fanotify_init(2) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | BUGS | SEE ALSO | COLOPHON


FANOTIFY_INIT(2)        Linux Programmer's Manual       FANOTIFY_INIT(2)

NAME         top

       fanotify_init - create and initialize fanotify group

SYNOPSIS         top

       #include <fcntl.h>
       #include <sys/fanotify.h>

       int fanotify_init(unsigned int flags, unsigned int
       event_f_flags);

DESCRIPTION         top

       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_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)).

       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, and FAN_MOVE, 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 for the directory
              entry modification events the reported file handle
              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.  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).

       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         top

       On success, fanotify_init() returns a new file descriptor.  On
       error, -1 is returned, and errno is set to indicate the error.

ERRORS         top

       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.

VERSIONS         top

       fanotify_init() was introduced in version 2.6.36 of the Linux
       kernel and enabled in version 2.6.37.

CONFORMING TO         top

       This system call is Linux-specific.

BUGS         top

       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 ALSO         top

       fanotify_mark(2), fanotify(7)

COLOPHON         top

       This page is part of release 5.10 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                          2020-11-01               FANOTIFY_INIT(2)

Pages that refer to this page: fanotify_mark(2)syscalls(2)proc(5)capabilities(7)fanotify(7)