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.
fanotify_mark(2),
fanotify(7)