man/man9/namei.9

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.\" Copyright (c) 1998, 1999 Eivind Eklund
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.Dd July 8, 2023
.Dt NAMEI 9
.Os
.Sh NAME
.Nm namei ,
.Nm NDINIT ,
.Nm NDFREE
.Nd pathname translation and lookup operations
.Sh SYNOPSIS
.In sys/param.h
.In sys/fcntl.h
.In sys/namei.h
.Ft int
.Fn namei "struct nameidata *ndp"
.Ft void
.Fo NDINIT
.Fa "struct nameidata *ndp" "u_long op" "u_long flags"
.Fa "enum uio_seg segflg" "const char *namep" "struct thread *td"
.Fc
.Ft void
.Fn NDFREE "struct nameidata *ndp" "const uint flags"
.Sh DESCRIPTION
The
.Nm
facility allows the client to perform pathname translation and lookup
operations.
The
.Nm
functions will increment the reference count for the vnode in question.
The reference count has to be decremented after use of the vnode, by
using either
.Xr vrele 9
or
.Xr vput 9 ,
depending on whether the
.Dv LOCKLEAF
flag was specified or not.
.Pp
The
.Fn NDINIT
function is used to initialize
.Nm
components.
It takes the following arguments:
.Bl -tag -width ".Fa segflg"
.It Fa ndp
The
.Vt "struct nameidata"
to initialize.
.It Fa op
The operation which
.Fn namei
will perform.
The following operations are valid:
.Dv LOOKUP , CREATE , DELETE ,
and
.Dv RENAME .
The latter three are just setup for those
effects; just calling
.Fn namei
will not result in
.Fn VOP_RENAME
being called.
.It Fa flags
Operation flags.
Several of these can be effective at the same time.
.It Fa segflg
UIO segment indicator.
This indicates if the name of the object is in userspace
.Pq Dv UIO_USERSPACE
or in the kernel address space
.Pq Dv UIO_SYSSPACE .
.It Fa namep
Pointer to the component's pathname buffer
(the file or directory name that will be looked up).
.It Fa td
The thread context to use for
.Nm
operations and locks.
.El
.Sh NAMEI OPERATION FLAGS
The
.Fn namei
function takes the following set of
.Dq "operation flags"
that influence its operation:
.Bl -tag -width ".Dv WANTPARENT"
.It Dv LOCKLEAF
Lock vnode on return with
.Dv LK_EXCLUSIVE
unless
.Dv LOCKSHARED
is also set.
The
.Xr VOP_UNLOCK 9
should be used
to release the lock (or
.Xr vput 9
which is equivalent to calling
.Xr VOP_UNLOCK 9
followed by
.Xr vrele 9 ,
all in one).
.It Dv LOCKPARENT
This flag lets the
.Fn namei
function return the parent (directory) vnode,
.Va ni_dvp
in locked state, unless it is identical to
.Va ni_vp ,
in which case
.Va ni_dvp
is not locked per se (but may be locked due to
.Dv LOCKLEAF ) .
If a lock is enforced, it should be released using
.Xr vput 9
or
.Xr VOP_UNLOCK 9
and
.Xr vrele 9 .
.It Dv LOCKSHARED
Lock vnode on return with
.Dv LK_SHARED .
The
.Xr VOP_UNLOCK 9
should be used
to release the lock (or
.Xr vput 9
which is equivalent to calling
.Xr VOP_UNLOCK 9
followed by
.Xr vrele 9 ,
all in one).
.It Dv WANTPARENT
This flag allows the
.Fn namei
function to return the parent (directory) vnode in an unlocked state.
The parent vnode must be released separately by using
.Xr vrele 9 .
.It Dv NOCACHE
Avoid
.Fn namei
creating this entry in the namecache if it is not
already present.
Normally,
.Fn namei
will add entries to the name cache
if they are not already there.
.It Dv FOLLOW
With this flag,
.Fn namei
will follow the symbolic link if the last part
of the path supplied is a symbolic link (i.e., it will return a vnode
for whatever the link points at, instead for the link itself).
.It Dv NOFOLLOW
Do not follow symbolic links (pseudo).
This flag is not looked for by the actual code, which looks for
.Dv FOLLOW .
.Dv NOFOLLOW
is used to indicate to the source code reader that symlinks
are intentionally not followed.
.It Dv SAVENAME
Do not free the pathname buffer at the end of the
.Fn namei
invocation; instead, free it later in
.Fn NDFREE
so that the caller may access the pathname buffer.
See below for details.
.It Dv SAVESTART
Retain an additional reference to the parent directory; do not free
the pathname buffer.
See below for details.
.El
.Sh ALLOCATED ELEMENTS
The
.Vt nameidata
structure is composed of the following fields:
.Bl -tag -width ".Va ni_cnd.cn_pnbuf"
.It Va ni_startdir
In the normal case, this is either the current directory or the root.
It is the current directory if the name passed in does not start with
.Ql /
and we have not gone through any symlinks with an absolute path, and
the root otherwise.
.Pp
In this case, it is only used by
.Fn vfs_lookup ,
and should not be
considered valid after a call to
.Fn namei .
.It Va ni_dvp
Vnode pointer to directory of the object on which lookup is performed.
This is available on successful return if
.Dv LOCKPARENT
or
.Dv WANTPARENT
is set.
It is locked if
.Dv LOCKPARENT
is set.
.It Va ni_vp
Vnode pointer to the resulting object,
.Dv NULL
otherwise.
The
.Va v_usecount
field of this vnode is incremented.
If
.Dv LOCKLEAF
is set, it is also locked.
.Pp
.It Va ni_cnd.cn_pnbuf
The pathname buffer contains the location of the file or directory
that will be used by the
.Nm
operations.
It is managed by the
.Xr uma 9
zone allocation interface.
.El
.Sh RETURN VALUES
If successful,
.Fn namei
will return 0, otherwise it will return an error.
.Sh FILES
.Bl -tag -width Pa
.It Pa src/sys/kern/vfs_lookup.c
.El
.Sh ERRORS
Errors which
.Fn namei
may return:
.Bl -tag -width Er
.It Bq Er ENOTDIR
A component of the specified pathname is not a directory when a directory is
expected.
.It Bq Er ENAMETOOLONG
A component of a pathname exceeded 255 characters,
or an entire pathname exceeded 1023 characters.
.It Bq Er ENOENT
A component of the specified pathname does not exist,
or the pathname is an empty string.
.It Bq Er EACCES
An attempt is made to access a file in a way forbidden by its file access
permissions.
.It Bq Er ELOOP
Too many symbolic links were encountered in translating the pathname.
.It Bq Er EISDIR
An attempt is made to open a directory with write mode specified.
.It Bq Er EINVAL
The last component of the pathname specified for a
.Dv DELETE
or
.Dv RENAME
operation is
.Ql \&. .
.It Bq Er EROFS
An attempt is made to modify a file or directory on a read-only file system.
.El
.Sh SEE ALSO
.Xr uio 9 ,
.Xr uma 9 ,
.Xr VFS 9 ,
.Xr vnode 9 ,
.Xr vput 9 ,
.Xr vref 9
.Sh AUTHORS
.An -nosplit
This manual page was written by
.An Eivind Eklund Aq Mt eivind@FreeBSD.org
and later significantly revised by
.An Hiten M. Pandya Aq Mt hmp@FreeBSD.org .
.Sh BUGS
The
.Dv LOCKPARENT
flag does not always result in the parent vnode being locked.
This results in complications when the
.Dv LOCKPARENT
is used.
In order to solve this for the cases where both
.Dv LOCKPARENT
and
.Dv LOCKLEAF
are used, it is necessary to resort to recursive locking.