linux/Documentation/filesystems/configfs.rst
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   1=======================================================
   2Configfs - Userspace-driven Kernel Object Configuration
   3=======================================================
   4
   5Joel Becker <joel.becker@oracle.com>
   6
   7Updated: 31 March 2005
   8
   9Copyright (c) 2005 Oracle Corporation,
  10        Joel Becker <joel.becker@oracle.com>
  11
  12
  13What is configfs?
  14=================
  15
  16configfs is a ram-based filesystem that provides the converse of
  17sysfs's functionality.  Where sysfs is a filesystem-based view of
  18kernel objects, configfs is a filesystem-based manager of kernel
  19objects, or config_items.
  20
  21With sysfs, an object is created in kernel (for example, when a device
  22is discovered) and it is registered with sysfs.  Its attributes then
  23appear in sysfs, allowing userspace to read the attributes via
  24readdir(3)/read(2).  It may allow some attributes to be modified via
  25write(2).  The important point is that the object is created and
  26destroyed in kernel, the kernel controls the lifecycle of the sysfs
  27representation, and sysfs is merely a window on all this.
  28
  29A configfs config_item is created via an explicit userspace operation:
  30mkdir(2).  It is destroyed via rmdir(2).  The attributes appear at
  31mkdir(2) time, and can be read or modified via read(2) and write(2).
  32As with sysfs, readdir(3) queries the list of items and/or attributes.
  33symlink(2) can be used to group items together.  Unlike sysfs, the
  34lifetime of the representation is completely driven by userspace.  The
  35kernel modules backing the items must respond to this.
  36
  37Both sysfs and configfs can and should exist together on the same
  38system.  One is not a replacement for the other.
  39
  40Using configfs
  41==============
  42
  43configfs can be compiled as a module or into the kernel.  You can access
  44it by doing::
  45
  46        mount -t configfs none /config
  47
  48The configfs tree will be empty unless client modules are also loaded.
  49These are modules that register their item types with configfs as
  50subsystems.  Once a client subsystem is loaded, it will appear as a
  51subdirectory (or more than one) under /config.  Like sysfs, the
  52configfs tree is always there, whether mounted on /config or not.
  53
  54An item is created via mkdir(2).  The item's attributes will also
  55appear at this time.  readdir(3) can determine what the attributes are,
  56read(2) can query their default values, and write(2) can store new
  57values.  Don't mix more than one attribute in one attribute file.
  58
  59There are two types of configfs attributes:
  60
  61* Normal attributes, which similar to sysfs attributes, are small ASCII text
  62  files, with a maximum size of one page (PAGE_SIZE, 4096 on i386).  Preferably
  63  only one value per file should be used, and the same caveats from sysfs apply.
  64  Configfs expects write(2) to store the entire buffer at once.  When writing to
  65  normal configfs attributes, userspace processes should first read the entire
  66  file, modify the portions they wish to change, and then write the entire
  67  buffer back.
  68
  69* Binary attributes, which are somewhat similar to sysfs binary attributes,
  70  but with a few slight changes to semantics.  The PAGE_SIZE limitation does not
  71  apply, but the whole binary item must fit in single kernel vmalloc'ed buffer.
  72  The write(2) calls from user space are buffered, and the attributes'
  73  write_bin_attribute method will be invoked on the final close, therefore it is
  74  imperative for user-space to check the return code of close(2) in order to
  75  verify that the operation finished successfully.
  76  To avoid a malicious user OOMing the kernel, there's a per-binary attribute
  77  maximum buffer value.
  78
  79When an item needs to be destroyed, remove it with rmdir(2).  An
  80item cannot be destroyed if any other item has a link to it (via
  81symlink(2)).  Links can be removed via unlink(2).
  82
  83Configuring FakeNBD: an Example
  84===============================
  85
  86Imagine there's a Network Block Device (NBD) driver that allows you to
  87access remote block devices.  Call it FakeNBD.  FakeNBD uses configfs
  88for its configuration.  Obviously, there will be a nice program that
  89sysadmins use to configure FakeNBD, but somehow that program has to tell
  90the driver about it.  Here's where configfs comes in.
  91
  92When the FakeNBD driver is loaded, it registers itself with configfs.
  93readdir(3) sees this just fine::
  94
  95        # ls /config
  96        fakenbd
  97
  98A fakenbd connection can be created with mkdir(2).  The name is
  99arbitrary, but likely the tool will make some use of the name.  Perhaps
 100it is a uuid or a disk name::
 101
 102        # mkdir /config/fakenbd/disk1
 103        # ls /config/fakenbd/disk1
 104        target device rw
 105
 106The target attribute contains the IP address of the server FakeNBD will
 107connect to.  The device attribute is the device on the server.
 108Predictably, the rw attribute determines whether the connection is
 109read-only or read-write::
 110
 111        # echo 10.0.0.1 > /config/fakenbd/disk1/target
 112        # echo /dev/sda1 > /config/fakenbd/disk1/device
 113        # echo 1 > /config/fakenbd/disk1/rw
 114
 115That's it.  That's all there is.  Now the device is configured, via the
 116shell no less.
 117
 118Coding With configfs
 119====================
 120
 121Every object in configfs is a config_item.  A config_item reflects an
 122object in the subsystem.  It has attributes that match values on that
 123object.  configfs handles the filesystem representation of that object
 124and its attributes, allowing the subsystem to ignore all but the
 125basic show/store interaction.
 126
 127Items are created and destroyed inside a config_group.  A group is a
 128collection of items that share the same attributes and operations.
 129Items are created by mkdir(2) and removed by rmdir(2), but configfs
 130handles that.  The group has a set of operations to perform these tasks
 131
 132A subsystem is the top level of a client module.  During initialization,
 133the client module registers the subsystem with configfs, the subsystem
 134appears as a directory at the top of the configfs filesystem.  A
 135subsystem is also a config_group, and can do everything a config_group
 136can.
 137
 138struct config_item
 139==================
 140
 141::
 142
 143        struct config_item {
 144                char                    *ci_name;
 145                char                    ci_namebuf[UOBJ_NAME_LEN];
 146                struct kref             ci_kref;
 147                struct list_head        ci_entry;
 148                struct config_item      *ci_parent;
 149                struct config_group     *ci_group;
 150                struct config_item_type *ci_type;
 151                struct dentry           *ci_dentry;
 152        };
 153
 154        void config_item_init(struct config_item *);
 155        void config_item_init_type_name(struct config_item *,
 156                                        const char *name,
 157                                        struct config_item_type *type);
 158        struct config_item *config_item_get(struct config_item *);
 159        void config_item_put(struct config_item *);
 160
 161Generally, struct config_item is embedded in a container structure, a
 162structure that actually represents what the subsystem is doing.  The
 163config_item portion of that structure is how the object interacts with
 164configfs.
 165
 166Whether statically defined in a source file or created by a parent
 167config_group, a config_item must have one of the _init() functions
 168called on it.  This initializes the reference count and sets up the
 169appropriate fields.
 170
 171All users of a config_item should have a reference on it via
 172config_item_get(), and drop the reference when they are done via
 173config_item_put().
 174
 175By itself, a config_item cannot do much more than appear in configfs.
 176Usually a subsystem wants the item to display and/or store attributes,
 177among other things.  For that, it needs a type.
 178
 179struct config_item_type
 180=======================
 181
 182::
 183
 184        struct configfs_item_operations {
 185                void (*release)(struct config_item *);
 186                int (*allow_link)(struct config_item *src,
 187                                  struct config_item *target);
 188                void (*drop_link)(struct config_item *src,
 189                                 struct config_item *target);
 190        };
 191
 192        struct config_item_type {
 193                struct module                           *ct_owner;
 194                struct configfs_item_operations         *ct_item_ops;
 195                struct configfs_group_operations        *ct_group_ops;
 196                struct configfs_attribute               **ct_attrs;
 197                struct configfs_bin_attribute           **ct_bin_attrs;
 198        };
 199
 200The most basic function of a config_item_type is to define what
 201operations can be performed on a config_item.  All items that have been
 202allocated dynamically will need to provide the ct_item_ops->release()
 203method.  This method is called when the config_item's reference count
 204reaches zero.
 205
 206struct configfs_attribute
 207=========================
 208
 209::
 210
 211        struct configfs_attribute {
 212                char                    *ca_name;
 213                struct module           *ca_owner;
 214                umode_t                  ca_mode;
 215                ssize_t (*show)(struct config_item *, char *);
 216                ssize_t (*store)(struct config_item *, const char *, size_t);
 217        };
 218
 219When a config_item wants an attribute to appear as a file in the item's
 220configfs directory, it must define a configfs_attribute describing it.
 221It then adds the attribute to the NULL-terminated array
 222config_item_type->ct_attrs.  When the item appears in configfs, the
 223attribute file will appear with the configfs_attribute->ca_name
 224filename.  configfs_attribute->ca_mode specifies the file permissions.
 225
 226If an attribute is readable and provides a ->show method, that method will
 227be called whenever userspace asks for a read(2) on the attribute.  If an
 228attribute is writable and provides a ->store  method, that method will be
 229called whenever userspace asks for a write(2) on the attribute.
 230
 231struct configfs_bin_attribute
 232=============================
 233
 234::
 235
 236        struct configfs_bin_attribute {
 237                struct configfs_attribute       cb_attr;
 238                void                            *cb_private;
 239                size_t                          cb_max_size;
 240        };
 241
 242The binary attribute is used when the one needs to use binary blob to
 243appear as the contents of a file in the item's configfs directory.
 244To do so add the binary attribute to the NULL-terminated array
 245config_item_type->ct_bin_attrs, and the item appears in configfs, the
 246attribute file will appear with the configfs_bin_attribute->cb_attr.ca_name
 247filename.  configfs_bin_attribute->cb_attr.ca_mode specifies the file
 248permissions.
 249The cb_private member is provided for use by the driver, while the
 250cb_max_size member specifies the maximum amount of vmalloc buffer
 251to be used.
 252
 253If binary attribute is readable and the config_item provides a
 254ct_item_ops->read_bin_attribute() method, that method will be called
 255whenever userspace asks for a read(2) on the attribute.  The converse
 256will happen for write(2). The reads/writes are bufferred so only a
 257single read/write will occur; the attributes' need not concern itself
 258with it.
 259
 260struct config_group
 261===================
 262
 263A config_item cannot live in a vacuum.  The only way one can be created
 264is via mkdir(2) on a config_group.  This will trigger creation of a
 265child item::
 266
 267        struct config_group {
 268                struct config_item              cg_item;
 269                struct list_head                cg_children;
 270                struct configfs_subsystem       *cg_subsys;
 271                struct list_head                default_groups;
 272                struct list_head                group_entry;
 273        };
 274
 275        void config_group_init(struct config_group *group);
 276        void config_group_init_type_name(struct config_group *group,
 277                                         const char *name,
 278                                         struct config_item_type *type);
 279
 280
 281The config_group structure contains a config_item.  Properly configuring
 282that item means that a group can behave as an item in its own right.
 283However, it can do more: it can create child items or groups.  This is
 284accomplished via the group operations specified on the group's
 285config_item_type::
 286
 287        struct configfs_group_operations {
 288                struct config_item *(*make_item)(struct config_group *group,
 289                                                 const char *name);
 290                struct config_group *(*make_group)(struct config_group *group,
 291                                                   const char *name);
 292                int (*commit_item)(struct config_item *item);
 293                void (*disconnect_notify)(struct config_group *group,
 294                                          struct config_item *item);
 295                void (*drop_item)(struct config_group *group,
 296                                  struct config_item *item);
 297        };
 298
 299A group creates child items by providing the
 300ct_group_ops->make_item() method.  If provided, this method is called from
 301mkdir(2) in the group's directory.  The subsystem allocates a new
 302config_item (or more likely, its container structure), initializes it,
 303and returns it to configfs.  Configfs will then populate the filesystem
 304tree to reflect the new item.
 305
 306If the subsystem wants the child to be a group itself, the subsystem
 307provides ct_group_ops->make_group().  Everything else behaves the same,
 308using the group _init() functions on the group.
 309
 310Finally, when userspace calls rmdir(2) on the item or group,
 311ct_group_ops->drop_item() is called.  As a config_group is also a
 312config_item, it is not necessary for a separate drop_group() method.
 313The subsystem must config_item_put() the reference that was initialized
 314upon item allocation.  If a subsystem has no work to do, it may omit
 315the ct_group_ops->drop_item() method, and configfs will call
 316config_item_put() on the item on behalf of the subsystem.
 317
 318Important:
 319   drop_item() is void, and as such cannot fail.  When rmdir(2)
 320   is called, configfs WILL remove the item from the filesystem tree
 321   (assuming that it has no children to keep it busy).  The subsystem is
 322   responsible for responding to this.  If the subsystem has references to
 323   the item in other threads, the memory is safe.  It may take some time
 324   for the item to actually disappear from the subsystem's usage.  But it
 325   is gone from configfs.
 326
 327When drop_item() is called, the item's linkage has already been torn
 328down.  It no longer has a reference on its parent and has no place in
 329the item hierarchy.  If a client needs to do some cleanup before this
 330teardown happens, the subsystem can implement the
 331ct_group_ops->disconnect_notify() method.  The method is called after
 332configfs has removed the item from the filesystem view but before the
 333item is removed from its parent group.  Like drop_item(),
 334disconnect_notify() is void and cannot fail.  Client subsystems should
 335not drop any references here, as they still must do it in drop_item().
 336
 337A config_group cannot be removed while it still has child items.  This
 338is implemented in the configfs rmdir(2) code.  ->drop_item() will not be
 339called, as the item has not been dropped.  rmdir(2) will fail, as the
 340directory is not empty.
 341
 342struct configfs_subsystem
 343=========================
 344
 345A subsystem must register itself, usually at module_init time.  This
 346tells configfs to make the subsystem appear in the file tree::
 347
 348        struct configfs_subsystem {
 349                struct config_group     su_group;
 350                struct mutex            su_mutex;
 351        };
 352
 353        int configfs_register_subsystem(struct configfs_subsystem *subsys);
 354        void configfs_unregister_subsystem(struct configfs_subsystem *subsys);
 355
 356A subsystem consists of a toplevel config_group and a mutex.
 357The group is where child config_items are created.  For a subsystem,
 358this group is usually defined statically.  Before calling
 359configfs_register_subsystem(), the subsystem must have initialized the
 360group via the usual group _init() functions, and it must also have
 361initialized the mutex.
 362
 363When the register call returns, the subsystem is live, and it
 364will be visible via configfs.  At that point, mkdir(2) can be called and
 365the subsystem must be ready for it.
 366
 367An Example
 368==========
 369
 370The best example of these basic concepts is the simple_children
 371subsystem/group and the simple_child item in
 372samples/configfs/configfs_sample.c. It shows a trivial object displaying
 373and storing an attribute, and a simple group creating and destroying
 374these children.
 375
 376Hierarchy Navigation and the Subsystem Mutex
 377============================================
 378
 379There is an extra bonus that configfs provides.  The config_groups and
 380config_items are arranged in a hierarchy due to the fact that they
 381appear in a filesystem.  A subsystem is NEVER to touch the filesystem
 382parts, but the subsystem might be interested in this hierarchy.  For
 383this reason, the hierarchy is mirrored via the config_group->cg_children
 384and config_item->ci_parent structure members.
 385
 386A subsystem can navigate the cg_children list and the ci_parent pointer
 387to see the tree created by the subsystem.  This can race with configfs'
 388management of the hierarchy, so configfs uses the subsystem mutex to
 389protect modifications.  Whenever a subsystem wants to navigate the
 390hierarchy, it must do so under the protection of the subsystem
 391mutex.
 392
 393A subsystem will be prevented from acquiring the mutex while a newly
 394allocated item has not been linked into this hierarchy.   Similarly, it
 395will not be able to acquire the mutex while a dropping item has not
 396yet been unlinked.  This means that an item's ci_parent pointer will
 397never be NULL while the item is in configfs, and that an item will only
 398be in its parent's cg_children list for the same duration.  This allows
 399a subsystem to trust ci_parent and cg_children while they hold the
 400mutex.
 401
 402Item Aggregation Via symlink(2)
 403===============================
 404
 405configfs provides a simple group via the group->item parent/child
 406relationship.  Often, however, a larger environment requires aggregation
 407outside of the parent/child connection.  This is implemented via
 408symlink(2).
 409
 410A config_item may provide the ct_item_ops->allow_link() and
 411ct_item_ops->drop_link() methods.  If the ->allow_link() method exists,
 412symlink(2) may be called with the config_item as the source of the link.
 413These links are only allowed between configfs config_items.  Any
 414symlink(2) attempt outside the configfs filesystem will be denied.
 415
 416When symlink(2) is called, the source config_item's ->allow_link()
 417method is called with itself and a target item.  If the source item
 418allows linking to target item, it returns 0.  A source item may wish to
 419reject a link if it only wants links to a certain type of object (say,
 420in its own subsystem).
 421
 422When unlink(2) is called on the symbolic link, the source item is
 423notified via the ->drop_link() method.  Like the ->drop_item() method,
 424this is a void function and cannot return failure.  The subsystem is
 425responsible for responding to the change.
 426
 427A config_item cannot be removed while it links to any other item, nor
 428can it be removed while an item links to it.  Dangling symlinks are not
 429allowed in configfs.
 430
 431Automatically Created Subgroups
 432===============================
 433
 434A new config_group may want to have two types of child config_items.
 435While this could be codified by magic names in ->make_item(), it is much
 436more explicit to have a method whereby userspace sees this divergence.
 437
 438Rather than have a group where some items behave differently than
 439others, configfs provides a method whereby one or many subgroups are
 440automatically created inside the parent at its creation.  Thus,
 441mkdir("parent") results in "parent", "parent/subgroup1", up through
 442"parent/subgroupN".  Items of type 1 can now be created in
 443"parent/subgroup1", and items of type N can be created in
 444"parent/subgroupN".
 445
 446These automatic subgroups, or default groups, do not preclude other
 447children of the parent group.  If ct_group_ops->make_group() exists,
 448other child groups can be created on the parent group directly.
 449
 450A configfs subsystem specifies default groups by adding them using the
 451configfs_add_default_group() function to the parent config_group
 452structure.  Each added group is populated in the configfs tree at the same
 453time as the parent group.  Similarly, they are removed at the same time
 454as the parent.  No extra notification is provided.  When a ->drop_item()
 455method call notifies the subsystem the parent group is going away, it
 456also means every default group child associated with that parent group.
 457
 458As a consequence of this, default groups cannot be removed directly via
 459rmdir(2).  They also are not considered when rmdir(2) on the parent
 460group is checking for children.
 461
 462Dependent Subsystems
 463====================
 464
 465Sometimes other drivers depend on particular configfs items.  For
 466example, ocfs2 mounts depend on a heartbeat region item.  If that
 467region item is removed with rmdir(2), the ocfs2 mount must BUG or go
 468readonly.  Not happy.
 469
 470configfs provides two additional API calls: configfs_depend_item() and
 471configfs_undepend_item().  A client driver can call
 472configfs_depend_item() on an existing item to tell configfs that it is
 473depended on.  configfs will then return -EBUSY from rmdir(2) for that
 474item.  When the item is no longer depended on, the client driver calls
 475configfs_undepend_item() on it.
 476
 477These API cannot be called underneath any configfs callbacks, as
 478they will conflict.  They can block and allocate.  A client driver
 479probably shouldn't calling them of its own gumption.  Rather it should
 480be providing an API that external subsystems call.
 481
 482How does this work?  Imagine the ocfs2 mount process.  When it mounts,
 483it asks for a heartbeat region item.  This is done via a call into the
 484heartbeat code.  Inside the heartbeat code, the region item is looked
 485up.  Here, the heartbeat code calls configfs_depend_item().  If it
 486succeeds, then heartbeat knows the region is safe to give to ocfs2.
 487If it fails, it was being torn down anyway, and heartbeat can gracefully
 488pass up an error.
 489
 490Committable Items
 491=================
 492
 493Note:
 494     Committable items are currently unimplemented.
 495
 496Some config_items cannot have a valid initial state.  That is, no
 497default values can be specified for the item's attributes such that the
 498item can do its work.  Userspace must configure one or more attributes,
 499after which the subsystem can start whatever entity this item
 500represents.
 501
 502Consider the FakeNBD device from above.  Without a target address *and*
 503a target device, the subsystem has no idea what block device to import.
 504The simple example assumes that the subsystem merely waits until all the
 505appropriate attributes are configured, and then connects.  This will,
 506indeed, work, but now every attribute store must check if the attributes
 507are initialized.  Every attribute store must fire off the connection if
 508that condition is met.
 509
 510Far better would be an explicit action notifying the subsystem that the
 511config_item is ready to go.  More importantly, an explicit action allows
 512the subsystem to provide feedback as to whether the attributes are
 513initialized in a way that makes sense.  configfs provides this as
 514committable items.
 515
 516configfs still uses only normal filesystem operations.  An item is
 517committed via rename(2).  The item is moved from a directory where it
 518can be modified to a directory where it cannot.
 519
 520Any group that provides the ct_group_ops->commit_item() method has
 521committable items.  When this group appears in configfs, mkdir(2) will
 522not work directly in the group.  Instead, the group will have two
 523subdirectories: "live" and "pending".  The "live" directory does not
 524support mkdir(2) or rmdir(2) either.  It only allows rename(2).  The
 525"pending" directory does allow mkdir(2) and rmdir(2).  An item is
 526created in the "pending" directory.  Its attributes can be modified at
 527will.  Userspace commits the item by renaming it into the "live"
 528directory.  At this point, the subsystem receives the ->commit_item()
 529callback.  If all required attributes are filled to satisfaction, the
 530method returns zero and the item is moved to the "live" directory.
 531
 532As rmdir(2) does not work in the "live" directory, an item must be
 533shutdown, or "uncommitted".  Again, this is done via rename(2), this
 534time from the "live" directory back to the "pending" one.  The subsystem
 535is notified by the ct_group_ops->uncommit_object() method.
 536