fanotify_init
Section: System Calls (2)
Updated: 2023-02-05
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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 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).
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 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.
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)
Index
- NAME
-
- LIBRARY
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- ERRORS
-
- VERSIONS
-
- STANDARDS
-
- BUGS
-
- SEE ALSO
-
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Time: 00:42:35 GMT, April 20, 2024