX This should probably move to the top of function. */
                if (cnp->cn_flags & SAVESTART)
                        panic("lookup: SAVESTART");
                return (0);
        }

        if (cnp->cn_flags & ISDOTDOT)
                panic ("relookup: lookup on dot-dot");

        /*
         * We now have a segment name to search for, and a directory to search.
         */
#ifdef NAMEI_DIAGNOSTIC
        vprint("search in:", dp);
#endif
        if ((error = VOP_LOOKUP(dp, vpp, cnp)) != 0) {
                KASSERT(*vpp == NULL, ("leaf should be empty"));
                if (error != EJUSTRETURN)
                        goto bad;
                /*
                 * If creating and at end of pathname, then can consider
                 * allowing file to be created.
                 */
                if (rdonly) {
                        error = EROFS;
                        goto bad;
                }
                /* ASSERT(dvp == ndp->ni_startdir) */
                if (cnp->cn_flags & SAVESTART)
                        VREF(dvp);
                if ((cnp->cn_flags & LOCKPARENT) == 0)
                        VOP_UNLOCK(dp, 0);
                /*
                 * This is a temporary assert to make sure I know what the
                 * behavior here was.
                 */
                KASSERT((cnp->cn_flags & (WANTPARENT|LOCKPARENT)) != 0,
                   ("relookup: Unhandled case."));
                /*
                 * We return with ni_vp NULL to indicate that the entry
                 * doesn't currently exist, leaving a pointer to the
                 * (possibly locked) directory vnode in ndp->ni_dvp.
                 */
                return (0);
        }

        dp = *vpp;

        /*
         * Disallow directory write attempts on read-only filesystems.
         */
        if (rdonly &&
            (cnp->cn_nameiop == DELETE || cnp->cn_nameiop == RENAME)) {
                if (dvp == dp)
                        vrele(dvp);
                else
                        vput(dvp);
                error = EROFS;
                goto bad;
        }
        /*
         * Set the parent lock/ref state to the requested state.
         */
        if ((cnp->cn_flags & LOCKPARENT) == 0 && dvp != dp) {
                if (wantparent)
                        VOP_UNLOCK(dvp, 0);
                else
                        vput(dvp);
        } else if (!wantparent)
                vrele(dvp);
        /*
         * Check for symbolic link
         */
        KASSERT(dp->v_type != VLNK || !(cnp->cn_flags & FOLLOW),
            ("relookup: symlink found.\n"));

        /* ASSERT(dvp == ndp->ni_startdir) */
        if (cnp->cn_flags & SAVESTART)
                VREF(dvp);
        
        if ((cnp->cn_flags & LOCKLEAF) == 0)
                VOP_UNLOCK(dp, 0);
        return (0);
bad:
        vput(dp);
        *vpp = NULL;
        return (error);
}

/*
 * Free data allocated by namei(); see namei(9) for details.
 */
void
NDFREE(struct nameidata *ndp, const u_int flags)
{
        int unlock_dvp;
        int unlock_vp;

        unlock_dvp = 0;
        unlock_vp = 0;

        if (!(flags & NDF_NO_FREE_PNBUF) &&
            (ndp->ni_cnd.cn_flags & HASBUF)) {
                uma_zfree(namei_zone, ndp->ni_cnd.cn_pnbuf);
                ndp->ni_cnd.cn_flags &= ~HASBUF;
        }
        if (!(flags & NDF_NO_VP_UNLOCK) &&
            (ndp->ni_cnd.cn_flags & LOCKLEAF) && ndp->ni_vp)
                unlock_vp = 1;
        if (!(flags & NDF_NO_VP_RELE) && ndp->ni_vp) {
                if (unlock_vp) {
                        vput(ndp->ni_vp);
                        unlock_vp = 0;
                } else
                        vrele(ndp->ni_vp);
                ndp->ni_vp = NULL;
        }
        if (unlock_vp)
                VOP_UNLOCK(ndp->ni_vp, 0);
        if (!(flags & NDF_NO_DVP_UNLOCK) &&
            (ndp->ni_cnd.cn_flags & LOCKPARENT) &&
            ndp->ni_dvp != ndp->ni_vp)
                unlock_dvp = 1;
        if (!(flags & NDF_NO_DVP_RELE) &&
            (ndp->ni_cnd.cn_flags & (LOCKPARENT|WANTPARENT))) {
                if (unlock_dvp) {
                        vput(ndp->ni_dvp);
                        unlock_dvp = 0;
                } else
                        vrele(ndp->ni_dvp);
                ndp->ni_dvp = NULL;
        }
        if (unlock_dvp)
                VOP_UNLOCK(ndp->ni_dvp, 0);
        if (!(flags & NDF_NO_STARTDIR_RELE) &&
            (ndp->ni_cnd.cn_flags & SAVESTART)) {
                vrele(ndp->ni_startdir);
                ndp->ni_startdir = NULL;
        }
}

/*
 * Determine if there is a suitable alternate filename under the specified
 * prefix for the specified path.  If the create flag is set, then the
 * alternate prefix will be used so long as the parent directory exists.
 * This is used by the various compatiblity ABIs so that Linux binaries prefer
 * files under /compat/linux for example.  The chosen path (whether under
 * the prefix or under /) is returned in a kernel malloc'd buffer pointed
 * to by pathbuf.  The caller is responsible for free'ing the buffer from
 * the M_TEMP bucket if one is returned.
 */
int
kern_alternate_path(struct thread *td, const char *prefix, const char *path,
    enum uio_seg pathseg, char **pathbuf, int create, int dirfd)
{
        struct nameidata nd, ndroot;
        char *ptr, *buf, *cp;
        size_t len, sz;
        int error;

        buf = (char *) malloc(MAXPATHLEN, M_TEMP, M_WAITOK);
        *pathbuf = buf;

        /* Copy the prefix into the new pathname as a starting point. */
        len = strlcpy(buf, prefix, MAXPATHLEN);
        if (len >= MAXPATHLEN) {
                *pathbuf = NULL;
                free(buf, M_TEMP);
                return (EINVAL);
        }
        sz = MAXPATHLEN - len;
        ptr = buf + len;

        /* Append the filename to the prefix. */
        if (pathseg == UIO_SYSSPACE)
                error = copystr(path, ptr, sz, &len);
        else
                error = copyinstr(path, ptr, sz, &len);

        if (error) {
                *pathbuf = NULL;
                free(buf, M_TEMP);
                return (error);
        }

        /* Only use a prefix with absolute pathnames. */
        if (*ptr != '/') {
                error = EINVAL;
                goto keeporig;
        }

        if (dirfd != AT_FDCWD) {
                /*
                 * We want the original because the "prefix" is
                 * included in the already opened dirfd.
                 */
                bcopy(ptr, buf, len);
                return (0);
        }

        /*
         * We know that there is a / somewhere in this pathname.
         * Search backwards for it, to find the file's parent dir
         * to see if it exists in the alternate tree. If it does,
         * and we want to create a file (cflag is set). We don't
         * need to worry about the root comparison in this case.
         */

        if (create) {
                for (cp = &ptr[len] - 1; *cp != '/'; cp--);
                *cp = '\0';

                NDINIT(&nd, LOOKUP, FOLLOW | MPSAFE, UIO_SYSSPACE, buf, td);
                error = namei(&nd);
                *cp = '/';
                if (error != 0)
                        goto keeporig;
        } else {
                NDINIT(&nd, LOOKUP, FOLLOW | MPSAFE, UIO_SYSSPACE, buf, td);

                error = namei(&nd);
                if (error != 0)
                        goto keeporig;

                /*
                 * We now compare the vnode of the prefix to the one
                 * vnode asked. If they resolve to be the same, then we
                 * ignore the match so that the real root gets used.
                 * This avoids the problem of traversing "../.." to find the
                 * root directory and never finding it, because "/" resolves
                 * to the emulation root directory. This is expensive :-(
                 */
                NDINIT(&ndroot, LOOKUP, FOLLOW | MPSAFE, UIO_SYSSPACE, prefix,
                    td);

                /* We shouldn't ever get an error from this namei(). */
                error = namei(&ndroot);
                if (error == 0) {
                        if (nd.ni_vp == ndroot.ni_vp)
                                error = ENOENT;

                        NDFREE(&ndroot, NDF_ONLY_PNBUF);
                        vrele(ndroot.ni_vp);
                        VFS_UNLOCK_GIANT(NDHASGIANT(&ndroot));
                }
        }

        NDFREE(&nd, NDF_ONLY_PNBUF);
        vrele(nd.ni_vp);
        VFS_UNLOCK_GIANT(NDHASGIANT(&nd));

keeporig:
        /* If there was an error, use the original path name. */
        if (error)
                bcopy(ptr, buf, len);
        return (error);
}