FANOTIFY_INIT(2) Linux Programmer's Manual FANOTIFY_INIT(2)
NAME
fanotify_init - create and initialize fanotify group
SYNOPSIS
#include <fcntl.h>
#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.
In the current implementation, the number of fanotify groups per user
is limited to 128. This limit cannot be overridden.
Calling fanotify_init() requires the CAP_SYS_ADMIN capability. This
constraint might be relaxed in future versions of the API. Therefore,
certain additional capability checks have been implemented as indicated
below.
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.
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.
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 of 16384 events for the event queue. Use of
this flag requires the CAP_SYS_ADMIN capability.
FAN_UNLIMITED_MARKS
Remove the limit of 8192 marks. 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)).
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, and FAN_MOVE, or in events
such as FAN_ATTRIB, FAN_DELETE_SELF, and FAN_MOVE_SELF. All the
events above require an fanotify group that identifies filesys-
tem objects by file handles. Note that for the directory entry
modification events the reported file handle identifies the mod-
ified 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 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. For other events that occur on a directory ob-
ject, the reported file handle is that of the directory object
itself and the reported name is '.'. For other events that oc-
cur 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 re-
ported 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 de-
tails.
FAN_REPORT_DFID_NAME
This is a synonym for (FAN_REPORT_DIR_FID|FAN_REPORT_NAME).
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 kernel version 4.20)
defines all allowable bits for flags.
EMFILE The number of fanotify groups for this user exceeds 128.
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 the
CAP_SYS_ADMIN capability.
VERSIONS
fanotify_init() was introduced in version 2.6.36 of the Linux kernel
and enabled in version 2.6.37.
CONFORMING TO
This system call is Linux-specific.
BUGS
The following bug was present in Linux kernels before version 3.18:
* The O_CLOEXEC is ignored when passed in event_f_flags.
The following bug was present in Linux kernels before version 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)
COLOPHON
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