==============================
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General notification mechanism
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==============================
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The general notification mechanism is built on top of the standard pipe driver
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whereby it effectively splices notification messages from the kernel into pipes
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opened by userspace. This can be used in conjunction with::
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* Key/keyring notifications
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The notifications buffers can be enabled by:
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"General setup"/"General notification queue"
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(CONFIG_WATCH_QUEUE)
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This document has the following sections:
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.. contents:: :local:
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Overview
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========
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This facility appears as a pipe that is opened in a special mode. The pipe's
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internal ring buffer is used to hold messages that are generated by the kernel.
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These messages are then read out by read(). Splice and similar are disabled on
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such pipes due to them wanting to, under some circumstances, revert their
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additions to the ring - which might end up interleaved with notification
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messages.
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The owner of the pipe has to tell the kernel which sources it would like to
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watch through that pipe. Only sources that have been connected to a pipe will
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insert messages into it. Note that a source may be bound to multiple pipes and
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insert messages into all of them simultaneously.
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Filters may also be emplaced on a pipe so that certain source types and
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subevents can be ignored if they're not of interest.
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A message will be discarded if there isn't a slot available in the ring or if
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no preallocated message buffer is available. In both of these cases, read()
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will insert a WATCH_META_LOSS_NOTIFICATION message into the output buffer after
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the last message currently in the buffer has been read.
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Note that when producing a notification, the kernel does not wait for the
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consumers to collect it, but rather just continues on. This means that
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notifications can be generated whilst spinlocks are held and also protects the
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kernel from being held up indefinitely by a userspace malfunction.
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Message Structure
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=================
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Notification messages begin with a short header::
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struct watch_notification {
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__u32 type:24;
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__u32 subtype:8;
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__u32 info;
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};
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"type" indicates the source of the notification record and "subtype" indicates
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the type of record from that source (see the Watch Sources section below). The
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type may also be "WATCH_TYPE_META". This is a special record type generated
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internally by the watch queue itself. There are two subtypes:
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* WATCH_META_REMOVAL_NOTIFICATION
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* WATCH_META_LOSS_NOTIFICATION
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The first indicates that an object on which a watch was installed was removed
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or destroyed and the second indicates that some messages have been lost.
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"info" indicates a bunch of things, including:
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* The length of the message in bytes, including the header (mask with
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WATCH_INFO_LENGTH and shift by WATCH_INFO_LENGTH__SHIFT). This indicates
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the size of the record, which may be between 8 and 127 bytes.
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* The watch ID (mask with WATCH_INFO_ID and shift by WATCH_INFO_ID__SHIFT).
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This indicates that caller's ID of the watch, which may be between 0
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and 255. Multiple watches may share a queue, and this provides a means to
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distinguish them.
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* A type-specific field (WATCH_INFO_TYPE_INFO). This is set by the
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notification producer to indicate some meaning specific to the type and
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subtype.
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Everything in info apart from the length can be used for filtering.
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The header can be followed by supplementary information. The format of this is
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at the discretion is defined by the type and subtype.
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Watch List (Notification Source) API
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====================================
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A "watch list" is a list of watchers that are subscribed to a source of
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notifications. A list may be attached to an object (say a key or a superblock)
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or may be global (say for device events). From a userspace perspective, a
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non-global watch list is typically referred to by reference to the object it
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belongs to (such as using KEYCTL_NOTIFY and giving it a key serial number to
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watch that specific key).
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To manage a watch list, the following functions are provided:
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* ::
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void init_watch_list(struct watch_list *wlist,
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void (*release_watch)(struct watch *wlist));
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Initialise a watch list. If ``release_watch`` is not NULL, then this
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indicates a function that should be called when the watch_list object is
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destroyed to discard any references the watch list holds on the watched
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object.
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* ``void remove_watch_list(struct watch_list *wlist);``
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This removes all of the watches subscribed to a watch_list and frees them
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and then destroys the watch_list object itself.
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Watch Queue (Notification Output) API
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=====================================
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A "watch queue" is the buffer allocated by an application that notification
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records will be written into. The workings of this are hidden entirely inside
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of the pipe device driver, but it is necessary to gain a reference to it to set
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a watch. These can be managed with:
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* ``struct watch_queue *get_watch_queue(int fd);``
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Since watch queues are indicated to the kernel by the fd of the pipe that
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implements the buffer, userspace must hand that fd through a system call.
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This can be used to look up an opaque pointer to the watch queue from the
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system call.
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* ``void put_watch_queue(struct watch_queue *wqueue);``
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This discards the reference obtained from ``get_watch_queue()``.
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Watch Subscription API
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======================
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A "watch" is a subscription on a watch list, indicating the watch queue, and
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thus the buffer, into which notification records should be written. The watch
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queue object may also carry filtering rules for that object, as set by
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userspace. Some parts of the watch struct can be set by the driver::
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struct watch {
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union {
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u32 info_id; /* ID to be OR'd in to info field */
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...
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};
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void *private; /* Private data for the watched object */
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u64 id; /* Internal identifier */
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...
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};
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The ``info_id`` value should be an 8-bit number obtained from userspace and
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shifted by WATCH_INFO_ID__SHIFT. This is OR'd into the WATCH_INFO_ID field of
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struct watch_notification::info when and if the notification is written into
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the associated watch queue buffer.
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The ``private`` field is the driver's data associated with the watch_list and
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is cleaned up by the ``watch_list::release_watch()`` method.
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The ``id`` field is the source's ID. Notifications that are posted with a
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different ID are ignored.
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The following functions are provided to manage watches:
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* ``void init_watch(struct watch *watch, struct watch_queue *wqueue);``
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Initialise a watch object, setting its pointer to the watch queue, using
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appropriate barriering to avoid lockdep complaints.
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* ``int add_watch_to_object(struct watch *watch, struct watch_list *wlist);``
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Subscribe a watch to a watch list (notification source). The
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driver-settable fields in the watch struct must have been set before this
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is called.
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* ::
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int remove_watch_from_object(struct watch_list *wlist,
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struct watch_queue *wqueue,
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u64 id, false);
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Remove a watch from a watch list, where the watch must match the specified
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watch queue (``wqueue``) and object identifier (``id``). A notification
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(``WATCH_META_REMOVAL_NOTIFICATION``) is sent to the watch queue to
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indicate that the watch got removed.
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* ``int remove_watch_from_object(struct watch_list *wlist, NULL, 0, true);``
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Remove all the watches from a watch list. It is expected that this will be
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called preparatory to destruction and that the watch list will be
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inaccessible to new watches by this point. A notification
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(``WATCH_META_REMOVAL_NOTIFICATION``) is sent to the watch queue of each
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subscribed watch to indicate that the watch got removed.
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Notification Posting API
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========================
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To post a notification to watch list so that the subscribed watches can see it,
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the following function should be used::
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void post_watch_notification(struct watch_list *wlist,
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struct watch_notification *n,
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const struct cred *cred,
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u64 id);
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The notification should be preformatted and a pointer to the header (``n``)
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should be passed in. The notification may be larger than this and the size in
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units of buffer slots is noted in ``n->info & WATCH_INFO_LENGTH``.
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The ``cred`` struct indicates the credentials of the source (subject) and is
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passed to the LSMs, such as SELinux, to allow or suppress the recording of the
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note in each individual queue according to the credentials of that queue
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(object).
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The ``id`` is the ID of the source object (such as the serial number on a key).
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Only watches that have the same ID set in them will see this notification.
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Watch Sources
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=============
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Any particular buffer can be fed from multiple sources. Sources include:
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* WATCH_TYPE_KEY_NOTIFY
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Notifications of this type indicate changes to keys and keyrings, including
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the changes of keyring contents or the attributes of keys.
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See Documentation/security/keys/core.rst for more information.
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Event Filtering
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===============
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Once a watch queue has been created, a set of filters can be applied to limit
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the events that are received using::
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struct watch_notification_filter filter = {
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...
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};
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ioctl(fd, IOC_WATCH_QUEUE_SET_FILTER, &filter)
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The filter description is a variable of type::
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struct watch_notification_filter {
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__u32 nr_filters;
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__u32 __reserved;
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struct watch_notification_type_filter filters[];
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};
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Where "nr_filters" is the number of filters in filters[] and "__reserved"
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should be 0. The "filters" array has elements of the following type::
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struct watch_notification_type_filter {
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__u32 type;
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__u32 info_filter;
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__u32 info_mask;
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__u32 subtype_filter[8];
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};
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Where:
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* ``type`` is the event type to filter for and should be something like
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"WATCH_TYPE_KEY_NOTIFY"
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* ``info_filter`` and ``info_mask`` act as a filter on the info field of the
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notification record. The notification is only written into the buffer if::
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(watch.info & info_mask) == info_filter
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This could be used, for example, to ignore events that are not exactly on
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the watched point in a mount tree.
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* ``subtype_filter`` is a bitmask indicating the subtypes that are of
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interest. Bit 0 of subtype_filter[0] corresponds to subtype 0, bit 1 to
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subtype 1, and so on.
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If the argument to the ioctl() is NULL, then the filters will be removed and
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all events from the watched sources will come through.
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Userspace Code Example
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======================
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A buffer is created with something like the following::
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pipe2(fds, O_TMPFILE);
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ioctl(fds[1], IOC_WATCH_QUEUE_SET_SIZE, 256);
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It can then be set to receive keyring change notifications::
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keyctl(KEYCTL_WATCH_KEY, KEY_SPEC_SESSION_KEYRING, fds[1], 0x01);
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The notifications can then be consumed by something like the following::
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static void consumer(int rfd, struct watch_queue_buffer *buf)
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{
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unsigned char buffer[128];
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ssize_t buf_len;
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while (buf_len = read(rfd, buffer, sizeof(buffer)),
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buf_len > 0
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) {
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void *p = buffer;
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void *end = buffer + buf_len;
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while (p < end) {
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union {
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struct watch_notification n;
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unsigned char buf1[128];
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} n;
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size_t largest, len;
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largest = end - p;
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if (largest > 128)
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largest = 128;
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memcpy(&n, p, largest);
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len = (n->info & WATCH_INFO_LENGTH) >>
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WATCH_INFO_LENGTH__SHIFT;
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if (len == 0 || len > largest)
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return;
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switch (n.n.type) {
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case WATCH_TYPE_META:
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got_meta(&n.n);
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case WATCH_TYPE_KEY_NOTIFY:
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saw_key_change(&n.n);
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break;
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}
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p += len;
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}
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}
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}
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