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Commit 0795e7c031 ("[AFS]: Update the AFS fs documentation.") adds a new
section listing the build configuration options that need to be enabled for
the AFS file system.
The documentation refers to CONFIG_AFS, but the option is called
CONFIG_AFS_FS, since the beginning of Linux's git history.
Refer to the config option with the correct name.
Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20230720094301.9888-1-lukas.bulwahn@gmail.com
252 lines
7.8 KiB
ReStructuredText
252 lines
7.8 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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====================
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kAFS: AFS FILESYSTEM
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====================
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.. Contents:
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- Overview.
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- Usage.
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- Mountpoints.
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- Dynamic root.
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- Proc filesystem.
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- The cell database.
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- Security.
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- The @sys substitution.
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Overview
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========
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This filesystem provides a fairly simple secure AFS filesystem driver. It is
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under development and does not yet provide the full feature set. The features
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it does support include:
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(*) Security (currently only AFS kaserver and KerberosIV tickets).
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(*) File reading and writing.
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(*) Automounting.
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(*) Local caching (via fscache).
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It does not yet support the following AFS features:
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(*) pioctl() system call.
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Compilation
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===========
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The filesystem should be enabled by turning on the kernel configuration
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options::
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CONFIG_AF_RXRPC - The RxRPC protocol transport
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CONFIG_RXKAD - The RxRPC Kerberos security handler
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CONFIG_AFS_FS - The AFS filesystem
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Additionally, the following can be turned on to aid debugging::
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CONFIG_AF_RXRPC_DEBUG - Permit AF_RXRPC debugging to be enabled
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CONFIG_AFS_DEBUG - Permit AFS debugging to be enabled
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They permit the debugging messages to be turned on dynamically by manipulating
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the masks in the following files::
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/sys/module/af_rxrpc/parameters/debug
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/sys/module/kafs/parameters/debug
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Usage
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=====
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When inserting the driver modules the root cell must be specified along with a
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list of volume location server IP addresses::
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modprobe rxrpc
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modprobe kafs rootcell=cambridge.redhat.com:172.16.18.73:172.16.18.91
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The first module is the AF_RXRPC network protocol driver. This provides the
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RxRPC remote operation protocol and may also be accessed from userspace. See:
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Documentation/networking/rxrpc.rst
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The second module is the kerberos RxRPC security driver, and the third module
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is the actual filesystem driver for the AFS filesystem.
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Once the module has been loaded, more modules can be added by the following
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procedure::
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echo add grand.central.org 18.9.48.14:128.2.203.61:130.237.48.87 >/proc/fs/afs/cells
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Where the parameters to the "add" command are the name of a cell and a list of
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volume location servers within that cell, with the latter separated by colons.
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Filesystems can be mounted anywhere by commands similar to the following::
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mount -t afs "%cambridge.redhat.com:root.afs." /afs
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mount -t afs "#cambridge.redhat.com:root.cell." /afs/cambridge
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mount -t afs "#root.afs." /afs
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mount -t afs "#root.cell." /afs/cambridge
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Where the initial character is either a hash or a percent symbol depending on
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whether you definitely want a R/W volume (percent) or whether you'd prefer a
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R/O volume, but are willing to use a R/W volume instead (hash).
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The name of the volume can be suffixes with ".backup" or ".readonly" to
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specify connection to only volumes of those types.
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The name of the cell is optional, and if not given during a mount, then the
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named volume will be looked up in the cell specified during modprobe.
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Additional cells can be added through /proc (see later section).
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Mountpoints
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===========
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AFS has a concept of mountpoints. In AFS terms, these are specially formatted
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symbolic links (of the same form as the "device name" passed to mount). kAFS
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presents these to the user as directories that have a follow-link capability
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(i.e.: symbolic link semantics). If anyone attempts to access them, they will
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automatically cause the target volume to be mounted (if possible) on that site.
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Automatically mounted filesystems will be automatically unmounted approximately
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twenty minutes after they were last used. Alternatively they can be unmounted
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directly with the umount() system call.
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Manually unmounting an AFS volume will cause any idle submounts upon it to be
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culled first. If all are culled, then the requested volume will also be
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unmounted, otherwise error EBUSY will be returned.
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This can be used by the administrator to attempt to unmount the whole AFS tree
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mounted on /afs in one go by doing::
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umount /afs
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Dynamic Root
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============
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A mount option is available to create a serverless mount that is only usable
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for dynamic lookup. Creating such a mount can be done by, for example::
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mount -t afs none /afs -o dyn
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This creates a mount that just has an empty directory at the root. Attempting
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to look up a name in this directory will cause a mountpoint to be created that
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looks up a cell of the same name, for example::
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ls /afs/grand.central.org/
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Proc Filesystem
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===============
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The AFS module creates a "/proc/fs/afs/" directory and populates it:
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(*) A "cells" file that lists cells currently known to the afs module and
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their usage counts::
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[root@andromeda ~]# cat /proc/fs/afs/cells
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USE NAME
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3 cambridge.redhat.com
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(*) A directory per cell that contains files that list volume location
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servers, volumes, and active servers known within that cell::
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[root@andromeda ~]# cat /proc/fs/afs/cambridge.redhat.com/servers
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USE ADDR STATE
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4 172.16.18.91 0
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[root@andromeda ~]# cat /proc/fs/afs/cambridge.redhat.com/vlservers
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ADDRESS
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172.16.18.91
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[root@andromeda ~]# cat /proc/fs/afs/cambridge.redhat.com/volumes
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USE STT VLID[0] VLID[1] VLID[2] NAME
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1 Val 20000000 20000001 20000002 root.afs
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The Cell Database
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=================
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The filesystem maintains an internal database of all the cells it knows and the
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IP addresses of the volume location servers for those cells. The cell to which
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the system belongs is added to the database when modprobe is performed by the
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"rootcell=" argument or, if compiled in, using a "kafs.rootcell=" argument on
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the kernel command line.
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Further cells can be added by commands similar to the following::
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echo add CELLNAME VLADDR[:VLADDR][:VLADDR]... >/proc/fs/afs/cells
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echo add grand.central.org 18.9.48.14:128.2.203.61:130.237.48.87 >/proc/fs/afs/cells
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No other cell database operations are available at this time.
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Security
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========
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Secure operations are initiated by acquiring a key using the klog program. A
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very primitive klog program is available at:
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https://people.redhat.com/~dhowells/rxrpc/klog.c
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This should be compiled by::
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make klog LDLIBS="-lcrypto -lcrypt -lkrb4 -lkeyutils"
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And then run as::
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./klog
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Assuming it's successful, this adds a key of type RxRPC, named for the service
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and cell, e.g.: "afs@<cellname>". This can be viewed with the keyctl program or
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by cat'ing /proc/keys::
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[root@andromeda ~]# keyctl show
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Session Keyring
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-3 --alswrv 0 0 keyring: _ses.3268
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2 --alswrv 0 0 \_ keyring: _uid.0
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111416553 --als--v 0 0 \_ rxrpc: afs@CAMBRIDGE.REDHAT.COM
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Currently the username, realm, password and proposed ticket lifetime are
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compiled into the program.
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It is not required to acquire a key before using AFS facilities, but if one is
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not acquired then all operations will be governed by the anonymous user parts
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of the ACLs.
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If a key is acquired, then all AFS operations, including mounts and automounts,
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made by a possessor of that key will be secured with that key.
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If a file is opened with a particular key and then the file descriptor is
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passed to a process that doesn't have that key (perhaps over an AF_UNIX
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socket), then the operations on the file will be made with key that was used to
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open the file.
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The @sys Substitution
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=====================
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The list of up to 16 @sys substitutions for the current network namespace can
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be configured by writing a list to /proc/fs/afs/sysname::
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[root@andromeda ~]# echo foo amd64_linux_26 >/proc/fs/afs/sysname
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or cleared entirely by writing an empty list::
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[root@andromeda ~]# echo >/proc/fs/afs/sysname
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The current list for current network namespace can be retrieved by::
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[root@andromeda ~]# cat /proc/fs/afs/sysname
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foo
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amd64_linux_26
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When @sys is being substituted for, each element of the list is tried in the
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order given.
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By default, the list will contain one item that conforms to the pattern
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"<arch>_linux_26", amd64 being the name for x86_64.
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