NAMEI(9)

NAME
namei, lookup, relookup, NDINIT - pathname lookup

SYNOPSIS
#include <sys/namei.h>
#include <sys/uio.h>
#include <sys/vnode.h>
int
namei(struct nameidata *ndp);
int
lookup(struct nameidata *ndp);
int
relookup(struct vnode *dvp, struct vnode **vpp, struct componentname *cnp);
void
NDINIT(struct nameidata *ndp, u_long op, u_long flags, enum uio_seg segflg, const char *namep);

DESCRIPTION
The namei interface is used to convert pathnames to file system vnodes. The name of the interface is actually a contraction of the words name and inode for name-to-inode conversion, in the days before the vfs(9) interface was implemented.

The arguments passed to the functions are encapsulated in the nameidata structure. It has the following structure:

    struct nameidata {
            /*
             * Arguments to namei/lookup.
             */
            const char *ni_dirp;            /* pathname pointer */
            enum    uio_seg ni_segflg;      /* location of pathname */
            /*
             * Arguments to lookup.
             */
            struct  vnode *ni_startdir;     /* starting directory */
            struct  vnode *ni_rootdir;      /* logical root directory */
            /*
             * Results: returned from/manipulated by lookup
             */
            struct  vnode *ni_vp;           /* vnode of result */
            struct  vnode *ni_dvp;          /* vnode of intermediate dir */
            /*
             * Shared between namei and lookup/commit routines.
             */
            size_t  ni_pathlen;             /* remaining chars in path */
            const char *ni_next;            /* next location in pathname */
            u_long  ni_loopcnt;             /* count of symlinks encountered */
            /*
             * Lookup parameters
             */
            struct componentname {
                    /*
                     * Arguments to lookup.
                     */
                    u_long  cn_nameiop;     /* namei operation */
                    u_long  cn_flags;       /* flags to namei */
                    kauth_cred_t cn_cred;   /* credentials */
                    /*
                     * Shared between lookup and commit routines.
                     */
                    char    *cn_pnbuf;      /* pathname buffer */
                    const 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() */
            } ni_cnd;
    };

The namei interface accesses vnode operations by passing arguments in the partially initialised componentname structure ni_cnd. This structure describes the subset of information from the nameidata structure that is passed through to the vnode operations. See vnodeops(9) for more information. The details of the componentname structure are not absolutely necessary since the members are initialised by the helper macro NDINIT(). It is useful to know the operations and flags as specified in vnodeops(9).

The namei interface overloads ni_cnd.cn_flags with some additional flags. These flags should be specific to the namei interface and ignored by vnode operations. However, due to the historic close relationship between the namei interface and the vnode operations, these flags are sometimes used (and set) by vnode operations, particularly VOP_LOOKUP(). The additional flags are:

NOCROSSMOUNT
    do not cross mount points
RDONLY
    lookup with read-only semantics
HASBUF
    caller has allocated pathname buffer ni_cnd.cn_pnbuf
SAVENAME
    save pathname buffer
SAVESTART
    save starting directory
ISDOTDOT
    current pathname component is ..
MAKEENTRY
    add entry to the name cache
ISLASTCN
    this is last component of pathname
ISSYMLINK
    symlink needs interpretation
ISWHITEOUT
    found whiteout
DOWHITEOUT
    do whiteouts
REQUIREDIR
    must be a directory
CREATEDIR
    trailing slashes are ok
PARAMASK
    mask of parameter descriptors

If the caller of namei() sets the SAVENAME flag, then it must free the buffer. If VOP_LOOKUP() sets the flag, then the buffer must be freed by either the commit routine or the VOP_ABORT() routine. The SAVESTART flag is set only by the callers of namei(). It implies SAVENAME plus the addition of saving the parent directory that contains the name in ni_startdir. It allows repeated calls to lookup() for the name being sought. The caller is responsible for releasing the buffer and for invoking vrele() on ni_startdir.

All access to the namei interface must be in process context. Pathname lookups cannot be done in interrupt context.

FUNCTIONS

namei(ndp)
    Convert a pathname into a pointer to a vnode. The pathname is specified by ndp-ni_dirp and is of length ndp-ni_pathlen. The ndp-segflg flags defines whether the name in ndp-ni_dirp is an address in kernel space (UIO_SYSSPACE) or an address in user space (UIO_USERSPACE).

    The vnode for the pathname is returned in ndp-ni_vp. The parent directory is returned locked in ndp-ni_dvp iff LOCKPARENT is specified.

    If ndp-ni_cnd.cn_flags has the FOLLOW flag set then symbolic links are 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.

lookup(ndp)
    Search for a pathname. This is a very central and rather complicated routine.

    The pathname is specified by ndp-ni_dirp and is of length ndp-ni_pathlen. The starting directory is taken from ndp-ni_startdir. The pathname is descended until done, or a symbolic link is encountered.

    The semantics of lookup() are altered by the operation specified by ndp-ni_cnd.cn_nameiop. When CREATE, RENAME, or DELETE is specified, information usable in creating, renaming, or deleting a directory entry may be calculated.

    If the target of the pathname exists and LOCKLEAF is set, the target is returned locked in ndp-ni_vp, otherwise it is returned unlocked.

relookup(dvp, vpp, cnp)
    Reacquire a path name component is a directory. This is a quicker way to lookup a pathname component when the parent directory is known. The locked parent directory vnode is specified by dvp and the pathname component by cnp. The vnode of the pathname is returned in the address specified by vpp.

NDINIT(ndp, op, flags, segflg, namep)
    Initialise a nameidata structure pointed to by ndp for use by the namei interface. It saves having to deal with the componentname structure inside ndp. The operation and flags are specified by op and flags respectively. These are the values to which ndp-ni_cnd.cn_nameiop and ndp-ni_cnd.cn_flags are respectively set. The segment flags which defines whether the pathname is in kernel address space or user address space is specified by segflag. The argument namep is a pointer to the pathname that ndp-ni_dirp is set to.

    This routine stores the credentials of the calling thread (curlwp) in ndp. In the rare case that another set of credentials is required for the namei operation, ndp-ni_cnd.cn_cred must be set manually.

CODE REFERENCES
This section describes places within the NetBSD source tree where actual code implementing or using the name lookup subsystem can be found. All pathnames are relative to /usr/src.

The name lookup subsystem is implemented within the file sys/kern/vfs_lookup.c.

SEE ALSO
intro(9), namecache(9), vfs(9), vnode(9), vnodeops(9)

BUGS
It is unfortunate that much of the namei interface makes assumptions on the underlying vnode operations. These assumptions are an artefact of the introduction of the vfs interface to split a file system interface which was historically designed as a tightly coupled module.