Ubuntu Manpage: fanotify_init - create and initialize fanotify group

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

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

       Linux.

HISTORY

       Linux 2.6.37.

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.