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NAME

     VOP_LOOKUP - lookup a component of a pathname

SYNOPSIS

     #include <sys/param.h>
     #include <sys/vnode.h>
     #include <sys/namei.h>

     int
     VOP_LOOKUP(struct vnode *dvp, struct vnode **vpp,
             struct componentname *cnp);

DESCRIPTION

     This entry point looks up a single pathname component in a given
     directory.

     Its arguments are:

     dvp  The locked vnode of the directory to search.

     vpp  The address of a variable where the resulting locked vnode should be
          stored.

     cnp  The pathname component to be searched for.

     Cnp is a pointer to a componentname structure defined as follows:

     struct componentname {
             /*
              * Arguments to lookup.
              */
             u_long  cn_nameiop;     /* namei operation */
             u_long  cn_flags;       /* flags to namei */
             struct  thread *cn_thread;      /* thread requesting lookup */
             struct  ucred *cn_cred; /* credentials */
             /*
              * Shared between lookup and commit routines.
              */
             char    *cn_pnbuf;      /* pathname buffer */
             char    *cn_nameptr;    /* pointer to looked up name */
             long    cn_namelen;     /* length of looked up component */
             u_long  cn_hash;        /* hash value of looked up name */
             long    cn_consume;     /* chars to consume in lookup() */
     };

     Convert a component of a pathname into a pointer to a locked vnode.  This
     is a very central and rather complicated routine.  If the file system is
     not maintained in a strict tree hierarchy, this can result in a deadlock
     situation.

     The cnp->cn_nameiop argument is LOOKUP, CREATE, RENAME, or DELETE
     depending on the intended use of the object.  When CREATE, RENAME, or
     DELETE is specified, information usable in creating, renaming, or
     deleting a directory entry may be calculated.

     Overall outline of VOP_LOOKUP:

           Check accessibility of directory.  Look for name in cache, if
           found, then return name.  Search for name in directory, goto to
           found or notfound as appropriate.

     notfound:

           If creating or renaming and at end of pathname, return EJUSTRETURN,
           leaving info on available slots else return ENOENT.

     found:

           If at end of path and deleting, return information to allow delete.
           If at end of path and renaming, lock target inode and return info
           to allow rename.  If not at end, add name to cache; if at end and
           neither creating nor deleting, add name to cache.

LOCKS

     The directory, dvp should be locked on entry.  If an error (note: the
     return value EJUSTRETURN is not considered an error) is detected, it will
     be returned locked.  Otherwise, it will be unlocked unless both
     LOCKPARENT and ISLASTCN are specified in cnp->cn_flags.  If an entry is
     found in the directory, it will be returned locked.

RETURN VALUES

     Zero is returned with *vpp set to the locked vnode of the file if the
     component is found.  If the component being searched for is ".", then the
     vnode just has an extra reference added to it with vref(9).  The caller
     must take care to release the locks appropriately in this case.

     If the component is not found and the operation is CREATE or RENAME, the
     flag ISLASTCN is specified and the operation would succeed, the special
     return value EJUSTRETURN is returned.  Otherwise, an appropriate error
     code is returned.

PSEUDOCODE

     int
     vop_lookup(struct vnode *dvp,
                struct vnode **vpp,
                struct componentname *cnp)
     {
         int error;
         int nameiop = cnp->cn_nameiop;
         int flags = cnp->cn_flags;
         int lockparent = flags & LOCKPARENT;
         int islastcn = flags & ISLASTCN;
         struct vnode *vp = NULL;

         /*
          * Check accessibility of directory.
          */
         if (dvp->v_type != VDIR)
             return ENOTDIR;

         error = VOP_ACCESS(dvp, VEXEC, cred, cnp->cn_thread);
         if (error)
             return (error);

         if (islastcn && (dvp->v_mount->mnt_flag & MNT_RDONLY) &&
             (cnp->cn_nameiop == DELETE || cnp->cn_nameiop == RENAME))
             return (EROFS);

         /*
          * Check name cache for directory/name pair.  This returns ENOENT
          * if the name is known not to exist, -1 if the name was found, or
          * zero if not.
          */
         error = cache_lookup(dvp, vpp, cnp);
         if (error) {
             int vpid;

             if (error = ENOENT)
                 return error;

             vp = *vpp;
             if (dvp == vp) {        /* lookup on "." */
                 VREF(vp);
                 error = 0;
             } else if (flags & ISDOTDOT) {
                 /*
                  * We need to unlock the directory before getting
                  * the locked vnode for ".." to avoid deadlocks.
                  */
                 VOP_UNLOCK(dvp);
                 error = vget(vp, 1);
                 if (!error) {
                     if (lockparent && islastcn)
                         error = VOP_LOCK(dvp);
                 }
             } else {
                 error = vget(vp, 1);
                 if (error || !(lockparent && islastcn)) {
                     VOP_UNLOCK(dvp);
                 }
             }

             /*
              * Check that the capability number did not change
              * while we were waiting for the lock.
              */
             if (!error) {
                 if (vpid == vp->v_id) {
                     /*
                      * dvp is locked if lockparent && islastcn.
                      * vp is locked.
                      */
                     return (0);
                 }
                 vput(vp);

                 if (dvp != vp && lockparent && islastcn)
                     VOP_UNLOCK(pdp);
             }

             /*
              * Re-lock dvp for the directory search below.
              */
             error = VOP_LOCK(dvp);
             if (error) {
                 return (error);
             }

             *vpp = NULL;
         }

         /*
          * Search dvp for the component cnp->cn_nameptr.
          */
         ...;

         if (!found) {
             if ((nameiop == CREATE || nameiop == RENAME)
                 && islastcn
                 && directory dvp has not been removed) {
                 /*
                  * Check for write access on directory.
                  */

                 /*
                  * Possibly record the position of a slot in the directory
                  * large enough for the new component name.  This can be
                  * recorded in the vnode private data for dvp.
                  * Set the SAVENAME flag to hold onto the pathname for use
                  * later in VOP_CREATE or VOP_RENAME.
                  */
                 cnp->cn_flags |= SAVENAME;
                 if (!lockparent)
                     /*
                      * Note that the extra data recorded above is only
                      * useful if lockparent is specified.
                      */
                     VOP_UNLOCK(dvp);

                 return EJUSTRETURN;
             }

             /*
              * Consider inserting name into cache.
              */
             if ((cnp->cn_flags & MAKEENTRY) && nameiop != CREATE)
                 cache_enter(dvp, NULL, cnp);

             return ENOENT;
         } else {
             /*
              * If deleting, and at end of pathname, return parameters
              * which can be used to remove file.  If the wantparent flag
              * isn’t set, we return only the directory, otherwise we go on
              * and lock the inode, being careful with ".".
              */
             if (nameiop == DELETE && islastcn) {
                 /*
                  * Check for write access on directory.
                  */
                 error = VOP_ACCESS(dvp, VWRITE, cred, cnp->cn_thread);
                 if (error)
                     return (error);

                 if (found entry is same as dvp) {
                     VREF(dvp);
                     *vpp = dvp;
                     return 0;
                 }

                 error = VFS_VGET(dvp->v_mount, ..., &vp);
                 if (error)
                     return error;

                 if (directory is sticky
                     && cred->cr_uid != 0
                     && cred->cr_uid != owner of dvp
                     && owner of vp != cred->cr_uid) {
                     vput(vp);
                     return EPERM;
                 }
                 *vpp = vp;
                 if (!lockparent)
                     VOP_UNLOCK(dvp);

                 return 0;
             }

             /*
              * If rewriting (RENAME), return the inode and the
              * information required to rewrite the present directory
              * Must get inode of directory entry to verify it’s a
              * regular file, or empty directory.
              */
             if (nameiop == RENAME && wantparent && islastcn) {
                 error = VOP_ACCESS(dvp, VWRITE, cred, cnp->cn_thread);
                 if (error)
                     return (error);

                 /*
                  * Check for "."
                  */
                 if (found entry is same as dvp)
                     return EISDIR;

                 error = VFS_VGET(dvp->v_mount, ..., &vp);
                 if (error)
                     return error;
                 *vpp = vp;
                 /*
                  * Save the name for use in VOP_RENAME later.
                  */
                 cnp->cn_flags |= SAVENAME;
                 if (!lockparent)
                     VOP_UNLOCK(dvp);

                 return 0;
             }

             /*
              * Step through the translation in the name.  We do not ‘vput’ the
              * directory because we may need it again if a symbolic link
              * is relative to the current directory.  Instead we save it
              * unlocked as "pdp".  We must get the target inode before unlocking
              * the directory to insure that the inode will not be removed
              * before we get it.  We prevent deadlock by always fetching
              * inodes from the root, moving down the directory tree. Thus
              * when following backward pointers ".." we must unlock the
              * parent directory before getting the requested directory.
              * There is a potential race condition here if both the current
              * and parent directories are removed before the VFS_VGET for the
              * inode associated with ".." returns.  We hope that this occurs
              * infrequently since we cannot avoid this race condition without
              * implementing a sophisticated deadlock detection algorithm.
              * Note also that this simple deadlock detection scheme will not
              * work if the file system has any hard links other than ".."
              * that point backwards in the directory structure.
              */
             if (flags & ISDOTDOT) {
                 VOP_UNLOCK(dvp);    /* race to get the inode */
                 error = VFS_VGET(dvp->v_mount, ..., &vp);
                 if (error) {
                     VOP_LOCK(dvp);
                     return (error);
                 }
                 if (lockparent && islastcn) {
                     error = VOP_LOCK(dvp);
                     if (error) {
                         vput(vp);
                         return error;
                     }
                 }
                 *vpp = vp;
             } else if (found entry is same as dvp) {
                 VREF(dvp);  /* we want ourself, ie "." */
                 *vpp = dvp;
             } else {
                 error = VFS_VGET(dvp->v_mount, ..., &vp);
                 if (error)
                     return (error);
                 if (!lockparent || !islastcn)
                     VOP_UNLOCK(dvp);
                 *vpp = vp;
             }

             /*
              * Insert name into cache if appropriate.
              */
             if (cnp->cn_flags & MAKEENTRY)
                 cache_enter(dvp, *vpp, cnp);
             return (0);
         }
     }

ERRORS

     [ENOTDIR]          The vnode dvp does not represent a directory.

     [ENOENT]           The component dvp was not found in this directory.

     [EACCES]           Access for the specified operation is denied.

     [EJUSTRETURN]      A CREATE or RENAME operation would be successful.

SEE ALSO

     vnode(9), VOP_ACCESS(9), VOP_CREATE(9), VOP_MKDIR(9), VOP_MKNOD(9),
     VOP_RENAME(9), VOP_SYMLINK(9)

HISTORY

     The function VOP_LOOKUP appeared in 4.3BSD.

AUTHORS

     This manual page was written by Doug Rabson, with some text from comments
     in ufs_lookup.c.