vop_lookup(9)

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 com
ponent */

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 directo
ry, 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 de
tected, 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 appro
priate 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 == RE
NAME))

return (EROFS);
/*
* Check name cache for directory/name pair. This re
turns 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 get
ting

* 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 path
name 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 != CRE
ATE)
cache_enter(dvp, NULL, cnp);
return ENOENT;
} else {
/*
* If deleting, and at end of pathname, return pa
rameters

* which can be used to remove file. If the want
parent flag

* isn't set, we return only the directory, other
wise 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 di
rectory

* 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 sym
bolic 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 un
lock the

* parent directory before getting the requested di
rectory.

* 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 con
dition 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 direc
tory.
[ENOENT] The component dvp was not found in this
directory.
[EACCES] Access for the specified operation is de
nied.
[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.
BSD November 24, 1997

BSD

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