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linux/Documentation/filesystems/9p.rst
Linus Torvalds bdafff62ae 9p update for 6.15-rc1 
- fix handling of bogus (negative/too long) replies
 - fix crash on mkdir with ACLs
 (... looks like nobody is using ACLs with semi-recent kernels...)
 - ipv6 support for trans=tcp
 - minor concurrency fix to make syzbot happy
 - minor cleanup
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Merge tag '9p-for-6.15-rc1' of https://github.com/martinetd/linux

Pull 9p updates from Dominique Martinet:

 - fix handling of bogus (negative/too long) replies

 - fix crash on mkdir with ACLs (... looks like nobody is using ACLs
   with semi-recent kernels...)

 - ipv6 support for trans=tcp

 - minor concurrency fix to make syzbot happy

 - minor cleanup

* tag '9p-for-6.15-rc1' of https://github.com/martinetd/linux:
  docs: fs/9p: Add missing "not" in cache documentation
  9p: Use hashtable.h for hash_errmap
  Documentation/fs/9p: fix broken link
  9p/trans_fd: mark concurrent read and writes to p9_conn->err
  9p/net: return error on bogus (longer than requested) replies
  9p/net: fix improper handling of bogus negative read/write replies
  fs/9p: fix NULL pointer dereference on mkdir
  net/9p/fd: support ipv6 for trans=tcp
2025-04-03 15:35:46 -07:00

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.. SPDX-License-Identifier: GPL-2.0
=======================================
v9fs: Plan 9 Resource Sharing for Linux
=======================================
About
=====
v9fs is a Unix implementation of the Plan 9 9p remote filesystem protocol.
This software was originally developed by Ron Minnich <rminnich@sandia.gov>
and Maya Gokhale. Additional development by Greg Watson
<gwatson@lanl.gov> and most recently Eric Van Hensbergen
<ericvh@gmail.com>, Latchesar Ionkov <lucho@ionkov.net> and Russ Cox
<rsc@swtch.com>.
The best detailed explanation of the Linux implementation and applications of
the 9p client is available in the form of a USENIX paper:
https://www.usenix.org/events/usenix05/tech/freenix/hensbergen.html
Other applications are described in the following papers:
* XCPU & Clustering
http://xcpu.org/papers/xcpu-talk.pdf
* KVMFS: control file system for KVM
http://xcpu.org/papers/kvmfs.pdf
* CellFS: A New Programming Model for the Cell BE
http://xcpu.org/papers/cellfs-talk.pdf
* PROSE I/O: Using 9p to enable Application Partitions
http://plan9.escet.urjc.es/iwp9/cready/PROSE_iwp9_2006.pdf
* VirtFS: A Virtualization Aware File System pass-through
https://kernel.org/doc/ols/2010/ols2010-pages-109-120.pdf
Usage
=====
For remote file server::
mount -t 9p 10.10.1.2 /mnt/9
For Plan 9 From User Space applications (https://9fans.github.io/plan9port/)::
mount -t 9p `namespace`/acme /mnt/9 -o trans=unix,uname=$USER
For server running on QEMU host with virtio transport::
mount -t 9p -o trans=virtio <mount_tag> /mnt/9
where mount_tag is the tag generated by the server to each of the exported
mount points. Each 9P export is seen by the client as a virtio device with an
associated "mount_tag" property. Available mount tags can be
seen by reading /sys/bus/virtio/drivers/9pnet_virtio/virtio<n>/mount_tag files.
USBG Usage
==========
To mount a 9p FS on a USB Host accessible via the gadget at runtime::
mount -t 9p -o trans=usbg,aname=/path/to/fs <device> /mnt/9
To mount a 9p FS on a USB Host accessible via the gadget as root filesystem::
root=<device> rootfstype=9p rootflags=trans=usbg,cache=loose,uname=root,access=0,dfltuid=0,dfltgid=0,aname=/path/to/rootfs
where <device> is the tag associated by the usb gadget transport.
It is defined by the configfs instance name.
USBG Example
============
The USB host exports a filesystem, while the gadget on the USB device
side makes it mountable.
Diod (9pfs server) and the forwarder are on the development host, where
the root filesystem is actually stored. The gadget is initialized during
boot (or later) on the embedded board. Then the forwarder will find it
on the USB bus and start forwarding requests.
In this case the 9p requests come from the device and are handled by the
host. The reason is that USB device ports are normally not available on
PCs, so a connection in the other direction would not work.
When using the usbg transport, for now there is no native usb host
service capable to handle the requests from the gadget driver. For
this we have to use the extra python tool p9_fwd.py from tools/usb.
Just start the 9pfs capable network server like diod/nfs-ganesha e.g.::
$ diod -f -n -d 0 -S -l 0.0.0.0:9999 -e $PWD
Optionally scan your bus if there are more then one usbg gadgets to find their path::
$ python $kernel_dir/tools/usb/p9_fwd.py list
Bus | Addr | Manufacturer | Product | ID | Path
--- | ---- | ---------------- | ---------------- | --------- | ----
2 | 67 | unknown | unknown | 1d6b:0109 | 2-1.1.2
2 | 68 | unknown | unknown | 1d6b:0109 | 2-1.1.3
Then start the python transport::
$ python $kernel_dir/tools/usb/p9_fwd.py --path 2-1.1.2 connect -p 9999
After that the gadget driver can be used as described above.
One use-case is to use it as an alternative to NFS root booting during
the development of embedded Linux devices.
Options
=======
============= ===============================================================
trans=name select an alternative transport. Valid options are
currently:
======== ============================================
unix specifying a named pipe mount point
tcp specifying a normal TCP/IP connection
fd used passed file descriptors for connection
(see rfdno and wfdno)
virtio connect to the next virtio channel available
(from QEMU with trans_virtio module)
rdma connect to a specified RDMA channel
usbg connect to a specified usb gadget channel
======== ============================================
uname=name user name to attempt mount as on the remote server. The
server may override or ignore this value. Certain user
names may require authentication.
aname=name aname specifies the file tree to access when the server is
offering several exported file systems.
cache=mode specifies a caching policy. By default, no caches are used.
The mode can be specified as a bitmask or by using one of the
preexisting common 'shortcuts'.
The bitmask is described below: (unspecified bits are reserved)
========== ====================================================
0b00000000 all caches disabled, mmap disabled
0b00000001 file caches enabled
0b00000010 meta-data caches enabled
0b00000100 writeback behavior (as opposed to writethrough)
0b00001000 loose caches (no explicit consistency with server)
0b10000000 fscache enabled for persistent caching
========== ====================================================
The current shortcuts and their associated bitmask are:
========= ====================================================
none 0b00000000 (no caching)
readahead 0b00000001 (only read-ahead file caching)
mmap 0b00000101 (read-ahead + writeback file cache)
loose 0b00001111 (non-coherent file and meta-data caches)
fscache 0b10001111 (persistent loose cache)
========= ====================================================
NOTE: only these shortcuts are tested modes of operation at the
moment, so using other combinations of bit-patterns is not
known to work. Work on better cache support is in progress.
IMPORTANT: loose caches (and by extension at the moment fscache)
do not necessarily validate cached values on the server. In other
words changes on the server are not guaranteed to be reflected
on the client system. Only use this mode of operation if you
have an exclusive mount and the server will not modify the
filesystem underneath you.
debug=n specifies debug level. The debug level is a bitmask.
===== ================================
0x01 display verbose error messages
0x02 developer debug (DEBUG_CURRENT)
0x04 display 9p trace
0x08 display VFS trace
0x10 display Marshalling debug
0x20 display RPC debug
0x40 display transport debug
0x80 display allocation debug
0x100 display protocol message debug
0x200 display Fid debug
0x400 display packet debug
0x800 display fscache tracing debug
===== ================================
rfdno=n the file descriptor for reading with trans=fd
wfdno=n the file descriptor for writing with trans=fd
msize=n the number of bytes to use for 9p packet payload
port=n port to connect to on the remote server
noextend force legacy mode (no 9p2000.u or 9p2000.L semantics)
version=name Select 9P protocol version. Valid options are:
======== ==============================
9p2000 Legacy mode (same as noextend)
9p2000.u Use 9P2000.u protocol
9p2000.L Use 9P2000.L protocol
======== ==============================
dfltuid attempt to mount as a particular uid
dfltgid attempt to mount with a particular gid
afid security channel - used by Plan 9 authentication protocols
nodevmap do not map special files - represent them as normal files.
This can be used to share devices/named pipes/sockets between
hosts. This functionality will be expanded in later versions.
directio bypass page cache on all read/write operations
ignoreqv ignore qid.version==0 as a marker to ignore cache
noxattr do not offer xattr functions on this mount.
access there are four access modes.
user
if a user tries to access a file on v9fs
filesystem for the first time, v9fs sends an
attach command (Tattach) for that user.
This is the default mode.
<uid>
allows only user with uid=<uid> to access
the files on the mounted filesystem
any
v9fs does single attach and performs all
operations as one user
clien
ACL based access check on the 9p client
side for access validation
cachetag cache tag to use the specified persistent cache.
cache tags for existing cache sessions can be listed at
/sys/fs/9p/caches. (applies only to cache=fscache)
============= ===============================================================
Behavior
========
This section aims at describing 9p 'quirks' that can be different
from a local filesystem behaviors.
- Setting O_NONBLOCK on a file will make client reads return as early
as the server returns some data instead of trying to fill the read
buffer with the requested amount of bytes or end of file is reached.
Resources
=========
Protocol specifications are maintained on github:
http://ericvh.github.com/9p-rfc/
9p client and server implementations are listed on
http://9p.cat-v.org/implementations
A 9p2000.L server is being developed by LLNL and can be found
at http://code.google.com/p/diod/
There are user and developer mailing lists available through the v9fs project
on sourceforge (http://sourceforge.net/projects/v9fs).
News and other information is maintained on a Wiki.
(http://sf.net/apps/mediawiki/v9fs/index.php).
Bug reports are best issued via the mailing list.
For more information on the Plan 9 Operating System check out
http://plan9.bell-labs.com/plan9
For information on Plan 9 from User Space (Plan 9 applications and libraries
ported to Linux/BSD/OSX/etc) check out https://9fans.github.io/plan9port/
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