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.Dd March 18, 2017
.Dt NAMEI 9
.Os
.Sh NAME
.Nm namei ,
.Nm NDINIT ,
.Nm NDAT ,
.Nm namei_simple_kernel ,
.Nm namei_simple_user ,
.Nm relookup ,
.Nm lookup_for_nfsd ,
.Nm lookup_for_nfsd_index
.Nd pathname lookup
.Sh SYNOPSIS
.In sys/namei.h
.In sys/uio.h
.In sys/vnode.h
.Fn NDINIT "struct nameidata *ndp" "u_long op" "u_long flags" \
"struct pathbuf *pathbuf"
.Fn NDAT "struct nameidata *ndp" "struct vnode *dvp"
.Ft int
.Fn namei "struct nameidata *ndp"
.Ft int
.Fn namei_simple_kernel "const char *path" "namei_simple_flags_t sflags" \
"struct vnode **ret"
.Ft int
.Fn namei_simple_user "const char *path" "namei_simple_flags_t sflags" \
"struct vnode **ret"
.Ft int
.Fn relookup "struct vnode *dvp" "struct vnode **vpp" \
"struct componentname *cnp"
.Ft int
.Fn lookup_for_nfsd "struct nameidata *ndp" "struct vnode *startdir" \
"int neverfollow"
.Ft int
.Fn lookup_for_nfsd_index "struct nameidata *ndp"
.Sh DESCRIPTION
The
.Nm
interface is used to convert pathnames to file system vnodes.
The
name of the interface is actually a contraction of the words
.Em name
and
.Em inode
for name-to-inode conversion, in the days before the
.Xr vfs 9
interface was implemented.
.Pp
All access to the
.Nm
interface must be in process context.
Pathname lookups cannot be done in interrupt context.
.Pp
In the general form of
.Nm ,
a caller must:
.Bl -enum -compact
.It
Allocate storage for a
.Ft struct nameidata
object
.Fa nd .
.It
Initialize
.Fa nd
with
.Fn NDINIT
and optionally
.Fn NDAT
to specify the arguments to a lookup.
.It
Call
.Fn namei
and handle failure if it returns a nonzero error code.
.It
Read the resulting vnode out of
.Fa nd Ns Li .ni_vp .
If requested with
.Dv LOCKPARENT ,
read the directory vnode out of
.Fa nd Ns Li .ni_dvp .
.It
For directory operations, use the
.Ft struct componentname
object stored at
.Fa nd Ns Li .ni_cnd .
.El
.Pp
The other fields of
.Ft struct nameidata
should not be examined or altered directly.
.Pp
Note that the
.Xr nfs 4
code misuses
.Ft struct nameidata
and currently has an incestuous relationship with the
.Nm
code.
This is gradually being cleaned up.
.Pp
The
.Ft struct componentname
type has the following layout:
.Bd -literal
struct componentname {
	/*
	 * Arguments to VOP_LOOKUP and directory VOP routines.
	 */
	uint32_t	cn_nameiop;	/* namei operation */
	uint32_t	cn_flags;	/* flags to namei */
	kauth_cred_t 	cn_cred;	/* credentials */
	const char 	*cn_nameptr;	/* pointer to looked up name */
	size_t		cn_namelen;	/* length of looked up comp */
	/*
	 * Side result from VOP_LOOKUP.
	 */
	size_t		cn_consume;	/* chars to consume in lookup */
};
.Ed
.Pp
This structure contains the information about a single directory
component name, along with certain other information required by vnode
operations.
See
.Xr vnodeops 9
for more information about these vnode operations.
.Pp
The members:
.Bl -tag -offset indent -width cn_consumexx -compact
.It cn_nameiop
The type of operation in progress; indicates the basic operating mode
of namei.
May be one of
.Dv LOOKUP ,
.Dv CREATE ,
.Dv DELETE ,
or
.Dv RENAME .
These modes are described below.
.It cn_flags
Additional flags affecting the operation of namei.
These are described below as well.
.It cn_cred
The credentials to use for the lookup or other operation the
.Em componentname
is passed to.
This may match the credentials of the current process or it may not,
depending on where the original operation request came from and how it
has been routed.
.It cn_nameptr
The name of this directory component, followed by the rest of the path
being looked up.
.It cn_namelen
The length of the name of this directory component.
The name is not in general null terminated, although the complete
string (the full remaining path) always is.
.It cn_consume
This field starts at zero; it may be set to a larger value by
implementations of
.Xr VOP_LOOKUP 9
to indicate how many more characters beyond
.Em cn_namelen
are being consumed.
New uses of this feature are discouraged and should be discussed.
.El
.Ss Operating modes
Each lookup happens in one of the following modes, specified by
callers of
.Nm
with
.Fn NDINIT
and specified internally by
.Nm
to
.Xr VOP_LOOKUP 9 :
.Bl -bullet -compact
.It
Callers of
.Nm
specify the mode for the last component of a lookup.
.It
Internally,
.Nm
recursively calls
.Xr VOP_LOOKUP 9
in
.Dv LOOKUP
mode for each directory component, and then finally calls
.Xr VOP_LOOKUP 9
in the caller-specified mode for the last component.
.El
Each mode can fail in different ways \(em for example,
.Dv LOOKUP
mode fails with
.Er ENOENT
if no entry exists, but
.Dv CREATE
mode succeeds with a
.Dv NULL
vnode.
.Bl -tag -width LOOKUP
.It Dv LOOKUP
Yield the vnode for an existing entry.
Callers specify
.Dv LOOKUP
for operations on existing vnodes:
.Xr stat 2 ,
.Xr open 2
without
.Dv O_CREATE ,
etc.
.Pp
File systems:
.Bl -dash -compact
.It
MUST refuse if user lacks lookup permission for directory.
.It
SHOULD use
.Xr namecache 9
to cache lookup results.
.El
.Pp
.Bl -tag -compact -width ENAMETOOLONG
.It Bq Er ENOENT
No entry exists.
.El
.It Dv CREATE
Yield the vnode for an existing entry; or, if there is none, yield
.Dv NULL
and hint that it will soon be created.
Callers specify
.Dv CREATE
for operations that may create directory entries:
.Xr mkdir 2 ,
.Xr open 2
with
.Dv O_CREATE ,
etc.
.Pp
File systems:
.Bl -dash -compact
.It
MUST refuse if user lacks lookup permission for directory.
.It
MUST refuse if no entry exists and user lacks write permission for
directory.
.It
MUST refuse if no entry exists and file system is read-only.
.It
SHOULD NOT use
.Xr namecache 9
to cache negative lookup results.
.It
SHOULD save lookup hints internally in the directory for a subsequent
operation to create a directory entry.
.El
.Pp
.Bl -tag -compact -width ENAMETOOLONG
.It Bq Er EPERM
The user lacks lookup permission for the directory.
.It Bq Er EPERM
No entry exists and the user lacks write permission for the directory.
.It Bq Er EROFS
No entry exists and the file system is read-only.
.El
.It Dv DELETE
Yield the vnode of an existing entry, and hint that it will soon be
deleted.
Callers specify
.Dv DELETE
for operations that delete directory entries:
.Xr unlink 2 ,
.Xr rmdir 2 ,
etc.
.Pp
File systems:
.Bl -dash -compact
.It
MUST refuse if user lacks lookup permission for directory.
.It
MUST refuse if entry exists and user lacks write permission for
directory.
.It
MUST refuse if entry exists and file system is read-only.
.It
SHOULD NOT use
.Xr namecache 9
to cache lookup results.
.It
SHOULD save lookup hints internally in the directory for a subsequent
operation to delete a directory entry.
.El
.Pp
.Bl -tag -compact -width ENAMETOOLONG
.It Bq Er ENOENT
No entry exists.
.It Bq Er EPERM
The user lacks lookup permission for the directory.
.It Bq Er EPERM
An entry exists and the user lacks write permission for the directory.
.It Bq Er EROFS
An entry exists and the file system is read-only.
.El
.It Dv RENAME
Yield the vnode of an existing entry, and hint that it will soon be
overwritten; or, if there is none, yield
.Dv NULL ,
and hint that it will soon be created.
.Pp
Callers specify
.Dv RENAME
for an entry that is about to be created or overwritten, namely for the
target of
.Xr rename 2 .
.Pp
File systems:
.Bl -dash -compact
.It
MUST refuse if user lacks lookup permission for directory.
.It
MUST refuse if user lacks write permission for directory.
.It
MUST refuse if file system is read-only.
.It
SHOULD NOT use
.Xr namecache 9
to cache lookup results.
.It
SHOULD save lookup hints internally in the directory for a subsequent
operation to create or overwrite a directory entry.
.El
.Pp
.Bl -tag -compact -width ENAMETOOLONG
.It Bq Er EPERM
The user lacks lookup permission for the directory.
.It Bq Er EPERM
The user lacks write permission for the directory.
.It Bq Er EROFS
The file system is read-only.
.El
.El
.Pp
If a caller decides not to perform an operation it hinted at by a
destructive operating mode
.Pq Dv CREATE , Dv DELETE , No or Dv RENAME ,
it SHOULD call
.Xr VOP_ABORTOP 9
to release the hints.
If a file system fails to perform such an operation, it SHOULD call
.Xr VOP_ABORTOP 9
to release the hints.
However, the current code is inconsistent about this, and every
implementation of
.Xr VOP_ABORTOP 9
does nothing.
.Ss Flags
The following flags may be specified by
.Em callers
of
.Nm ,
and MUST NOT be used by file systems:
.Bl -tag -width NOCROSSMOUNT
.It Dv FOLLOW
Follow symbolic links in the last path component.
Used by operations that do not address symbolic links directly, such as
.Xr stat 2 .
(Does not affect symbolic links found in the middle of a path.)
.It Dv NOFOLLOW
Do not follow symbolic links in the last path component.
Used by operations that address symbolic links directly, such as
.Xr lstat 2 .
.Pp
Note: The value of
.Dv NOFOLLOW
is 0.
We define the constant to let callers say either
.Dv FOLLOW
or
.Dv NOFOLLOW
explicitly.
.It Dv LOCKLEAF
On successful lookup, lock the vnode, if any, in
.Fa ndp Ns Li ->ni_vp .
Without this flag, it would be unlocked.
.It Dv LOCKPARENT
On successful lookup, lock and return the directory vnode in
.Fa ndp Ns Li ->ni_dvp .
Without this flag, it is not returned at all.
.It Dv TRYEMULROOT
If set, the path is looked up in the emulation root of the current
process first.
If that fails, the system root is used.
.It Dv EMULROOTSET
Indicates that the caller has set
.Fa ndp Ns Li ->ni_erootdir
prior to calling
.Nm .
This is only useful or permitted when the emulation in the current
process is partway through being set up.
.It Dv NOCHROOT
Bypass normal
.Xr chroot 8
handling for absolute paths.
.It Dv NOCROSSMOUNT
Do not cross mount points.
.It Dv RDONLY
Enforce read-only behavior.
.It Dv CREATEDIR
Accept slashes after a component name that does not exist.
This only makes sense in
.Dv CREATE
mode and when creating a directory.
.It Dv NOCACHE
Do not cache the lookup result for the last component name.
This is used only with the
.Dv RENAME
mode for the target; the cache entry would be invalidated immediately.
.El
.Pp
The following flag may be set by a caller of
.Nm
and tested by a file system in
.Xr VOP_LOOKUP 9
or other subsequent directory operations:
.Bl -tag -width NOCROSSMOUNT
.It Dv DOWHITEOUT
Allow whiteouts to be seen as objects instead of functioning as
.Dq nothing there .
.El
.Pp
The following flags are set by namei for calling
.Xr VOP_LOOKUP 9 :
.Bl -tag -width NOCROSSMOUNT
.It Dv ISDOTDOT
The current pathname component is
.Dq Li .. .
May be tested by subsequent directory operations too.
.It Dv ISLASTCN
The current pathname component is the last component found in the
pathname.
Guaranteed to remain set in subsequent directory operations.
.It Dv REQUIREDIR
The current object to be looked up must be a directory.
May not be used by subsequent directory operations.
.It Dv MAKEENTRY
The lookup result for the current pathname component should be added to
the
.Xr namecache 9 .
May be used to make additional caching decisions, e.g. to store an
mtime for determining whether our cache for a remote vnode is stale.
May not be used by subsequent directory operatoins.
.El
.Pp
A file system may set the following flag on return from
.Xr VOP_LOOKUP 9
for use by
.Nm ,
.Xr namecache 9 ,
and subsequent directory operations:
.Bl -tag -width NOCROSSMOUNT
.It Dv ISWHITEOUT
The object at the current pathname component is a whiteout.
.El
.Pp
The following additional historic flags have been removed from
.Nx
and should be handled as follows if porting code from elsewhere:
.Bl -tag -width NOCROSSMOUNT
.It Dv INRENAME
Part of a misbegotten and incorrect locking scheme.
Any file-system-level code using this is presumptively incorrect.
File systems should use the
.Xr genfs_rename 9
interface to handle locking in
.Fn VOP_RENAME .
.It Dv INRELOOKUP
Used at one point for signaling to
.Xr puffs 3
to work around a protocol deficiency that was later rectified.
.It Dv ISSYMLINK
Useless internal state.
.It Dv SAVESTART
Unclean setting affect vnode reference counting.
Now effectively never in effect.
Any code referring to this is suspect.
.It Dv SAVENAME
Unclean setting relating to responsibility for freeing pathname buffers
in the days before the
.Em pathbuf
structure.
Now effectively always in effect; the caller of
.Nm
owns the
.Em pathbuf
structure and is always responsible for destroying it.
.It Dv HASBUF
Related to SAVENAME.
Any uses can be replaced with
.Dq true .
.El
.Sh FUNCTIONS
.Bl -tag -width abcd
.It Fn NDINIT "ndp" "op" "flags" "pathbuf"
Initialise a nameidata structure pointed to by
.Fa ndp
for use by the
.Nm
interface.
The operating mode and flags (as documented above) are specified by
.Fa op
and
.Fa flags
respectively.
The pathname is passed as a pathbuf structure, which should be
initialized using one of the
.Xr pathbuf 9
operations.
Destroying the pathbuf is the responsibility of the caller; this must
not be done until the caller is finished with all of the
.Nm
results and all of the nameidata contents except for the result vnode.
.Pp
This routine stores the credentials of the calling thread
.Va ( curlwp )
in
.Fa ndp .
.Fn NDINIT
sets the credentials using
.Xr kauth_cred_get 9 .
In the rare case that another set of credentials is required for the
namei operation,
.Em ndp-\*[Gt]ni_cnd.cn_cred
must be set manually after
.Fn NDINIT .
.It Fn NDAT "ndp" "dvp"
This macro is used after
.Fn NDINIT
to set the starting directory.
This supersedes the current process's current working directory as the
initial point of departure for looking up relative paths.
This mechanism is used by
.Xr openat 2
and related calls.
.It Fn namei "ndp"
Convert a pathname into a pointer to a vnode.
The nameidata structure pointed to by
.Fa ndp
should be initialized with the
.Fn NDINIT
macro, and perhaps also the
.Fn NDAT
macro.
Direct initialization of members of struct nameidata is
.Em not
supported and may (will) break silently in the future.
.Pp
The vnode for the pathname is returned in
.Em ndp-\*[Gt]ni_vp .
The parent directory is returned locked in
.Em ndp-\*[Gt]ni_dvp
iff
.Dv LOCKPARENT
is specified.
.Pp
Any or all of the flags documented above as set by the caller can be
enabled by passing them (OR'd together) as the
.Fa flags
argument of
.Fn NDINIT .
As discussed above every such call should explicitly contain either
.Dv FOLLOW
or
.Dv NOFOLLOW
to control the behavior regarding final symbolic links.
.It Fn namei_simple_kernel "path" "sflags" "ret"
Look up the path
.Fa path
and translate it to a vnode, returned in
.Fa ret .
The
.Fa path
argument must be a kernel
.Pq Dv UIO_SYSSPACE
pointer.
The
.Fa sflags
argument chooses the precise behavior.
It may be set to one of the following symbols:
.Bl -tag -offset indent -width NSM_NOFOLLOW_TRYEMULROOT -compact
.It Dv NSM_NOFOLLOW_NOEMULROOT
.It Dv NSM_NOFOLLOW_TRYEMULROOT
.It Dv NSM_FOLLOW_NOEMULROOT
.It Dv NSM_FOLLOW_TRYEMULROOT
.El
These select (or not) the
.Dv FOLLOW/NOFOLLOW
and
.Dv TRYEMULROOT
flags.
Other flags are not available through this interface, which is
nonetheless sufficient for more than half the
.Fn namei
usage in the kernel.
Note that the encoding of
.Fa sflags
has deliberately been arranged to be type-incompatible with anything
else.
This prevents various possible accidents while the
.Fn namei
interface is being rototilled.
.It Fn namei_simple_user "path" "sflags" "ret"
This function is the same as
.Fn namei_simple_kernel
except that the
.Fa path
argument shall be a user pointer
.Pq Dv UIO_USERSPACE
rather than a kernel pointer.
.It Fn 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
.Fa dvp
and the pathname component by
.Fa cnp .
The vnode of the pathname is returned in the address specified by
.Fa vpp .
Note that one may only use
.Fn relookup
to repeat a lookup of a final path component previously done by
.Nm ,
and one must use the same
.Em componentname
structure that call produced.
Otherwise the behavior is undefined and likely adverse.
.It Fn lookup_for_nfsd "ndp" "startdir" "neverfollow"
This is a private entry point into
.Nm
used by the NFS server code.
It looks up a path starting from
.Fa startdir .
If
.Fa neverfollow
is set,
.Em any
symbolic link (not just at the end of the path) will cause an error.
Otherwise, it follows symlinks normally.
It should not be used by new code.
.It Fn lookup_for_nfsd_index "ndp"
This is a (second) private entry point into
.Nm
used by the NFS server code.
It looks up a single path component.
It should not be used by new code.
.El
.Sh INTERNALS
The
.Em nameidata
structure has the following layout:
.Bd -literal
struct nameidata {
	/*
	 * Arguments to namei.
	 */
	struct vnode *ni_atdir;		/* startup dir, cwd if null */
	struct pathbuf *ni_pathbuf;	/* pathname container */
	char *ni_pnbuf;			/* extra pathname buffer ref (XXX) */
	/*
	 * Internal starting state. (But see notes.)
	 */
	struct	vnode *ni_rootdir;	/* logical root directory */
	struct	vnode *ni_erootdir;	/* emulation root directory */
	/*
	 * Results from namei.
	 */
	struct	vnode *ni_vp;		/* vnode of result */
	struct	vnode *ni_dvp;		/* vnode of intermediate directory */
	/*
	 * Internal current state.
	 */
	size_t		ni_pathlen;	/* remaining chars in path */
	const char	*ni_next;	/* next location in pathname */
	unsigned int	ni_loopcnt;	/* count of symlinks encountered */
	/*
	 * Lookup parameters: this structure describes the subset of
	 * information from the nameidata structure that is passed
	 * through the VOP interface.
	 */
	struct componentname ni_cnd;
};
.Ed
.Pp
These fields are:
.Bl -tag -offset indent -width ni_erootdirx -compact
.It ni_atdir
The directory to use for the starting point of relative paths.
If null, the current process's current directory is used.
This is initialized to
.Dv NULL
by
.Fn NDINIT
and set by
.Fn NDAT .
.It ni_pathbuf
The abstract path buffer in use, passed as an argument to
.Fn NDINIT .
The name pointers that appear elsewhere, such as in the
.Em componentname
structure, point into this buffer.
It is owned by the caller and must not be destroyed until all
.Nm
operations are complete.
See
.Xr pathbuf 9 .
.It ni_pnbuf
This is the name pointer used during
.Nm .
It points into
.Fa ni_pathbuf .
It is not initialized until entry into
.Nm .
.It ni_rootdir
The root directory to use as the starting point for absolute paths.
This is retrieved from the current process's current root directory
when
.Nm
starts up.
It is not initialized by
.Fn NDINIT .
.It ni_erootdir
The root directory to use as the emulation root, for processes running
in emulation.
This is retrieved from the current process's emulation root directory
when
.Nm
starts up and not initialized by
.Fn NDINIT .
As described elsewhere, it may be set by the caller if the
.Dv EMULROOTSET
flag is used, but this should only be done when the current process's
emulation root directory is not yet initialized.
(And ideally in the future things would be tidied so that this is not
necessary.)
.It ni_vp
.It ni_dvp
Returned vnodes, as described above.
These only contain valid values if
.Nm
returns successfully.
.It ni_pathlen
The length of the full current remaining path string in
.Fa ni_pnbuf .
This is not initialized by
.Fn NDINIT
and is used only internally.
.It ni_next
The remaining part of the path, after the current component found in
the
.Em componentname
structure.
This is not initialized by
.Fn NDINIT
and is used only internally.
.It ni_loopcnt
The number of symbolic links encountered (and traversed) so far.
If this exceeds a limit,
.Nm
fails with
.Er ELOOP .
This is not initialized by
.Fn NDINIT
and is used only internally.
.It ni_cnd
The
.Em componentname
structure holding the current directory component, and also the
mode, flags, and credentials.
The mode, flags, and credentials are initialized by
.Fn NDINIT ;
the rest is not initialized until
.Nm
runs.
.El
.Pp
There is also a
.Em namei_state
structure that is hidden within
.Pa vfs_lookup.c .
This contains the following additional state:
.Bl -tag -offset indent -width attempt_retry -compact
.It docache
A flag indicating whether to cache the last pathname component.
.It rdonly
The read-only state, initialized from the
.Dv RDONLY
flag.
.It slashes
The number of trailing slashes found after the current pathname
component.
.It attempt_retry
Set on some error cases (and not others) to indicate that a failure in
the emulation root should be followed by a retry in the real system
root.
.El
.Pp
The state in
.Em namei_state
is genuinely private to
.Nm .
Note that much of the state in
.Em nameidata
should also be private, but is currently not because it is misused in
some fashion by outside code, usually
.Xr nfs 4 .
.Pp
The control flow within the
.Nm
portions of
.Pa vfs_lookup.c
is as follows.
.Bl -tag -width namei_tryemulrootXX
.It Fn namei
does a complete path lookup by calling
.Fn namei_init ,
.Fn namei_tryemulroot ,
and
.Fn namei_cleanup .
.It Fn namei_init
sets up the basic internal state and makes some (precondition-type)
assertions.
.It Fn namei_cleanup
makes some postcondition-type assertions; it currently does nothing
besides this.
.It Fn namei_tryemulroot
handles
.Dv TRYEMULROOT
by calling
.Fn namei_oneroot
once or twice as needed, and attends to making sure the original
pathname is preserved for the second try.
.It Fn namei_oneroot
does a complete path search from a single root directory.
It begins with
.Fn namei_start ,
then calls
.Fn lookup_once
(and if necessary,
.Fn namei_follow )
repeatedly until done.
It also handles returning the result vnode(s) in the requested state.
.It Fn namei_start
sets up the initial state and locking; it calls
.Fn namei_getstartdir .
.It Fn namei_getstartdir
initializes the root directory state (both
.Fa ni_rootdir
and
.Fa ni_erootdir )
and picks the starting directory, consuming the leading slashes of an
absolute path and handling the magic
.Dq /../
string for bypassing the emulation root.
A different version
.Fn namei_getstartdir_for_nfsd
is used for lookups coming from
.Xr nfsd 8
as those are required to have different semantics.
.It Fn lookup_once
calls
.Fn VOP_LOOKUP
for one path component, also handling any needed crossing of mount
points (either up or down) and coping with locking requirements.
.It Fn lookup_parsepath
is called prior to each
.Fn lookup_once
call to examine the pathname and find where the next component
starts.
.It Fn namei_follow
reads the contents of a symbolic link and updates both the path buffer
and the search directory accordingly.
.El
.Pp
As a final note be advised that the magic return value associated with
.Dv CREATE
mode is different for
.Nm
than it is for
.Fn VOP_LOOKUP .
The latter
.Dq fails
with
.Er EJUSTRETURN .
.Nm
translates this into succeeding and returning a null vnode.
.Sh CODE REFERENCES
The name lookup subsystem is implemented within the file
.Pa sys/kern/vfs_lookup.c .
.Sh SEE ALSO
.Xr intro 9 ,
.Xr namecache 9 ,
.Xr vfs 9 ,
.Xr vnode 9 ,
.Xr vnodeops 9
.Sh BUGS
There should be no such thing as operating modes.
Only
.Dv LOOKUP
is actually needed.
The behavior where removing an object looks it up within
.Nm
and then calls into the file system (which must look it up again
internally or cache state from
.Fn VOP_LOOKUP )
is particularly contorted.
.Pp
Most of the flags are equally bogus.
.Pp
Most of the contents of the
.Em nameidata
structure should be private and hidden within
.Nm ;
currently it cannot be because of abuse elsewhere.
.Pp
The
.Dv EMULROOTSET
flag is messy.
.Pp
There is no good way to support file systems that want to use
a more elaborate pathname schema than the customary slash-delimited
components.