Provide a man page for VOP_SETLABEL(9).

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+.\"-
+.\" Copyright (c) 2021 Robert N. M. Watson
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+.\" $FreeBSD$
+.\"
+.Dd February 27, 2021
+.Dt VOP_SETLABEL 9
+.Os
+.Sh NAME
+.Nm VOP_SETLABEL
+.Nd persistently store an updated MAC label on a vnode
+.Sh SYNOPSIS
+.In sys/param.h
+.In sys/vnode.h
+.In security/mac.h
+.Ft int
+.Fn VOP_SETLABEL "struct vnode *vp" "label *label"
+.Sh DESCRIPTION
+This vnode call is made by
+.Xr mac 9
+file relabeling operation has been authorized, and the filesystem must now be
+updated.
+.Ss Single-Label vs. Multi-Label Filesystems
+Filesystems that do not implement per-file labels -- known as single-label
+filesystems -- can simply leave the
+.Xr vnode 9
+operation undefined.
+These filesystems must not set the
+.Dv MNT_MULTLABEL
+flag in their
+.Vt struct mount .
+.Pp
+Filesystems that do implement per-vnode label storage -- known as multi-label
+filesystems -- will set the
+.Dv MNT_MULTILABEL
+flag in their
+.Vt struct mount .
+The UFS filesystem uses a superblock flag to persisently configure whether a
+specific filesystem implements a label for each
+.Xr vnode 9 ,
+and then keys various behaviors on whether that flag is set.
+.Ss Extended Attributes
+If the filesystem implements extended attributes, then the MAC Framework's
+.Fn vop_stdsetlabel_ea
+function can be used, and maps operations into a series of
+.Xr VOP_OPENEXTATTR 9 ,
+.Xr VOP_WRITEEXTATTR 9 ,
+and
+.Xr VOP_CLOSEEXTATTR 9 .
+.Pp
+Filesystems will also need to call
+.Fn mac_vnode_create_extattr
+when a new filesystem object is created, so that suitable extended attributes
+can be written out, and
+.Fn mac_vnode_associate_extattr
+when a
+.Xr vnode 9
+is associated with a filesystem object for the first time.
+These utility functions use
+.Xr VOP_OPENEXTATTR 9 ,
+.Xr VOP_READEXTATTR 9 ,
+.Xr VOP_WRITEEXTATTR 9 ,
+and
+.Xr VOP_CLOSEEXTATTR 9
+as required.
+.Pp
+.Ss Locking and Crash Safety
+In all cases, it is important that exclusive
+.Xr vnode 9
+locks be held to prevent concurrent access when a MAC label may not yet be
+initialized.
+It is also important that operations are ordered so that a system crash does
+not leave a file improperly labeled.
+For example, the extended attribute for a newly created file must be written
+to disk before the file is linked by its parent directory, so that there is
+no opportunity for a crash to lead to an unlabeled file.
+.Sh LOCKS
+The vnode will be locked on entry and should remain locked on return.
+.Sh RETURN VALUES
+If the MAC label is successfully set, then zero is returned.
+Otherwise, an appropriate error code is returned.
+.Sh ERRORS
+.Bl -tag -width Er
+.It Bq Er EOPNOTSUPP
+The file system does not support
+.Fn VOP_SETLABEL .
+.It Bq Er ENOSPC
+The file system is out of space.
+.It Bq Er EROFS
+The file system is read-only.
+.El
+.Pp
+Depending on the underlying implementation of
+.Fn VOP_SETLABEL ,
+other errors may also be possible.
+.Sh SEE ALSO
+.Xr VOP_CLOSEEXTATTR 9 ,
+.Xr VOP_OPENEXTATTR 9 ,
+.Xr VOP_READEXTATTR 9 ,
+.Xr VOP_WRITEXTATTR 9 ,
+.Xr mac 9 ,
+.Xr mount 9 ,
+.Xr vnode 9 ,
+.Sh AUTHORS
+This manual page was written by
+.An Robert Watson .