fanotify_init - create and initialize fanotify group
#include <fcntl.h>
#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.
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 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.
- 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.
- 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 structure 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 that
are interested in directory entry events, such as FAN_CREATE,
FAN_ATTRIB, FAN_MOVE, and FAN_DELETE for example.
Note that the use of directory modification events are not supported when
monitoring a mount point. 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
information.
- 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 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)).
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 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
CONFIG_FANOTIFY.
- EPERM
- The operation is not permitted because the caller lacks the
CAP_SYS_ADMIN capability.
fanotify_init() was introduced in version 2.6.36 of the Linux kernel and
enabled in version 2.6.37.
This system call is Linux-specific.
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.
fanotify_mark(2),
fanotify(7)