Documentation/watch_queue.rst

   1 ==============================
   2 General notification mechanism
   3 ==============================
   4
   5 The general notification mechanism is built on top of the standard pipe driver
   6 whereby it effectively splices notification messages from the kernel into pipes
   7 opened by userspace.  This can be used in conjunction with::
   8
   9   * Key/keyring notifications
  10
  11
  12 The notifications buffers can be enabled by:
  13
  14         "General setup"/"General notification queue"
  15         (CONFIG_WATCH_QUEUE)
  16
  17 This document has the following sections:
  18
  19 .. contents:: :local:
  20
  21
  22 Overview
  23 ========
  24
  25 This facility appears as a pipe that is opened in a special mode.  The pipe's
  26 internal ring buffer is used to hold messages that are generated by the kernel.
  27 These messages are then read out by read().  Splice and similar are disabled on
  28 such pipes due to them wanting to, under some circumstances, revert their
  29 additions to the ring - which might end up interleaved with notification
  30 messages.
  31
  32 The owner of the pipe has to tell the kernel which sources it would like to
  33 watch through that pipe.  Only sources that have been connected to a pipe will
  34 insert messages into it.  Note that a source may be bound to multiple pipes and
  35 insert messages into all of them simultaneously.
  36
  37 Filters may also be emplaced on a pipe so that certain source types and
  38 subevents can be ignored if they're not of interest.
  39
  40 A message will be discarded if there isn't a slot available in the ring or if
  41 no preallocated message buffer is available.  In both of these cases, read()
  42 will insert a WATCH_META_LOSS_NOTIFICATION message into the output buffer after
  43 the last message currently in the buffer has been read.
  44
  45 Note that when producing a notification, the kernel does not wait for the
  46 consumers to collect it, but rather just continues on.  This means that
  47 notifications can be generated whilst spinlocks are held and also protects the
  48 kernel from being held up indefinitely by a userspace malfunction.
  49
  50
  51 Message Structure
  52 =================
  53
  54 Notification messages begin with a short header::
  55
  56         struct watch_notification {
  57                 __u32   type:24;
  58                 __u32   subtype:8;
  59                 __u32   info;
  60         };
  61
  62 "type" indicates the source of the notification record and "subtype" indicates
  63 the type of record from that source (see the Watch Sources section below).  The
  64 type may also be "WATCH_TYPE_META".  This is a special record type generated
  65 internally by the watch queue itself.  There are two subtypes:
  66
  67   * WATCH_META_REMOVAL_NOTIFICATION
  68   * WATCH_META_LOSS_NOTIFICATION
  69
  70 The first indicates that an object on which a watch was installed was removed
  71 or destroyed and the second indicates that some messages have been lost.
  72
  73 "info" indicates a bunch of things, including:
  74
  75   * The length of the message in bytes, including the header (mask with
  76     WATCH_INFO_LENGTH and shift by WATCH_INFO_LENGTH__SHIFT).  This indicates
  77     the size of the record, which may be between 8 and 127 bytes.
  78
  79   * The watch ID (mask with WATCH_INFO_ID and shift by WATCH_INFO_ID__SHIFT).
  80     This indicates that caller's ID of the watch, which may be between 0
  81     and 255.  Multiple watches may share a queue, and this provides a means to
  82     distinguish them.
  83
  84   * A type-specific field (WATCH_INFO_TYPE_INFO).  This is set by the
  85     notification producer to indicate some meaning specific to the type and
  86     subtype.
  87
  88 Everything in info apart from the length can be used for filtering.
  89
  90 The header can be followed by supplementary information.  The format of this is
  91 at the discretion is defined by the type and subtype.
  92
  93
  94 Watch List (Notification Source) API
  95 ====================================
  96
  97 A "watch list" is a list of watchers that are subscribed to a source of
  98 notifications.  A list may be attached to an object (say a key or a superblock)
  99 or may be global (say for device events).  From a userspace perspective, a
 100 non-global watch list is typically referred to by reference to the object it
 101 belongs to (such as using KEYCTL_NOTIFY and giving it a key serial number to
 102 watch that specific key).
 103
 104 To manage a watch list, the following functions are provided:
 105
 106   * ::
 107
 108         void init_watch_list(struct watch_list *wlist,
 109                              void (*release_watch)(struct watch *wlist));
 110
 111     Initialise a watch list.  If ``release_watch`` is not NULL, then this
 112     indicates a function that should be called when the watch_list object is
 113     destroyed to discard any references the watch list holds on the watched
 114     object.
 115
 116   * ``void remove_watch_list(struct watch_list *wlist);``
 117
 118     This removes all of the watches subscribed to a watch_list and frees them
 119     and then destroys the watch_list object itself.
 120
 121
 122 Watch Queue (Notification Output) API
 123 =====================================
 124
 125 A "watch queue" is the buffer allocated by an application that notification
 126 records will be written into.  The workings of this are hidden entirely inside
 127 of the pipe device driver, but it is necessary to gain a reference to it to set
 128 a watch.  These can be managed with:
 129
 130   * ``struct watch_queue *get_watch_queue(int fd);``
 131
 132     Since watch queues are indicated to the kernel by the fd of the pipe that
 133     implements the buffer, userspace must hand that fd through a system call.
 134     This can be used to look up an opaque pointer to the watch queue from the
 135     system call.
 136
 137   * ``void put_watch_queue(struct watch_queue *wqueue);``
 138
 139     This discards the reference obtained from ``get_watch_queue()``.
 140
 141
 142 Watch Subscription API
 143 ======================
 144
 145 A "watch" is a subscription on a watch list, indicating the watch queue, and
 146 thus the buffer, into which notification records should be written.  The watch
 147 queue object may also carry filtering rules for that object, as set by
 148 userspace.  Some parts of the watch struct can be set by the driver::
 149
 150         struct watch {
 151                 union {
 152                         u32             info_id;        /* ID to be OR'd in to info field */
 153                         ...
 154                 };
 155                 void                    *private;       /* Private data for the watched object */
 156                 u64                     id;             /* Internal identifier */
 157                 ...
 158         };
 159
 160 The ``info_id`` value should be an 8-bit number obtained from userspace and
 161 shifted by WATCH_INFO_ID__SHIFT.  This is OR'd into the WATCH_INFO_ID field of
 162 struct watch_notification::info when and if the notification is written into
 163 the associated watch queue buffer.
 164
 165 The ``private`` field is the driver's data associated with the watch_list and
 166 is cleaned up by the ``watch_list::release_watch()`` method.
 167
 168 The ``id`` field is the source's ID.  Notifications that are posted with a
 169 different ID are ignored.
 170
 171 The following functions are provided to manage watches:
 172
 173   * ``void init_watch(struct watch *watch, struct watch_queue *wqueue);``
 174
 175     Initialise a watch object, setting its pointer to the watch queue, using
 176     appropriate barriering to avoid lockdep complaints.
 177
 178   * ``int add_watch_to_object(struct watch *watch, struct watch_list *wlist);``
 179
 180     Subscribe a watch to a watch list (notification source).  The
 181     driver-settable fields in the watch struct must have been set before this
 182     is called.
 183
 184   * ::
 185
 186         int remove_watch_from_object(struct watch_list *wlist,
 187                                      struct watch_queue *wqueue,
 188                                      u64 id, false);
 189
 190     Remove a watch from a watch list, where the watch must match the specified
 191     watch queue (``wqueue``) and object identifier (``id``).  A notification
 192     (``WATCH_META_REMOVAL_NOTIFICATION``) is sent to the watch queue to
 193     indicate that the watch got removed.
 194
 195   * ``int remove_watch_from_object(struct watch_list *wlist, NULL, 0, true);``
 196
 197     Remove all the watches from a watch list.  It is expected that this will be
 198     called preparatory to destruction and that the watch list will be
 199     inaccessible to new watches by this point.  A notification
 200     (``WATCH_META_REMOVAL_NOTIFICATION``) is sent to the watch queue of each
 201     subscribed watch to indicate that the watch got removed.
 202
 203
 204 Notification Posting API
 205 ========================
 206
 207 To post a notification to watch list so that the subscribed watches can see it,
 208 the following function should be used::
 209
 210         void post_watch_notification(struct watch_list *wlist,
 211                                      struct watch_notification *n,
 212                                      const struct cred *cred,
 213                                      u64 id);
 214
 215 The notification should be preformatted and a pointer to the header (``n``)
 216 should be passed in.  The notification may be larger than this and the size in
 217 units of buffer slots is noted in ``n->info & WATCH_INFO_LENGTH``.
 218
 219 The ``cred`` struct indicates the credentials of the source (subject) and is
 220 passed to the LSMs, such as SELinux, to allow or suppress the recording of the
 221 note in each individual queue according to the credentials of that queue
 222 (object).
 223
 224 The ``id`` is the ID of the source object (such as the serial number on a key).
 225 Only watches that have the same ID set in them will see this notification.
 226
 227
 228 Watch Sources
 229 =============
 230
 231 Any particular buffer can be fed from multiple sources.  Sources include:
 232
 233   * WATCH_TYPE_KEY_NOTIFY
 234
 235     Notifications of this type indicate changes to keys and keyrings, including
 236     the changes of keyring contents or the attributes of keys.
 237
 238     See Documentation/security/keys/core.rst for more information.
 239
 240
 241 Event Filtering
 242 ===============
 243
 244 Once a watch queue has been created, a set of filters can be applied to limit
 245 the events that are received using::
 246
 247         struct watch_notification_filter filter = {
 248                 ...
 249         };
 250         ioctl(fd, IOC_WATCH_QUEUE_SET_FILTER, &filter)
 251
 252 The filter description is a variable of type::
 253
 254         struct watch_notification_filter {
 255                 __u32   nr_filters;
 256                 __u32   __reserved;
 257                 struct watch_notification_type_filter filters[];
 258         };
 259
 260 Where "nr_filters" is the number of filters in filters[] and "__reserved"
 261 should be 0.  The "filters" array has elements of the following type::
 262
 263         struct watch_notification_type_filter {
 264                 __u32   type;
 265                 __u32   info_filter;
 266                 __u32   info_mask;
 267                 __u32   subtype_filter[8];
 268         };
 269
 270 Where:
 271
 272   * ``type`` is the event type to filter for and should be something like
 273     "WATCH_TYPE_KEY_NOTIFY"
 274
 275   * ``info_filter`` and ``info_mask`` act as a filter on the info field of the
 276     notification record.  The notification is only written into the buffer if::
 277
 278         (watch.info & info_mask) == info_filter
 279
 280     This could be used, for example, to ignore events that are not exactly on
 281     the watched point in a mount tree.
 282
 283   * ``subtype_filter`` is a bitmask indicating the subtypes that are of
 284     interest.  Bit 0 of subtype_filter[0] corresponds to subtype 0, bit 1 to
 285     subtype 1, and so on.
 286
 287 If the argument to the ioctl() is NULL, then the filters will be removed and
 288 all events from the watched sources will come through.
 289
 290
 291 Userspace Code Example
 292 ======================
 293
 294 A buffer is created with something like the following::
 295
 296         pipe2(fds, O_TMPFILE);
 297         ioctl(fds[1], IOC_WATCH_QUEUE_SET_SIZE, 256);
 298
 299 It can then be set to receive keyring change notifications::
 300
 301         keyctl(KEYCTL_WATCH_KEY, KEY_SPEC_SESSION_KEYRING, fds[1], 0x01);
 302
 303 The notifications can then be consumed by something like the following::
 304
 305         static void consumer(int rfd, struct watch_queue_buffer *buf)
 306         {
 307                 unsigned char buffer[128];
 308                 ssize_t buf_len;
 309
 310                 while (buf_len = read(rfd, buffer, sizeof(buffer)),
 311                        buf_len > 0
 312                        ) {
 313                         void *p = buffer;
 314                         void *end = buffer + buf_len;
 315                         while (p < end) {
 316                                 union {
 317                                         struct watch_notification n;
 318                                         unsigned char buf1[128];
 319                                 } n;
 320                                 size_t largest, len;
 321
 322                                 largest = end - p;
 323                                 if (largest > 128)
 324                                         largest = 128;
 325                                 memcpy(&n, p, largest);
 326
 327                                 len = (n->info & WATCH_INFO_LENGTH) >>
 328                                         WATCH_INFO_LENGTH__SHIFT;
 329                                 if (len == 0 || len > largest)
 330                                         return;
 331
 332                                 switch (n.n.type) {
 333                                 case WATCH_TYPE_META:
 334                                         got_meta(&n.n);
 335                                 case WATCH_TYPE_KEY_NOTIFY:
 336                                         saw_key_change(&n.n);
 337                                         break;
 338                                 }
 339
 340                                 p += len;
 341                         }
 342                 }
 343         }