fanotify_init

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



NAME
       fanotify_init - create and initialize fanotify group

SYNOPSIS
       #include <fcntl.h>
       #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.

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

CONFORMING TO
       This system call is Linux-specific.

BUGS
       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
       fanotify_mark(2), fanotify(7)

COLOPHON
       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)