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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.

SEE ALSO

       fanotify_mark(2), fanotify(7)