fanotify_init(2) System Calls Manual fanotify_init(2)
NAME
fanotify_init - create and initialize fanotify group
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#include <fcntl.h> /* Definition of O_* constants */
#include <sys/fanotify.h>
int fanotify_init(unsigned int flags, unsigned int event_f_flags);
DESCRIPTION
For an overview of the fanotify API, see fanotify(7).
fanotify_init() initializes a new fanotify group and returns a file de-
scriptor 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 an-
other application is permitted to access a file or directory. Permis-
sion to access filesystem objects is granted by writing to the file de-
scriptor.
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 speci-
fying 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 no-
tification class might be used by hierarchical storage managers,
for example. Use of this flag requires the CAP_SYS_ADMIN capa-
bility.
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 pro-
grams, 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 descrip-
tor. See the description of the O_CLOEXEC flag in open(2).
FAN_NONBLOCK
Enable the nonblocking flag (O_NONBLOCK) for the file descrip-
tor. 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 re-
quires 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 de-
scriptor 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 iden-
tify an object more suitable than a file descriptor. Addition-
ally, it may be used for applications monitoring a directory or
a filesystem that are interested in the directory entry modifi-
cation 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 identi-
fies 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 de-
tails.
FAN_REPORT_DIR_FID (since Linux 5.9)
Events for fanotify groups initialized with this flag will con-
tain (see exceptions below) additional information about a di-
rectory 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 informa-
tion 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 associ-
ated 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 con-
tain additional information about the name of the directory en-
try correlated to an event. This flag must be provided in con-
junction with the flag FAN_REPORT_DIR_FID. Providing this flag
value without FAN_REPORT_DIR_FID will result in the error EIN-
VAL. 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 sub-
stitutes 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 di-
rectory 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 informa-
tion 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 direc-
tory entry where the object was located at the time of the
event. The rationale behind this logic is that the reported di-
rectory 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 re-
ported or it will be reported without the directory entry infor-
mation. Note that there is no guarantee that the filesystem ob-
ject will be found at the location described by the directory
entry information at the time the event is received. See fan-
otify(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 con-
tain additional information about the child correlated with di-
rectory 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 in-
formation 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 in-
cludes 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_RE-
PORT_FID|FAN_REPORT_TARGET_FID).
FAN_REPORT_PIDFD (since Linux 5.15)
Events for fanotify groups initialized with this flag will con-
tain an additional information record alongside the generic fan-
otify_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 in-
terested 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 cre-
ation 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_NON-
BLOCK, and O_SYNC. Specifying any other flag in event_f_flags yields
the error EINVAL (but see BUGS).
RETURN VALUE
On success, fanotify_init() returns a new file descriptor. On error,
-1 is returned, and errno is set to indicate the error.
ERRORS
EINVAL An invalid value was passed in flags or event_f_flags.
FAN_ALL_INIT_FLAGS (deprecated since Linux 4.20) defines all al-
lowable 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 CON-
FIG_FANOTIFY.
EPERM The operation is not permitted because the caller lacks a re-
quired capability.
VERSIONS
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.
STANDARDS
This system call is Linux-specific.
BUGS
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.
SEE ALSO
fanotify_mark(2), fanotify(7)
Linux man-pages 6.03 2023-02-05 fanotify_init(2)
Generated by dwww version 1.15 on Wed Jun 26 10:58:27 CEST 2024.