FANOTIFY_INIT(2) Linux Programmer's Manual FANOTIFY_INIT(2)NAMEfanotify_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
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, and mounts for which fanotify events shall be cre‐
ated. 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 appli‐
cation 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.
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
notification class might be used by hierarchical storage man‐
agers, 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.
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 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.
VERSIONSfanotify_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 ALSOfanotify_mark(2), fanotify(7)COLOPHON
This page is part of release 4.14 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2017-09-15 FANOTIFY_INIT(2)
