when we mount and get zero cost if no rules are used in a mountpoint.
Add code to deref rules on unmount.
Switch from SLIST to TAILQ.
Drop SYSINIT, use SX_SYSINIT and static initializer of TAILQ instead.
Drop goto, a break will do.
Reduce double pointers to single pointers.
Combine reaping and destroying rulesets.
Avoid memory leaks in a some error cases.
Give DEVFS a proper inode called struct cdev_priv. It is important
to keep in mind that this "inode" is shared between all DEVFS
mountpoints, therefore it is protected by the global device mutex.
Link the cdev_priv's into a list, protected by the global device
mutex. Keep track of each cdev_priv's state with a flag bit and
of references from mountpoints with a dedicated usecount.
Reap the benefits of much improved kernel memory allocator and the
generally better defined device driver APIs to get rid of the tables
of pointers + serial numbers, their overflow tables, the atomics
to muck about in them and all the trouble that resulted in.
This makes RAM the only limit on how many devices we can have.
The cdev_priv is actually a super struct containing the normal cdev
as the "public" part, and therefore allocation and freeing has moved
to devfs_devs.c from kern_conf.c.
The overall responsibility is (to be) split such that kern/kern_conf.c
is the stuff that deals with drivers and struct cdev and fs/devfs
handles filesystems and struct cdev_priv and their private liason
exposed only in devfs_int.h.
Move the inode number from cdev to cdev_priv and allocate inode
numbers properly with unr. Local dirents in the mountpoints
(directories, symlinks) allocate inodes from the same pool to
guarantee against overlaps.
Various other fields are going to migrate from cdev to cdev_priv
in the future in order to hide them. A few fields may migrate
from devfs_dirent to cdev_priv as well.
Protect the DEVFS mountpoint with an sx lock instead of lockmgr,
this lock also protects the directory tree of the mountpoint.
Give each mountpoint a unique integer index, allocated with unr.
Use it into an array of devfs_dirent pointers in each cdev_priv.
Initially the array points to a single element also inside cdev_priv,
but as more devfs instances are mounted, the array is extended with
malloc(9) as necessary when the filesystem populates its directory
tree.
Retire the cdev alias lists, the cdev_priv now know about all the
relevant devfs_dirents (and their vnodes) and devfs_revoke() will
pick them up from there. We still spelunk into other mountpoints
and fondle their data without 100% good locking. It may make better
sense to vector the revoke event into the tty code and there do a
destroy_dev/make_dev on the tty's devices, but that's for further
study.
Lots of shuffling of stuff and churn of bits for no good reason[2].
XXX: There is still nothing preventing the dev_clone EVENTHANDLER
from being invoked at the same time in two devfs mountpoints. It
is not obvious what the best course of action is here.
XXX: comment out an if statement that lost its body, until I can
find out what should go there so it doesn't do damage in the meantime.
XXX: Leave in a few extra malloc types and KASSERTS to help track
down any remaining issues.
Much testing provided by: Kris
Much confusion caused by (races in): md(4)
[1] You are not supposed to understand anything past this point.
[2] This line should simplify life for the peanut gallery.
running" panics.
Previously, recursion through the "include" feature was prevented by
marking each ruleset as "running" when applied. This doesn't work for
the case where two DEVFS instances try to apply the same ruleset at
the same time.
Instead introduce the sysctl vfs.devfs.rule_depth (default == 1) which
limits how many levels of "include" we will traverse.
Be aware that traversal of "include" is recursive and kernel stack
size is limited.
MFC: after 3 days
event handler, dev_clone, which accepts a credential argument.
Implementors of the event can ignore it if they're not interested,
and most do. This avoids having multiple event handler types and
fall-back/precedence logic in devfs.
This changes the kernel API for /dev cloning, and may affect third
party packages containg cloning kernel modules.
Requested by: phk
MFC after: 3 days
process that caused the clone event to take place for the device driver
creating the device. This allows cloned device drivers to adapt the
device node based on security aspects of the process, such as the uid,
gid, and MAC label.
- Add a cred reference to struct cdev, so that when a device node is
instantiated as a vnode, the cloning credential can be exposed to
MAC.
- Add make_dev_cred(), a version of make_dev() that additionally
accepts the credential to stick in the struct cdev. Implement it and
make_dev() in terms of a back-end make_dev_credv().
- Add a new event handler, dev_clone_cred, which can be registered to
receive the credential instead of dev_clone, if desired.
- Modify the MAC entry point mac_create_devfs_device() to accept an
optional credential pointer (may be NULL), so that MAC policies can
inspect and act on the label or other elements of the credential
when initializing the skeleton device protections.
- Modify tty_pty.c to register clone_dev_cred and invoke make_dev_cred(),
so that the pty clone credential is exposed to the MAC Framework.
While currently primarily focussed on MAC policies, this change is also
a prerequisite for changes to allow ptys to be instantiated with the UID
of the process looking up the pty. This requires further changes to the
pty driver -- in particular, to immediately recycle pty nodes on last
close so that the credential-related state can be recreated on next
lookup.
Submitted by: Andrew Reisse <andrew.reisse@sparta.com>
Obtained from: TrustedBSD Project
Sponsored by: SPAWAR, SPARTA
MFC after: 1 week
MFC note: Merge to 6.x, but not 5.x for ABI reasons
these filesystems will support shared locks until they are explicitly
modified to do so. Careful review must be done to ensure that this
is safe for each individual filesystem.
Sponsored by: Isilon Systems, Inc.
List devfs_dirents rather than vnodes off their shared struct cdev, this
saves a pointer field in the vnode at the expense of a field in the
devfs_dirent. There are often 100 times more vnodes so this is bargain.
In addition it makes it harder for people to try to do stypid things like
"finding the vnode from cdev".
Since DEVFS handles all VCHR nodes now, we can do the vnode related
cleanup in devfs_reclaim() instead of in dev_rel() and vgonel().
Similarly, we can do the struct cdev related cleanup in dev_rel()
instead of devfs_reclaim().
rename idestroy_dev() to destroy_devl() for consistency.
Add LIST_ENTRY de_alias to struct devfs_dirent.
Remove v_specnext from struct vnode.
Change si_hlist to si_alist in struct cdev.
String new devfs vnodes' devfs_dirent on si_alist when
we create them and take them off in devfs_reclaim().
Fix devfs_revoke() accordingly. Also don't clear fields
devfs_reclaim() will clear when called from vgone();
Let devfs_reclaim() call dev_rel() instead of vgonel().
Move the usecount tracking from dev_rel() to devfs_reclaim(),
and let dev_rel() take a struct cdev argument instead of vnode.
Destroy SI_CHEAPCLONE devices in dev_rel() (instead of
devfs_reclaim()) when they are no longer used. (This
should maybe happen in devfs_close() instead.)
After disscussing things I have decided to take the easy and
consistent 90% solution instead of aiming for the very involved 99%
solution.
If we allow forceful unmounts of DEVFS we need to decide how to handle
the devices which are in use through this filesystem at the time.
We cannot just readopt the open devices in the main /dev instance since
that would open us to security issues.
For the majority of the devices, this is relatively straightforward
as we can just pretend they got revoke(2)'ed.
Some devices get tricky: /dev/console and /dev/tty for instance
does a sort of recursive open of the real console device. Other devices
may be mmap'ed (kill the processes ?).
And then there are disk devices which are mounted.
The correct thing here would be to recursively unmount the filesystems
mounte from devices from our DEVFS instance (forcefully) and if
this succeeds, complete the forcefully unmount of DEVFS. But if
one of the forceful unmounts fail we cannot complete the forceful
unmount of DEVFS, but we are likely to already have severed a lot
of stuff in the process of trying.
Event attempting this would be a lot of code for a very far out
corner-case which most people would never see or get in touch with.
It's just not worth it.
methods:
Read can see O_NONBLOCK and O_DIRECT.
Write can see O_NONBLOCK, O_DIRECT and O_FSYNC.
In addition O_DIRECT is shadowed as IO_DIRECT for now for backwards
compatibility.
fcntl.h.
This is in preparation for making the flags passed to device drivers be
consistently from fcntl.h for all entrypoints.
Today open, close and ioctl uses fcntl.h flags, while read and write
uses vnode.h flags.
split the conversion of the remaining three filesystems out from the root
mounting changes, so in one go:
cd9660:
Convert to nmount.
Add omount compat shims.
Remove dedicated rootfs mounting code.
Use vfs_mountedfrom()
Rely on vfs_mount.c calling VFS_STATFS()
nfs(client):
Convert to nmount (the simple way, mount_nfs(8) is still necessary).
Add omount compat shims.
Drop COMPAT_PRELITE2 mount arg compatibility.
ffs:
Convert to nmount.
Add omount compat shims.
Remove dedicated rootfs mounting code.
Use vfs_mountedfrom()
Rely on vfs_mount.c calling VFS_STATFS()
Remove vfs_omount() method, all filesystems are now converted.
Remove MNTK_WANTRDWR, handling RO/RW conversions is a filesystem
task, and they all do it now.
Change rootmounting to use DEVFS trampoline:
vfs_mount.c:
Mount devfs on /. Devfs needs no 'from' so this is clean.
symlink /dev to /. This makes it possible to lookup /dev/foo.
Mount "real" root filesystem on /.
Surgically move the devfs mountpoint from under the real root
filesystem onto /dev in the real root filesystem.
Remove now unnecessary getdiskbyname().
kern_init.c:
Don't do devfs mounting and rootvnode assignment here, it was
already handled by vfs_mount.c.
Remove now unused bdevvp(), addaliasu() and addalias(). Put the
few necessary lines in devfs where they belong. This eliminates the
second-last source of bogo vnodes, leaving only the lemming-syncer.
Remove rootdev variable, it doesn't give meaning in a global context and
was not trustworth anyway. Correct information is provided by
statfs(/).
doesn't. Most of the implementations have grown weeds for this so they
copy some fields from mnt_stat if the passed argument isn't that.
Fix this the cleaner way: Always call the implementation on mnt_stat
and copy that in toto to the VFS_STATFS argument if different.
initializations but we did have lofty goals and big ideals.
Adjust to more contemporary circumstances and gain type checking.
Replace the entire vop_t frobbing thing with properly typed
structures. The only casualty is that we can not add a new
VOP_ method with a loadable module. History has not given
us reason to belive this would ever be feasible in the the
first place.
Eliminate in toto VOCALL(), vop_t, VNODEOP_SET() etc.
Give coda correct prototypes and function definitions for
all vop_()s.
Generate a bit more data from the vnode_if.src file: a
struct vop_vector and protype typedefs for all vop methods.
Add a new vop_bypass() and make vop_default be a pointer
to another struct vop_vector.
Remove a lot of vfs_init since vop_vector is ready to use
from the compiler.
Cast various vop_mumble() to void * with uppercase name,
for instance VOP_PANIC, VOP_NULL etc.
Implement VCALL() by making vdesc_offset the offsetof() the
relevant function pointer in vop_vector. This is disgusting
but since the code is generated by a script comparatively
safe. The alternative for nullfs etc. would be much worse.
Fix up all vnode method vectors to remove casts so they
become typesafe. (The bulk of this is generated by scripts)
to a cdev and a devsw, doing all the relevant checks along the way.
Add the check to see if fp->f_vnode->v_rdev differs from our cached
fp->f_data copy of our cdev. If it does the device was revoked and
we return ENXIO.
The tunable vfs.devfs.fops controls this feature and defaults to off.
When enabled (vfs.devfs.fops=1 in loader), device vnodes opened
through a filedescriptor gets a special fops vector which instead
of the detour through the vnode layer goes directly to DEVFS.
Amongst other things this allows us to run Giant free read/write to
device drivers which have been weaned off D_NEEDGIANT.
Currently this means /dev/null, /dev/zero, disks, (and maybe the
random stuff ?)
On a 700MHz K7 machine this doubles the speed of
dd if=/dev/zero of=/dev/null bs=1 count=1000000
This roughly translates to shaving 2usec of each read/write syscall.
The poll/kqfilter paths need more work before they are giant free,
this work is ongoing in p4::phk_bufwork
Please test this and report any problems, LORs etc.
buf->b-dev.
Put a bio between the buf passed to dev_strategy() and the device driver
strategy routine in order to not clobber fields in the buf.
Assert copyright on vfs_bio.c and update copyright message to canonical
text. There is no legal difference between John Dysons two-clause
abbreviated BSD license and the canonical text.
We keep si_bsize_phys around for now as that is the simplest way to pull
the number out of disk device drivers in devfs_open(). The correct solution
would be to do an ioctl(DIOCGSECTORSIZE), but the point is probably mooth
when filesystems sit on GEOM, so don't bother for now.
jest, of most excellent fancy: he hath taught me lessons a thousand
times; and now, how abhorred in my imagination it is! my gorge rises
at it. Here were those hacks that I have curs'd I know not how
oft. Where be your kludges now? your workarounds? your layering
violations, that were wont to set the table on a roar?
Move the skeleton of specfs into devfs where it now belongs and
bury the rest.
frobbing the cdevsw.
In both cases we examine only the cdevsw and it is a good question if we
weren't better off copying those properties into the cdev in the first
place. This question will be revisited.
and refuse initializing filesystems with a wrong version. This will
aid maintenance activites on the 5-stable branch.
s/vfs_mount/vfs_omount/
s/vfs_nmount/vfs_mount/
Name our filesystems mount function consistently.
Eliminate the namiedata argument to both vfs_mount and vfs_omount.
It was originally there to save stack space. A few places abused
it to get hold of some credentials to pass around. Effectively
it is unused.
Reorganize the root filesystem selection code.
somewhat clearer, but more importantly allows for a consistent naming
scheme for suser_cred flags.
The old name is still defined, but will be removed in a few days (unless I
hear any complaints...)
Discussed with: rwatson, scottl
Requested by: jhb
the caller passes in a td that is curthread, and consistently pass 'td'
into vget(). Remove some bogus logic that passed in td or curthread
conditional on td being non-NULL, which seems redundant in the face of
the earlier assignment of td to curthread if td is NULL.
In devfs_symlink(), cache the passed thread in 'td' so we don't have
to keep retrieving it from the 'ap' structure, and assert that td is
curthread (since we dereference it to get thread-local td_ucred). Use
'td' in preference to curthread for later lockmgr calls, since they are
equal.
This is to allow filesystems to decide based on the passed thread
which vnode to return.
Several filesystems used curthread, they now use the passed thread.
The big lines are:
NODEV -> NULL
NOUDEV -> NODEV
udev_t -> dev_t
udev2dev() -> findcdev()
Various minor adjustments including handling of userland access to kernel
space struct cdev etc.
in various kernel objects to represent security data, we embed a
(struct label *) pointer, which now references labels allocated using
a UMA zone (mac_label.c). This allows the size and shape of struct
label to be varied without changing the size and shape of these kernel
objects, which become part of the frozen ABI with 5-STABLE. This opens
the door for boot-time selection of the number of label slots, and hence
changes to the bound on the number of simultaneous labeled policies
at boot-time instead of compile-time. This also makes it easier to
embed label references in new objects as required for locking/caching
with fine-grained network stack locking, such as inpcb structures.
This change also moves us further in the direction of hiding the
structure of kernel objects from MAC policy modules, not to mention
dramatically reducing the number of '&' symbols appearing in both the
MAC Framework and MAC policy modules, and improving readability.
While this results in minimal performance change with MAC enabled, it
will observably shrink the size of a number of critical kernel data
structures for the !MAC case, and should have a small (but measurable)
performance benefit (i.e., struct vnode, struct socket) do to memory
conservation and reduced cost of zeroing memory.
NOTE: Users of MAC must recompile their kernel and all MAC modules as a
result of this change. Because this is an API change, third party
MAC modules will also need to be updated to make less use of the '&'
symbol.
Suggestions from: bmilekic
Obtained from: TrustedBSD Project
Sponsored by: DARPA, Network Associates Laboratories
pick up the DEVFS inode number from the dev_t and find our directory
entry from that, we don't need to scan the directory to find it.
This also solves an issue with on-demand devices in subdirectories.
Submitted by: cognet
closely what function is really doing. Update all existing consumers
to use the new name.
Introduce a new vfs_stdsync function, which iterates over mount
point's vnodes and call FSYNC on each one of them in turn.
Make nwfs and smbfs use this new function instead of rolling their
own identical sync implementations.
Reviewed by: jeff
Remove extraneous uses of vop_null, instead defering to the default op.
Rename vnode type "vfs" to the more descriptive "syncer".
Fix formatting for various filesystems that use vop_print.
was used to control code which were conditional on DEVFS' precense
since this avoided the need for large-scale source pollution with
#include "opt_geom.h"
Now that we approach making DEVFS standard, replace these tests
with an #ifdef to facilitate mechanical removal once DEVFS becomes
non-optional.
No functional change by this commit.
access its controlling terminal.
In essense, history dictates that any process is allowed to open
/dev/tty for RW, irrespective of credential, because by definition
it is it's own controlling terminal.
Before DEVFS we relied on a hacky half-device thing (kern/tty_tty.c)
which did the magic deep down at device level, which at best was
disgusting from an architectural point of view.
My first shot at this was to use the cloning mechanism to simply
give people the right tty when they ask for /dev/tty, that's why
you get this, slightly counter intuitive result:
syv# ls -l /dev/tty `tty`
crw--w---- 1 u1 tty 5, 0 Jan 13 22:14 /dev/tty
crw--w---- 1 u1 tty 5, 0 Jan 13 22:14 /dev/ttyp0
Trouble is, when user u1 su(1)'s to user u2, he cannot open
/dev/ttyp0 anymore because he doesn't have permission to do so.
The above fix allows him to do that.
The interesting side effect is that one was previously only able
to access the controlling tty by indirection:
date > /dev/tty
but not by name:
date > `tty`
This is now possible, and that feels a lot more like DTRT.
PR: 46635
MFC candidate: could be.
kern/vfs_defaults.c it is wrong for the individual filesystems to use
the std* functions as that prevents override of the default.
Found by: src/tools/tools/vop_table
unused. Replace it with a dm_mount back-pointer to the struct mount
that the devfs_mount is associated with. Export that pointer to MAC
Framework entry points, where all current policies don't use the
pointer. This permits the SEBSD port of SELinux's FLASK/TE to compile
out-of-the-box on 5.0-CURRENT with full file system labeling support.
Approved by: re (murray)
Obtained from: TrustedBSD Project
Sponsored by: DARPA, Network Associates Laboratories
"refreshing" the label on the vnode before use, just get the label
right from inception. For single-label file systems, set the label
in the generic VFS getnewvnode() code; for multi-label file systems,
leave the labeling up to the file system. With UFS1/2, this means
reading the extended attribute during vfs_vget() as the inode is
pulled off disk, rather than hitting the extended attributes
frequently during operations later, improving performance. This
also corrects sematics for shared vnode locks, which were not
previously present in the system. This chances the cache
coherrency properties WRT out-of-band access to label data, but in
an acceptable form. With UFS1, there is a small race condition
during automatic extended attribute start -- this is not present
with UFS2, and occurs because EAs aren't available at vnode
inception. We'll introduce a work around for this shortly.
Approved by: re
Obtained from: TrustedBSD Project
Sponsored by: DARPA, Network Associates Laboratories
devfs VOP symlink creation by introducing a new entry point to determine
the label of the devfs_dirent prior to allocation of a vnode for the
symlink.
Obtained from: TrustedBSD Project
Sponsored by: DARPA, Network Associates Laboratories
v_tag is now const char * and should only be used for debugging.
Additionally:
1. All users of VT_NTS now check vfsconf->vf_type VFCF_NETWORK
2. The user of VT_PROCFS now checks for the new flag VV_PROCDEP, which
is propagated by pseudofs to all child vnodes if the fs sets PFS_PROCDEP.
Suggested by: phk
Reviewed by: bde, rwatson (earlier version)
these in the main filesystems. This does not change the resulting code
but makes the source a little bit more grepable.
Sponsored by: DARPA and NAI Labs.
- v_vflag is protected by the vnode lock and is used when synchronization
with VOP calls is needed.
- v_iflag is protected by interlock and is used for dealing with vnode
management issues. These flags include X/O LOCK, FREE, DOOMED, etc.
- All accesses to v_iflag and v_vflag have either been locked or marked with
mp_fixme's.
- Many ASSERT_VOP_LOCKED calls have been added where the locking was not
clear.
- Many functions in vfs_subr.c were restructured to provide for stronger
locking.
Idea stolen from: BSD/OS
kernel access control.
Teach devfs how to respond to pathconf() _POSIX_MAC_PRESENT queries,
allowing it to indicate to user processes that individual vnode labels
are available.
Obtained from: TrustedBSD Project
Sponsored by: DARPA, NAI Labs
kernel access control.
Instrument devfs to support per-dirent MAC labels. In particular,
invoke MAC framework when devfs directory entries are instantiated
due to make_dev() and related calls, and invoke the MAC framework
when vnodes are instantiated from these directory entries. Implement
vop_setlabel() for devfs, which pushes the label update into the
devfs directory entry for semi-persistant store. This permits the MAC
framework to assign labels to devices and directories as they are
instantiated, and export access control information via devfs vnodes.
Obtained from: TrustedBSD Project
Sponsored by: DARPA, NAI Labs
ruleset. If we do, that means there's a ruleset loop (10 includes 20
include 30 includes 10), which will quickly cause a double fault due
to stack overflow (since "include" is implemented by recursion).
(Previously, we only checked that X didn't include X.)
administrator to define certain properties of new devfs nodes before
they become visible to the userland. Both static (e.g., /dev/speaker)
and dynamic (e.g., /dev/bpf*, some removable devices) nodes are
supported. Each DEVFS mount may have a different ruleset assigned to
it, permitting different policies to be implemented for things like
jails.
Approved by: phk
make sure it's a correct operation for devfs, do it only in the
ISLASTCN case. If we don't, we are assuming that the final file will
be in devfs, which is not true if another partition is mounted on top
of devfs or with special filenames (like /dev/net/../../foo).
Reviewed by: phk
Setting of timestamps on devices had no effect visible to userland
because timestamps for devices were set in places that are never used.
This broke:
- update of file change time after a change of an attribute
- setting of file access and modification times.
The VA_UTIMES_NULL case did not work. Revs 1.31-1.32 were supposed to
fix this by copying correct bits from ufs, but had little or no effect
because the old checks were not removed.
general cleanup of the API. The entire API now consists of two functions
similar to the pre-KSE API. The suser() function takes a thread pointer
as its only argument. The td_ucred member of this thread must be valid
so the only valid thread pointers are curthread and a few kernel threads
such as thread0. The suser_cred() function takes a pointer to a struct
ucred as its first argument and an integer flag as its second argument.
The flag is currently only used for the PRISON_ROOT flag.
Discussed on: smp@
pathconf() variables for directories, and set st_size and st_blocks
(of struct stat) for directories as appropriate. Note that st_size is
always set to DEV_BSIZE, since the size of the directories is not
currently kept.
Reviewed by: phk, bde
Note ALL MODULES MUST BE RECOMPILED
make the kernel aware that there are smaller units of scheduling than the
process. (but only allow one thread per process at this time).
This is functionally equivalent to teh previousl -current except
that there is a thread associated with each process.
Sorry john! (your next MFC will be a doosie!)
Reviewed by: peter@freebsd.org, dillon@freebsd.org
X-MFC after: ha ha ha ha
linux_getdents uses VOP_READDIR( ..., &ncookies, &cookies ) instead of
VOP_READDIR( ..., NULL, NULL ) because it seems to need the offsets for
linux_dirent and sizeof(dirent) != sizeof(linux_dirent)...
PR: 29467
Submitted by: Michael Reifenberger <root@nihil.plaut.de>
Reviewed by: phk
Only tun0 -> tun32767 may now be opened as struct ifnet's if_unit
is a short.
It's now possible to open /dev/tun and get a handle back for an available
tun device (use devname to find out what you got).
The implementation uses rman by popular demand (and against my judgement)
to track opened devices and uses the new dev_depends() to ensure that
all make_dev()d devices go away before the module is unloaded.
Reviewed by: phk
dev_t. The dev_depends(dev_t, dev_t) function is for tying them
to each other.
When destroy_dev() is called on a dev_t, all dev_t's depending
on it will also be destroyed (depth first order).
Rewrite the make_dev_alias() to use this dependency facility.
kern/subr_disk.c:
Make the disk mini-layer use dependencies to make sure all
relevant dev_t's are removed when the disk disappears.
Make the disk mini-layer precreate some magic sub devices
which the disk/slice/label code expects to be there.
kern/subr_disklabel.c:
Remove some now unneeded variables.
kern/subr_diskmbr.c:
Remove some ancient, commented out code.
kern/subr_diskslice.c:
Minor cleanup. Use name from dev_t instead of dsname()
This fixes a number of warnings relating to removed cloned devices.
It also makes it possible to recreate deleted devices with
mknod(2). The major/minor arguments are ignored.
the number of references on the filesystem root vnode to be both
expected and released. Many filesystems hold an extra reference on
the filesystem root vnode, which must be accounted for when
determining if the filesystem is busy and then released if it isn't
busy. The old `skipvp' approach required individual filesystem
xxx_unmount functions to re-implement much of vflush()'s logic to
deal with the root vnode.
All 9 filesystems that hold an extra reference on the root vnode
got the logic wrong in the case of forced unmounts, so `umount -f'
would always fail if there were any extra root vnode references.
Fix this issue centrally in vflush(), now that we can.
This commit also fixes a vnode reference leak in devfs, which could
result in idle devfs filesystems that refuse to unmount.
Reviewed by: phk, bp
