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 5.09 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)procfs(5)capabilities(7)fanotify(7)