fanotify_init - create and initialize fanotify group
Standard C library (libc, -lc)
#include <fcntl.h> /* Definition of O_* constants */
#include <sys/fanotify.h>
int fanotify_init(unsigned int flags, unsigned int event_f_flags);
For an overview of the fanotify API, see fanotify(7).
fanotify_init() initializes a new fanotify group and
returns a file descriptor for the event queue associated with the group.
The file descriptor is used in calls to fanotify_mark(2) to
specify the files, directories, mounts, or filesystems for which fanotify
events shall be created. These events are received by reading from the file
descriptor. Some events are only informative, indicating that a file has
been accessed. Other events can be used to determine whether another
application is permitted to access a file or directory. Permission to access
filesystem objects is granted by writing to the file descriptor.
Multiple programs may be using the fanotify interface at the same
time to monitor the same files.
The number of fanotify groups per user is limited. See
fanotify(7) for details about this limit.
The flags argument contains a multi-bit field defining the
notification class of the listening application and further single bit
fields specifying the behavior of the file descriptor.
If multiple listeners for permission events exist, the
notification class is used to establish the sequence in which the listeners
receive the events.
Only one of the following notification classes may be specified in
flags:
- FAN_CLASS_PRE_CONTENT
- This value allows the receipt of events notifying that a file has been
accessed and events for permission decisions if a file may be accessed. It
is intended for event listeners that need to access files before they
contain their final data. This notification class might be used by
hierarchical storage managers, for example. Use of this flag requires the
CAP_SYS_ADMIN capability.
- FAN_CLASS_CONTENT
- This value allows the receipt of events notifying that a file has been
accessed and events for permission decisions if a file may be accessed. It
is intended for event listeners that need to access files when they
already contain their final content. This notification class might be used
by malware detection programs, for example. Use of this flag requires the
CAP_SYS_ADMIN capability.
- FAN_CLASS_NOTIF
- This is the default value. It does not need to be specified. This value
only allows the receipt of events notifying that a file has been accessed.
Permission decisions before the file is accessed are not possible.
Listeners with different notification classes will receive events
in the order FAN_CLASS_PRE_CONTENT, FAN_CLASS_CONTENT,
FAN_CLASS_NOTIF. The order of notification for listeners in the same
notification class is undefined.
The following bits can additionally be set in flags:
- FAN_CLOEXEC
- Set the close-on-exec flag (FD_CLOEXEC) on the new file descriptor.
See the description of the O_CLOEXEC flag in open(2).
- FAN_NONBLOCK
- Enable the nonblocking flag (O_NONBLOCK) for the file descriptor.
Reading from the file descriptor will not block. Instead, if no data is
available, read(2) fails with the error EAGAIN.
- FAN_UNLIMITED_QUEUE
- Remove the limit on the number of events in the event queue. See
fanotify(7) for details about this limit. Use of this flag requires
the CAP_SYS_ADMIN capability.
- FAN_UNLIMITED_MARKS
- Remove the limit on the number of fanotify marks per user. See
fanotify(7) for details about this limit. Use of this flag requires
the CAP_SYS_ADMIN capability.
- FAN_REPORT_TID
(since Linux 4.20)
- Report thread ID (TID) instead of process ID (PID) in the pid field
of the struct fanotify_event_metadata supplied to read(2)
(see fanotify(7)). Use of this flag requires the
CAP_SYS_ADMIN capability.
- FAN_ENABLE_AUDIT
(since Linux 4.15)
- Enable generation of audit log records about access mediation performed by
permission events. The permission event response has to be marked with the
FAN_AUDIT flag for an audit log record to be generated. Use of this
flag requires the CAP_AUDIT_WRITE capability.
- FAN_REPORT_FID
(since Linux 5.1)
- This value allows the receipt of events which contain additional
information about the underlying filesystem object correlated to an event.
An additional record of type FAN_EVENT_INFO_TYPE_FID encapsulates
the information about the object and is included alongside the generic
event metadata structure. The file descriptor that is used to represent
the object correlated to an event is instead substituted with a file
handle. It is intended for applications that may find the use of a file
handle to identify an object more suitable than a file descriptor.
Additionally, it may be used for applications monitoring a directory or a
filesystem that are interested in the directory entry modification events
FAN_CREATE, FAN_DELETE, FAN_MOVE, and
FAN_RENAME, or in events such as FAN_ATTRIB,
FAN_DELETE_SELF, and FAN_MOVE_SELF. All the events above
require an fanotify group that identifies filesystem objects by file
handles. Note that without the flag FAN_REPORT_TARGET_FID, for the
directory entry modification events, there is an information record that
identifies the modified directory and not the created/deleted/moved child
object. The use of FAN_CLASS_CONTENT or
FAN_CLASS_PRE_CONTENT is not permitted with this flag and will
result in the error EINVAL. See fanotify(7) for additional
details.
- FAN_REPORT_DIR_FID
(since Linux 5.9)
- Events for fanotify groups initialized with this flag will contain (see
exceptions below) additional information about a directory object
correlated to an event. An additional record of type
FAN_EVENT_INFO_TYPE_DFID encapsulates the information about the
directory object and is included alongside the generic event metadata
structure. For events that occur on a non-directory object, the additional
structure includes a file handle that identifies the parent directory
filesystem object. Note that there is no guarantee that the directory
filesystem object will be found at the location described by the file
handle information at the time the event is received. When combined with
the flag FAN_REPORT_FID, two records may be reported with events
that occur on a non-directory object, one to identify the non-directory
object itself and one to identify the parent directory object. Note that
in some cases, a filesystem object does not have a parent, for example,
when an event occurs on an unlinked but open file. In that case, with the
FAN_REPORT_FID flag, the event will be reported with only one
record to identify the non-directory object itself, because there is no
directory associated with the event. Without the FAN_REPORT_FID
flag, no event will be reported. See fanotify(7) for additional
details.
- FAN_REPORT_NAME
(since Linux 5.9)
- Events for fanotify groups initialized with this flag will contain
additional information about the name of the directory entry correlated to
an event. This flag must be provided in conjunction with the flag
FAN_REPORT_DIR_FID. Providing this flag value without
FAN_REPORT_DIR_FID will result in the error EINVAL. This
flag may be combined with the flag FAN_REPORT_FID. An additional
record of type FAN_EVENT_INFO_TYPE_DFID_NAME, which encapsulates
the information about the directory entry, is included alongside the
generic event metadata structure and substitutes the additional
information record of type FAN_EVENT_INFO_TYPE_DFID. The additional
record includes a file handle that identifies a directory filesystem
object followed by a name that identifies an entry in that directory. For
the directory entry modification events FAN_CREATE,
FAN_DELETE, and FAN_MOVE, the reported name is that of the
created/deleted/moved directory entry. The event FAN_RENAME may
contain two information records. One of type
FAN_EVENT_INFO_TYPE_OLD_DFID_NAME identifying the old directory
entry, and another of type FAN_EVENT_INFO_TYPE_NEW_DFID_NAME
identifying the new directory entry. For other events that occur on a
directory object, the reported file handle is that of the directory object
itself and the reported name is '.'. For other events that occur on a
non-directory object, the reported file handle is that of the parent
directory object and the reported name is the name of a directory entry
where the object was located at the time of the event. The rationale
behind this logic is that the reported directory file handle can be passed
to open_by_handle_at(2) to get an open directory file descriptor
and that file descriptor along with the reported name can be used to call
fstatat(2). The same rule that applies to record type
FAN_EVENT_INFO_TYPE_DFID also applies to record type
FAN_EVENT_INFO_TYPE_DFID_NAME: if a non-directory object has no
parent, either the event will not be reported or it will be reported
without the directory entry information. Note that there is no guarantee
that the filesystem object will be found at the location described by the
directory entry information at the time the event is received. See
fanotify(7) for additional details.
- FAN_REPORT_DFID_NAME
- This is a synonym for
(FAN_REPORT_DIR_FID|FAN_REPORT_NAME).
- FAN_REPORT_TARGET_FID
(since Linux 5.17)
- Events for fanotify groups initialized with this flag will contain
additional information about the child correlated with directory entry
modification events. This flag must be provided in conjunction with the
flags FAN_REPORT_FID, FAN_REPORT_DIR_FID and
FAN_REPORT_NAME. or else the error EINVAL will be returned.
For the directory entry modification events FAN_CREATE,
FAN_DELETE, FAN_MOVE, and FAN_RENAME, an additional
record of type FAN_EVENT_INFO_TYPE_FID, is reported in addition to
the information records of type FAN_EVENT_INFO_TYPE_DFID,
FAN_EVENT_INFO_TYPE_DFID_NAME,
FAN_EVENT_INFO_TYPE_OLD_DFID_NAME, and
FAN_EVENT_INFO_TYPE_NEW_DFID_NAME. The additional record includes a
file handle that identifies the filesystem child object that the directory
entry is referring to.
- FAN_REPORT_DFID_NAME_TARGET
- This is a synonym for
(FAN_REPORT_DFID_NAME|FAN_REPORT_FID|FAN_REPORT_TARGET_FID).
- FAN_REPORT_PIDFD
(since Linux 5.15)
- Events for fanotify groups initialized with this flag will contain an
additional information record alongside the generic
fanotify_event_metadata structure. This information record will be
of type FAN_EVENT_INFO_TYPE_PIDFD and will contain a pidfd for the
process that was responsible for generating an event. A pidfd returned in
this information record object is no different to the pidfd that is
returned when calling pidfd_open(2). Usage of this information
record are for applications that may be interested in reliably determining
whether the process responsible for generating an event has been recycled
or terminated. The use of the FAN_REPORT_TID flag along with
FAN_REPORT_PIDFD is currently not supported and attempting to do so
will result in the error EINVAL being returned. This limitation is
currently imposed by the pidfd API as it currently only supports the
creation of pidfds for thread-group leaders. Creating pidfds for
non-thread-group leaders may be supported at some point in the future, so
this restriction may eventually be lifted. For more details on information
records, see fanotify(7).
The event_f_flags argument defines the file status flags
that will be set on the open file descriptions that are created for fanotify
events. For details of these flags, see the description of the flags
values in open(2). event_f_flags includes a multi-bit field
for the access mode. This field can take the following values:
- O_RDONLY
- This value allows only read access.
- O_WRONLY
- This value allows only write access.
- O_RDWR
- This value allows read and write access.
Additional bits can be set in event_f_flags. The most
useful values are:
- O_LARGEFILE
- Enable support for files exceeding 2 GB. Failing to set this flag
will result in an EOVERFLOW error when trying to open a large file
which is monitored by an fanotify group on a 32-bit system.
- O_CLOEXEC
(since Linux 3.18)
- Enable the close-on-exec flag for the file descriptor. See the description
of the O_CLOEXEC flag in open(2) for reasons why this may be
useful.
The following are also allowable: O_APPEND, O_DSYNC,
O_NOATIME, O_NONBLOCK, and O_SYNC. Specifying any other
flag in event_f_flags yields the error EINVAL (but see
BUGS).
On success, fanotify_init() returns a new file descriptor.
On error, -1 is returned, and errno is set to indicate the error.
- EINVAL
- An invalid value was passed in flags or event_f_flags.
FAN_ALL_INIT_FLAGS (deprecated since Linux 4.20) defines all
allowable bits for flags.
- EMFILE
- The number of fanotify groups for this user exceeds the limit. See
fanotify(7) for details about this limit.
- EMFILE
- The per-process limit on the number of open file descriptors has been
reached.
- ENOMEM
- The allocation of memory for the notification group failed.
- ENOSYS
- This kernel does not implement fanotify_init(). The fanotify API is
available only if the kernel was configured with
CONFIG_FANOTIFY.
- EPERM
- The operation is not permitted because the caller lacks a required
capability.
fanotify_init() was introduced in Linux 2.6.36 and enabled
in Linux 2.6.37.
Prior to Linux 5.13, calling fanotify_init() required the
CAP_SYS_ADMIN capability. Since Linux 5.13, users may call
fanotify_init() without the CAP_SYS_ADMIN capability to create
and initialize an fanotify group with limited functionality.
- The limitations imposed on an
event listener created by a user without the
- CAP_SYS_ADMIN capability are as follows:
- •
- The user cannot request for an unlimited event queue by using
FAN_UNLIMITED_QUEUE.
- •
- The user cannot request for an unlimited number of marks by using
FAN_UNLIMITED_MARKS.
- •
- The user cannot request to use either notification classes
FAN_CLASS_CONTENT or FAN_CLASS_PRE_CONTENT. This means that
user cannot request permission events.
- •
- The user is required to create a group that identifies filesystem objects
by file handles, for example, by providing the FAN_REPORT_FID
flag.
- •
- The user is limited to only mark inodes. The ability to mark a mount or
filesystem via fanotify_mark() through the use of
FAN_MARK_MOUNT or FAN_MARK_FILESYSTEM is not permitted.
- •
- The event object in the event queue is limited in terms of the information
that is made available to the unprivileged user. A user will also not
receive the pid that generated the event, unless the listening process
itself generated the event.
This system call is Linux-specific.
The following bug was present before Linux 3.18:
- •
- The O_CLOEXEC is ignored when passed in event_f_flags.
The following bug was present before Linux 3.14:
- •
- The event_f_flags argument is not checked for invalid flags. Flags
that are intended only for internal use, such as FMODE_EXEC, can be
set, and will consequently be set for the file descriptors returned when
reading from the fanotify file descriptor.