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251 lines
7.8 KiB
251 lines
7.8 KiB
.. 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 - 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: [email protected] |
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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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