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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | STANDARDS | HISTORY | BUGS | SEE ALSO | COLOPHON |
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fanotify_init(2) System Calls Manual fanotify_init(2)
fanotify_init - create and initialize fanotify group
Standard C library (libc, -lc)
#include <fcntl.h> /* Definition of O_* constants */
#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.
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, 5.15.154, and 5.10.220)
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 and 5.10.220)
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).
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 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.
Prior to Linux 5.13 (and 5.10.220), calling fanotify_init()
required the CAP_SYS_ADMIN capability. Since Linux 5.13 (and
5.10.220), 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.
Linux.
Linux 2.6.37.
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.
fanotify_mark(2), fanotify(7)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
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2025-02-02. If you discover any rendering problems in this HTML
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[email protected]
Linux man-pages 6.10 2024-07-23 fanotify_init(2)
Pages that refer to this page: fanotify_mark(2), syscalls(2), proc_pid_fdinfo(5), capabilities(7), fanotify(7)
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HTML rendering created 2025-02-02 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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