• David Howells's avatar
    CacheFiles: A cache that backs onto a mounted filesystem · 9ae326a6
    David Howells authored
    Add an FS-Cache cache-backend that permits a mounted filesystem to be used as a
    backing store for the cache.
    
    CacheFiles uses a userspace daemon to do some of the cache management - such as
    reaping stale nodes and culling.  This is called cachefilesd and lives in
    /sbin.  The source for the daemon can be downloaded from:
    
    	http://people.redhat.com/~dhowells/cachefs/cachefilesd.c
    
    And an example configuration from:
    
    	http://people.redhat.com/~dhowells/cachefs/cachefilesd.conf
    
    The filesystem and data integrity of the cache are only as good as those of the
    filesystem providing the backing services.  Note that CacheFiles does not
    attempt to journal anything since the journalling interfaces of the various
    filesystems are very specific in nature.
    
    CacheFiles creates a misc character device - "/dev/cachefiles" - that is used
    to communication with the daemon.  Only one thing may have this open at once,
    and whilst it is open, a cache is at least partially in existence.  The daemon
    opens this and sends commands down it to control the cache.
    
    CacheFiles is currently limited to a single cache.
    
    CacheFiles attempts to maintain at least a certain percentage of free space on
    the filesystem, shrinking the cache by culling the objects it contains to make
    space if necessary - see the "Cache Culling" section.  This means it can be
    placed on the same medium as a live set of data, and will expand to make use of
    spare space and automatically contract when the set of data requires more
    space.
    
    ============
    REQUIREMENTS
    ============
    
    The use of CacheFiles and its daemon requires the following features to be
    available in the system and in the cache filesystem:
    
    	- dnotify.
    
    	- extended attributes (xattrs).
    
    	- openat() and friends.
    
    	- bmap() support on files in the filesystem (FIBMAP ioctl).
    
    	- The use of bmap() to detect a partial page at the end of the file.
    
    It is strongly recommended that the "dir_index" option is enabled on Ext3
    filesystems being used as a cache.
    
    =============
    CONFIGURATION
    =============
    
    The cache is configured by a script in /etc/cachefilesd.conf.  These commands
    set up cache ready for use.  The following script commands are available:
    
     (*) brun <N>%
     (*) bcull <N>%
     (*) bstop <N>%
     (*) frun <N>%
     (*) fcull <N>%
     (*) fstop <N>%
    
    	Configure the culling limits.  Optional.  See the section on culling
    	The defaults are 7% (run), 5% (cull) and 1% (stop) respectively.
    
    	The commands beginning with a 'b' are file space (block) limits, those
    	beginning with an 'f' are file count limits.
    
     (*) dir <path>
    
    	Specify the directory containing the root of the cache.  Mandatory.
    
     (*) tag <name>
    
    	Specify a tag to FS-Cache to use in distinguishing multiple caches.
    	Optional.  The default is "CacheFiles".
    
     (*) debug <mask>
    
    	Specify a numeric bitmask to control debugging in the kernel module.
    	Optional.  The default is zero (all off).  The following values can be
    	OR'd into the mask to collect various information:
    
    		1	Turn on trace of function entry (_enter() macros)
    		2	Turn on trace of function exit (_leave() macros)
    		4	Turn on trace of internal debug points (_debug())
    
    	This mask can also be set through sysfs, eg:
    
    		echo 5 >/sys/modules/cachefiles/parameters/debug
    
    ==================
    STARTING THE CACHE
    ==================
    
    The cache is started by running the daemon.  The daemon opens the cache device,
    configures the cache and tells it to begin caching.  At that point the cache
    binds to fscache and the cache becomes live.
    
    The daemon is run as follows:
    
    	/sbin/cachefilesd [-d]* [-s] [-n] [-f <configfile>]
    
    The flags are:
    
     (*) -d
    
    	Increase the debugging level.  This can be specified multiple times and
    	is cumulative with itself.
    
     (*) -s
    
    	Send messages to stderr instead of syslog.
    
     (*) -n
    
    	Don't daemonise and go into background.
    
     (*) -f <configfile>
    
    	Use an alternative configuration file rather than the default one.
    
    ===============
    THINGS TO AVOID
    ===============
    
    Do not mount other things within the cache as this will cause problems.  The
    kernel module contains its own very cut-down path walking facility that ignores
    mountpoints, but the daemon can't avoid them.
    
    Do not create, rename or unlink files and directories in the cache whilst the
    cache is active, as this may cause the state to become uncertain.
    
    Renaming files in the cache might make objects appear to be other objects (the
    filename is part of the lookup key).
    
    Do not change or remove the extended attributes attached to cache files by the
    cache as this will cause the cache state management to get confused.
    
    Do not create files or directories in the cache, lest the cache get confused or
    serve incorrect data.
    
    Do not chmod files in the cache.  The module creates things with minimal
    permissions to prevent random users being able to access them directly.
    
    =============
    CACHE CULLING
    =============
    
    The cache may need culling occasionally to make space.  This involves
    discarding objects from the cache that have been used less recently than
    anything else.  Culling is based on the access time of data objects.  Empty
    directories are culled if not in use.
    
    Cache culling is done on the basis of the percentage of blocks and the
    percentage of files available in the underlying filesystem.  There are six
    "limits":
    
     (*) brun
     (*) frun
    
         If the amount of free space and the number of available files in the cache
         rises above both these limits, then culling is turned off.
    
     (*) bcull
     (*) fcull
    
         If the amount of available space or the number of available files in the
         cache falls below either of these limits, then culling is started.
    
     (*) bstop
     (*) fstop
    
         If the amount of available space or the number of available files in the
         cache falls below either of these limits, then no further allocation of
         disk space or files is permitted until culling has raised things above
         these limits again.
    
    These must be configured thusly:
    
    	0 <= bstop < bcull < brun < 100
    	0 <= fstop < fcull < frun < 100
    
    Note that these are percentages of available space and available files, and do
    _not_ appear as 100 minus the percentage displayed by the "df" program.
    
    The userspace daemon scans the cache to build up a table of cullable objects.
    These are then culled in least recently used order.  A new scan of the cache is
    started as soon as space is made in the table.  Objects will be skipped if
    their atimes have changed or if the kernel module says it is still using them.
    
    ===============
    CACHE STRUCTURE
    ===============
    
    The CacheFiles module will create two directories in the directory it was
    given:
    
     (*) cache/
    
     (*) graveyard/
    
    The active cache objects all reside in the first directory.  The CacheFiles
    kernel module moves any retired or culled objects that it can't simply unlink
    to the graveyard from which the daemon will actually delete them.
    
    The daemon uses dnotify to monitor the graveyard directory, and will delete
    anything that appears therein.
    
    The module represents index objects as directories with the filename "I..." or
    "J...".  Note that the "cache/" directory is itself a special index.
    
    Data objects are represented as files if they have no children, or directories
    if they do.  Their filenames all begin "D..." or "E...".  If represented as a
    directory, data objects will have a file in the directory called "data" that
    actually holds the data.
    
    Special objects are similar to data objects, except their filenames begin
    "S..." or "T...".
    
    If an object has children, then it will be represented as a directory.
    Immediately in the representative directory are a collection of directories
    named for hash values of the child object keys with an '@' prepended.  Into
    this directory, if possible, will be placed the representations of the child
    objects:
    
    	INDEX     INDEX      INDEX                             DATA FILES
    	========= ========== ================================= ================
    	cache/@4a/I03nfs/@30/Ji000000000000000--fHg8hi8400
    	cache/@4a/I03nfs/@30/Ji000000000000000--fHg8hi8400/@75/Es0g000w...DB1ry
    	cache/@4a/I03nfs/@30/Ji000000000000000--fHg8hi8400/@75/Es0g000w...N22ry
    	cache/@4a/I03nfs/@30/Ji000000000000000--fHg8hi8400/@75/Es0g000w...FP1ry
    
    If the key is so long that it exceeds NAME_MAX with the decorations added on to
    it, then it will be cut into pieces, the first few of which will be used to
    make a nest of directories, and the last one of which will be the objects
    inside the last directory.  The names of the intermediate directories will have
    '+' prepended:
    
    	J1223/@23/+xy...z/+kl...m/Epqr
    
    Note that keys are raw data, and not only may they exceed NAME_MAX in size,
    they may also contain things like '/' and NUL characters, and so they may not
    be suitable for turning directly into a filename.
    
    To handle this, CacheFiles will use a suitably printable filename directly and
    "base-64" encode ones that aren't directly suitable.  The two versions of
    object filenames indicate the encoding:
    
    	OBJECT TYPE	PRINTABLE	ENCODED
    	===============	===============	===============
    	Index		"I..."		"J..."
    	Data		"D..."		"E..."
    	Special		"S..."		"T..."
    
    Intermediate directories are always "@" or "+" as appropriate.
    
    Each object in the cache has an extended attribute label that holds the object
    type ID (required to distinguish special objects) and the auxiliary data from
    the netfs.  The latter is used to detect stale objects in the cache and update
    or retire them.
    
    Note that CacheFiles will erase from the cache any file it doesn't recognise or
    any file of an incorrect type (such as a FIFO file or a device file).
    
    ==========================
    SECURITY MODEL AND SELINUX
    ==========================
    
    CacheFiles is implemented to deal properly with the LSM security features of
    the Linux kernel and the SELinux facility.
    
    One of the problems that CacheFiles faces is that it is generally acting on
    behalf of a process, and running in that process's context, and that includes a
    security context that is not appropriate for accessing the cache - either
    because the files in the cache are inaccessible to that process, or because if
    the process creates a file in the cache, that file may be inaccessible to other
    processes.
    
    The way CacheFiles works is to temporarily change the security context (fsuid,
    fsgid and actor security label) that the process acts as - without changing the
    security context of the process when it the target of an operation performed by
    some other process (so signalling and suchlike still work correctly).
    
    When the CacheFiles module is asked to bind to its cache, it:
    
     (1) Finds the security label attached to the root cache directory and uses
         that as the security label with which it will create files.  By default,
         this is:
    
    	cachefiles_var_t
    
     (2) Finds the security label of the process which issued the bind request
         (presumed to be the cachefilesd daemon), which by default will be:
    
    	cachefilesd_t
    
         and asks LSM to supply a security ID as which it should act given the
         daemon's label.  By default, this will be:
    
    	cachefiles_kernel_t
    
         SELinux transitions the daemon's security ID to the module's security ID
         based on a rule of this form in the policy.
    
    	type_transition <daemon's-ID> kernel_t : process <module's-ID>;
    
         For instance:
    
    	type_transition cachefilesd_t kernel_t : process cachefiles_kernel_t;
    
    The module's security ID gives it permission to create, move and remove files
    and directories in the cache, to find and access directories and files in the
    cache, to set and access extended attributes on cache objects, and to read and
    write files in the cache.
    
    The daemon's security ID gives it only a very restricted set of permissions: it
    may scan directories, stat files and erase files and directories.  It may
    not read or write files in the cache, and so it is precluded from accessing the
    data cached therein; nor is it permitted to create new files in the cache.
    
    There are policy source files available in:
    
    	http://people.redhat.com/~dhowells/fscache/cachefilesd-0.8.tar.bz2
    
    and later versions.  In that tarball, see the files:
    
    	cachefilesd.te
    	cachefilesd.fc
    	cachefilesd.if
    
    They are built and installed directly by the RPM.
    
    If a non-RPM based system is being used, then copy the above files to their own
    directory and run:
    
    	make -f /usr/share/selinux/devel/Makefile
    	semodule -i cachefilesd.pp
    
    You will need checkpolicy and selinux-policy-devel installed prior to the
    build.
    
    By default, the cache is located in /var/fscache, but if it is desirable that
    it should be elsewhere, than either the above policy files must be altered, or
    an auxiliary policy must be installed to label the alternate location of the
    cache.
    
    For instructions on how to add an auxiliary policy to enable the cache to be
    located elsewhere when SELinux is in enforcing mode, please see:
    
    	/usr/share/doc/cachefilesd-*/move-cache.txt
    
    When the cachefilesd rpm is installed; alternatively, the document can be found
    in the sources.
    
    ==================
    A NOTE ON SECURITY
    ==================
    
    CacheFiles makes use of the split security in the task_struct.  It allocates
    its own task_security structure, and redirects current->act_as to point to it
    when it acts on behalf of another process, in that process's context.
    
    The reason it does this is that it calls vfs_mkdir() and suchlike rather than
    bypassing security and calling inode ops directly.  Therefore the VFS and LSM
    may deny the CacheFiles access to the cache data because under some
    circumstances the caching code is running in the security context of whatever
    process issued the original syscall on the netfs.
    
    Furthermore, should CacheFiles create a file or directory, the security
    parameters with that object is created (UID, GID, security label) would be
    derived from that process that issued the system call, thus potentially
    preventing other processes from accessing the cache - including CacheFiles's
    cache management daemon (cachefilesd).
    
    What is required is to temporarily override the security of the process that
    issued the system call.  We can't, however, just do an in-place change of the
    security data as that affects the process as an object, not just as a subject.
    This means it may lose signals or ptrace events for example, and affects what
    the process looks like in /proc.
    
    So CacheFiles makes use of a logical split in the security between the
    objective security (task->sec) and the subjective security (task->act_as).  The
    objective security holds the intrinsic security properties of a process and is
    never overridden.  This is what appears in /proc, and is what is used when a
    process is the target of an operation by some other process (SIGKILL for
    example).
    
    The subjective security holds the active security properties of a process, and
    may be overridden.  This is not seen externally, and is used whan a process
    acts upon another object, for example SIGKILLing another process or opening a
    file.
    
    LSM hooks exist that allow SELinux (or Smack or whatever) to reject a request
    for CacheFiles to run in a context of a specific security label, or to create
    files and directories with another security label.
    
    This documentation is added by the patch to:
    
    	Documentation/filesystems/caching/cachefiles.txt
    Signed-Off-By: 's avatarDavid Howells <dhowells@redhat.com>
    Acked-by: 's avatarSteve Dickson <steved@redhat.com>
    Acked-by: 's avatarTrond Myklebust <Trond.Myklebust@netapp.com>
    Acked-by: 's avatarAl Viro <viro@zeniv.linux.org.uk>
    Tested-by: 's avatarDaire Byrne <Daire.Byrne@framestore.com>
    9ae326a6