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NAME

       fanotify - monitoring filesystem events

DESCRIPTION

       The  fanotify  API provides notification and interception of filesystem events.  Use cases
       include virus scanning and hierarchical storage management.  In the original fanotify API,
       only  a  limited  set  of  events  was supported.  In particular, there was no support for
       create, delete, and move events.  The support for those events was  added  in  Linux  5.1.
       (See inotify(7) for details of an API that did notify those events pre Linux 5.1.)

       Additional  capabilities compared to the inotify(7) API include the ability to monitor all
       of the objects in a mounted filesystem, the ability to make access  permission  decisions,
       and the possibility to read or modify files before access by other applications.

       The  following  system  calls  are used with this API: fanotify_init(2), fanotify_mark(2),
       read(2), write(2), and close(2).

   fanotify_init(), fanotify_mark(), and notification groups
       The fanotify_init(2) system call creates and initializes an  fanotify  notification  group
       and returns a file descriptor referring to it.

       An  fanotify  notification  group  is a kernel-internal object that holds a list of files,
       directories, filesystems, and mounts for which events shall be created.

       For each entry in an fanotify notification group, two bit masks exist: the mark  mask  and
       the  ignore  mask.   The  mark  mask  defines  file activities for which an event shall be
       created.  The ignore mask defines activities  for  which  no  event  shall  be  generated.
       Having these two types of masks permits a filesystem, mount, or directory to be marked for
       receiving events, while at the same time ignoring events  for  specific  objects  under  a
       mount or directory.

       The  fanotify_mark(2)  system  call  adds  a  file,  directory,  filesystem, or mount to a
       notification group and specifies which events shall be reported (or ignored),  or  removes
       or modifies such an entry.

       A  possible  usage  of the ignore mask is for a file cache.  Events of interest for a file
       cache are modification of a file and closing of the same.  Hence, the cached directory  or
       mount  is to be marked to receive these events.  After receiving the first event informing
       that a file has been modified, the corresponding cache  entry  will  be  invalidated.   No
       further  modification  events  for  this  file  are  of interest until the file is closed.
       Hence, the modify event can be added to the ignore mask.  Upon receiving the close  event,
       the  modify  event  can  be  removed  from the ignore mask and the file cache entry can be
       updated.

       The entries in the fanotify notification groups refer to files and directories  via  their
       inode  number  and  to  mounts via their mount ID.  If files or directories are renamed or
       moved within the same mount, the respective entries survive.  If files or directories  are
       deleted  or  moved  to  another  mount  or  if  filesystems  or  mounts are unmounted, the
       corresponding entries are deleted.

   The event queue
       As events occur on the filesystem objects monitored by a notification group, the  fanotify
       system  generates  events  that  are  collected in a queue.  These events can then be read
       (using read(2) or similar) from the fanotify file descriptor returned by fanotify_init(2).

       Two  types  of  events  are  generated:  notification  events   and   permission   events.
       Notification  events  are  merely  informative  and  require  no action to be taken by the
       receiving application with one exception: if a valid file descriptor is provided within  a
       generic  event, the file descriptor must be closed.  Permission events are requests to the
       receiving application to decide whether permission for a file  access  shall  be  granted.
       For  these  events,  the  recipient  must write a response which decides whether access is
       granted or not.

       An event is removed from the event queue of the fanotify group  when  it  has  been  read.
       Permission  events  that have been read are kept in an internal list of the fanotify group
       until either a permission decision  has  been  taken  by  writing  to  the  fanotify  file
       descriptor or the fanotify file descriptor is closed.

   Reading fanotify events
       Calling  read(2)  for the file descriptor returned by fanotify_init(2) blocks (if the flag
       FAN_NONBLOCK is not specified in the call to fanotify_init(2)) until either a  file  event
       occurs or the call is interrupted by a signal (see signal(7)).

       After  a  successful  read(2),  the  read  buffer  contains  one  or more of the following
       structures:

           struct fanotify_event_metadata {
               __u32 event_len;
               __u8 vers;
               __u8 reserved;
               __u16 metadata_len;
               __aligned_u64 mask;
               __s32 fd;
               __s32 pid;
           };

       Information records are supplemental pieces of information that may be provided  alongside
       the  generic fanotify_event_metadata structure.  The flags passed to fanotify_init(2) have
       influence over the type of information records that may be returned  for  an  event.   For
       example, if a notification group is initialized with FAN_REPORT_FID or FAN_REPORT_DIR_FID,
       then event listeners should also expect to  receive  a  fanotify_event_info_fid  structure
       alongside the fanotify_event_metadata structure, whereby file handles are used to identify
       filesystem objects rather than file descriptors.  Information records may also be stacked,
       meaning  that  using  the  various  FAN_REPORT_*  flags in conjunction with one another is
       supported.  In such cases, multiple information records  can  be  returned  for  an  event
       alongside  the  generic fanotify_event_metadata structure.  For example, if a notification
       group is initialized  with  FAN_REPORT_TARGET_FID  and  FAN_REPORT_PIDFD,  then  an  event
       listener  should  expect  to receive up to two fanotify_event_info_fid information records
       and   one   fanotify_event_info_pidfd   information   record   alongside    the    generic
       fanotify_event_metadata structure.  Importantly, fanotify provides no guarantee around the
       ordering of information records when a notification group is initialized  with  a  stacked
       based   configuration.    Each   information   record  has  a  nested  structure  of  type
       fanotify_event_info_header.  It is imperative for event listeners to inspect the info_type
       field of this structure in order to determine the type of information record that had been
       received for a given event.

       In cases where an fanotify group identifies filesystem  objects  by  file  handles,  event
       listeners  should  also  expect  to  receive  one  or more of the below information record
       objects alongside the generic fanotify_event_metadata structure within the read buffer:

           struct fanotify_event_info_fid {
               struct fanotify_event_info_header hdr;
               __kernel_fsid_t fsid;
               unsigned char handle[];
           };

       In cases where an fanotify group is initialized  with  FAN_REPORT_PIDFD,  event  listeners
       should  expect  to  receive  the  below  information  record  object alongside the generic
       fanotify_event_metadata structure within the read buffer:

           struct fanotify_event_info_pidfd {
                   struct fanotify_event_info_header hdr;
                   __s32 pidfd;
           };

       In case of a FAN_FS_ERROR event, an additional information  record  describing  the  error
       that  occurred  is returned alongside the generic fanotify_event_metadata structure within
       the read buffer.  This structure is defined as follows:

           struct fanotify_event_info_error {
               struct fanotify_event_info_header hdr;
               __s32 error;
               __u32 error_count;
           };

       All information records contain a nested  structure  of  type  fanotify_event_info_header.
       This  structure  holds  meta-information  about  the information record that may have been
       returned alongside the  generic  fanotify_event_metadata  structure.   This  structure  is
       defined as follows:

           struct fanotify_event_info_header {
                __u8 info_type;
                __u8 pad;
                __u16 len;
           };

       For  performance  reasons, it is recommended to use a large buffer size (for example, 4096
       bytes), so that multiple events can be retrieved by a single read(2).

       The return value of read(2) is the number of bytes placed in the buffer, or -1 in case  of
       an error (but see BUGS).

       The fields of the fanotify_event_metadata structure are as follows:

       event_len
              This  is  the  length  of the data for the current event and the offset to the next
              event in the buffer.  Unless  the  group  identifies  filesystem  objects  by  file
              handles, the value of event_len is always FAN_EVENT_METADATA_LEN.  For a group that
              identifies filesystem objects by file handles, event_len also includes the variable
              length file identifier records.

       vers   This  field  holds  a  version  number  for  the structure.  It must be compared to
              FANOTIFY_METADATA_VERSION to verify that the structures returned at run time  match
              the  structures  defined  at  compile time.  In case of a mismatch, the application
              should abandon trying to use the fanotify file descriptor.

       reserved
              This field is not used.

       metadata_len
              This is the length of the structure.  The field was introduced  to  facilitate  the
              implementation  of optional headers per event type.  No such optional headers exist
              in the current implementation.

       mask   This is a bit mask describing the event (see below).

       fd     This is an open file descriptor for the object being accessed,  or  FAN_NOFD  if  a
              queue overflow occurred.  With an fanotify group that identifies filesystem objects
              by file handles, applications should expect this value to be set  to  FAN_NOFD  for
              each  event  that  is  received.   The  file  descriptor  can be used to access the
              contents  of  the  monitored  file  or  directory.   The  reading  application   is
              responsible for closing this file descriptor.

              When  calling  fanotify_init(2),  the  caller  may  specify  (via the event_f_flags
              argument) various file status flags that are to be set on the open file description
              that  corresponds  to  this  file  descriptor.   In addition, the (kernel-internal)
              FMODE_NONOTIFY file status flag is set on the open  file  description.   This  flag
              suppresses  fanotify  event  generation.   Hence, when the receiver of the fanotify
              event accesses the notified file  or  directory  using  this  file  descriptor,  no
              additional events will be created.

       pid    If  flag  FAN_REPORT_TID was set in fanotify_init(2), this is the TID of the thread
              that caused the event.  Otherwise, this the PID of  the  process  that  caused  the
              event.

       A  program  listening  to  fanotify  events  can  compare  this PID to the PID returned by
       getpid(2), to determine whether the event is caused by the listener itself, or is due to a
       file access by another process.

       The  bit mask in mask indicates which events have occurred for a single filesystem object.
       Multiple bits may be set in this mask, if more than one event occurred for  the  monitored
       filesystem  object.   In particular, consecutive events for the same filesystem object and
       originating from the same process may be merged into a single event,  with  the  exception
       that two permission events are never merged into one queue entry.

       The bits that may appear in mask are as follows:

       FAN_ACCESS
              A file or a directory (but see BUGS) was accessed (read).

       FAN_OPEN
              A file or a directory was opened.

       FAN_OPEN_EXEC
              A  file  was  opened with the intent to be executed.  See NOTES in fanotify_mark(2)
              for additional details.

       FAN_ATTRIB
              A file or directory metadata was changed.

       FAN_CREATE
              A child file or directory was created in a watched parent.

       FAN_DELETE
              A child file or directory was deleted in a watched parent.

       FAN_DELETE_SELF
              A watched file or directory was deleted.

       FAN_FS_ERROR
              A filesystem error was detected.

       FAN_RENAME
              A file or directory has been moved to or from a watched parent directory.

       FAN_MOVED_FROM
              A file or directory has been moved from a watched parent directory.

       FAN_MOVED_TO
              A file or directory has been moved to a watched parent directory.

       FAN_MOVE_SELF
              A watched file or directory was moved.

       FAN_MODIFY
              A file was modified.

       FAN_CLOSE_WRITE
              A file that was opened for writing (O_WRONLY or O_RDWR) was closed.

       FAN_CLOSE_NOWRITE
              A file or directory that was opened read-only (O_RDONLY) was closed.

       FAN_Q_OVERFLOW
              The event queue exceeded the  limit  on  number  of  events.   This  limit  can  be
              overridden    by    specifying    the   FAN_UNLIMITED_QUEUE   flag   when   calling
              fanotify_init(2).

       FAN_ACCESS_PERM
              An application wants to read a file or directory,  for  example  using  read(2)  or
              readdir(2).   The reader must write a response (as described below) that determines
              whether the permission to access the filesystem object shall be granted.

       FAN_OPEN_PERM
              An application wants to open a file or directory.  The reader must write a response
              that  determines  whether  the  permission  to  open the filesystem object shall be
              granted.

       FAN_OPEN_EXEC_PERM
              An application wants to open a  file  for  execution.   The  reader  must  write  a
              response  that  determines whether the permission to open the filesystem object for
              execution shall be granted.  See NOTES in fanotify_mark(2) for additional details.

       To check for any close event, the following bit mask may be used:

       FAN_CLOSE
              A file was closed.  This is a synonym for:

                  FAN_CLOSE_WRITE | FAN_CLOSE_NOWRITE

       To check for any move event, the following bit mask may be used:

       FAN_MOVE
              A file or directory was moved.  This is a synonym for:

                  FAN_MOVED_FROM | FAN_MOVED_TO

       The following bits may appear in mask only in conjunction with other event type bits:

       FAN_ONDIR
              The events described in the mask have occurred on a  directory  object.   Reporting
              events   on  directories  requires  setting  this  flag  in  the  mark  mask.   See
              fanotify_mark(2) for additional details.  The FAN_ONDIR  flag  is  reported  in  an
              event  mask  only  if  the  fanotify  group  identifies  filesystem objects by file
              handles.

       Information records  that  are  supplied  alongside  the  generic  fanotify_event_metadata
       structure  will always contain a nested structure of type fanotify_event_info_header.  The
       fields of the fanotify_event_info_header are as follows:

       info_type
              A unique integer value representing the type of information record object  received
              for  an  event.   The  value  of  this  field  can  be set to one of the following:
              FAN_EVENT_INFO_TYPE_FID,  FAN_EVENT_INFO_TYPE_DFID,  FAN_EVENT_INFO_TYPE_DFID_NAME,
              or  FAN_EVENT_INFO_TYPE_PIDFD.   The  value  set for this field is dependent on the
              flags that have been supplied to fanotify_init(2).  Refer to the field  details  of
              each  information  record  object  type  below to understand the different cases in
              which the info_type values can be set.

       pad    This field is currently  not  used  by  any  information  record  object  type  and
              therefore is set to zero.

       len    The value of len is set to the size of the information record object, including the
              fanotify_event_info_header.  The total size of all additional  information  records
              is not expected to be larger than (event_len - metadata_len).

       The fields of the fanotify_event_info_fid structure are as follows:

       hdr    This  is  a  structure  of  type  fanotify_event_info_header.  For example, when an
              fanotify file descriptor is created  using  FAN_REPORT_FID,  a  single  information
              record  is  expected  to  be  attached  to  the event with info_type field value of
              FAN_EVENT_INFO_TYPE_FID.  When an fanotify file descriptor  is  created  using  the
              combination  of FAN_REPORT_FID and FAN_REPORT_DIR_FID, there may be two information
              records  attached   to   the   event:   one   with   info_type   field   value   of
              FAN_EVENT_INFO_TYPE_DFID,  identifying  a  parent  directory  object,  and one with
              info_type field value of FAN_EVENT_INFO_TYPE_FID, identifying a child object.  Note
              that  for the directory entry modification events FAN_CREATE, FAN_DELETE, FAN_MOVE,
              and FAN_RENAME, an information record identifying the  created/deleted/moved  child
              object  is  reported  only  if  an  fanotify  group  was  initialized with the flag
              FAN_REPORT_TARGET_FID.

       fsid   This is a unique identifier of the filesystem containing the object associated with
              the  event.   It is a structure of type __kernel_fsid_t and contains the same value
              as f_fsid when calling statfs(2).

       handle This field contains a variable-length structure of type struct file_handle.  It  is
              an opaque handle that corresponds to a specified object on a filesystem as returned
              by name_to_handle_at(2).  It  can  be  used  to  uniquely  identify  a  file  on  a
              filesystem  and can be passed as an argument to open_by_handle_at(2).  If the value
              of info_type field is FAN_EVENT_INFO_TYPE_DFID_NAME, the file handle is followed by
              a  null terminated string that identifies the created/deleted/moved directory entry
              name.   For  other  events  such  as  FAN_OPEN,  FAN_ATTRIB,  FAN_DELETE_SELF,  and
              FAN_MOVE_SELF,  if  the  value  of  info_type field is FAN_EVENT_INFO_TYPE_FID, the
              handle identifies the object correlated to the event.  If the  value  of  info_type
              field  is  FAN_EVENT_INFO_TYPE_DFID,  the  handle  identifies  the directory object
              correlated to  the  event  or  the  parent  directory  of  a  non-directory  object
              correlated    to    the    event.    If   the   value   of   info_type   field   is
              FAN_EVENT_INFO_TYPE_DFID_NAME, the handle identifies the same directory object that
              would  be reported with FAN_EVENT_INFO_TYPE_DFID and the file handle is followed by
              a null terminated string that identifies the name of  a  directory  entry  in  that
              directory, or '.' to identify the directory object itself.

       The fields of the fanotify_event_info_pidfd structure are as follows:

       hdr    This  is a structure of type fanotify_event_info_header.  When an fanotify group is
              initialized  using   FAN_REPORT_PIDFD,   the   info_type   field   value   of   the
              fanotify_event_info_header is set to FAN_EVENT_INFO_TYPE_PIDFD.

       pidfd  This  is  a  process  file  descriptor  that  refers to the process responsible for
              generating the event.  The returned process file descriptor is  no  different  from
              one  which  could  be  obtained  manually  if  pidfd_open(2)  were  to be called on
              fanotify_event_metadata.pid.  In the instance that an error is  encountered  during
              pidfd  creation,  one of two possible error types represented by a negative integer
              value may be returned in this pidfd field.  In cases where the process  responsible
              for  generating  the event has terminated prior to the event listener being able to
              read events from the  notification  queue,  FAN_NOPIDFD  is  returned.   The  pidfd
              creation  for  an  event is only performed at the time the events are read from the
              notification queue.  All other possible pidfd creation failures are represented  by
              FAN_EPIDFD.   Once  the  event listener has dealt with an event and the pidfd is no
              longer required, the pidfd should be closed via close(2).

       The fields of the fanotify_event_info_error structure are as follows:

       hdr    This is a structure of type fanotify_event_info_header.  The info_type field is set
              to FAN_EVENT_INFO_TYPE_ERROR.

       error  Identifies the type of error that occurred.

       error_count
              This is a counter of the number of errors suppressed since the last error was read.

       The  following  macros  are  provided  to  iterate over a buffer containing fanotify event
       metadata returned by a read(2) from an fanotify file descriptor:

       FAN_EVENT_OK(meta, len)
              This macro checks the remaining length len of the buffer meta against the length of
              the  metadata  structure and the event_len field of the first metadata structure in
              the buffer.

       FAN_EVENT_NEXT(meta, len)
              This macro uses the length  indicated  in  the  event_len  field  of  the  metadata
              structure  pointed  to  by  meta  to  calculate  the  address  of the next metadata
              structure that follows meta.  len is the number of bytes of metadata that currently
              remain  in  the buffer.  The macro returns a pointer to the next metadata structure
              that follows meta, and reduces len by the number of bytes in the metadata structure
              that has been skipped over (i.e., it subtracts meta->event_len from len).

       In addition, there is:

       FAN_EVENT_METADATA_LEN
              This  macro  returns  the size (in bytes) of the structure fanotify_event_metadata.
              This is the minimum size (and currently the only size) of any event metadata.

   Monitoring an fanotify file descriptor for events
       When an fanotify event occurs, the fanotify file descriptor  indicates  as  readable  when
       passed to epoll(7), poll(2), or select(2).

   Dealing with permission events
       For  permission events, the application must write(2) a structure of the following form to
       the fanotify file descriptor:

           struct fanotify_response {
               __s32 fd;
               __u32 response;
           };

       The fields of this structure are as follows:

       fd     This is the file descriptor from the structure fanotify_event_metadata.

       response
              This field indicates whether or not the permission is to  be  granted.   Its  value
              must  be  either FAN_ALLOW to allow the file operation or FAN_DENY to deny the file
              operation.

       If access is denied,  the  requesting  application  call  will  receive  an  EPERM  error.
       Additionally,  if  the notification group has been created with the FAN_ENABLE_AUDIT flag,
       then the FAN_AUDIT flag can be set in  the  response  field.   In  that  case,  the  audit
       subsystem will log information about the access decision to the audit logs.

   Monitoring filesystems for errors
       A  single  FAN_FS_ERROR  event is stored per filesystem at once.  Extra error messages are
       suppressed and accounted for in the error_count field of the existing  FAN_FS_ERROR  event
       record, but details about the errors are lost.

       Errors reported by FAN_FS_ERROR are generic errno values, but not all kinds of error types
       are reported by all filesystems.

       Errors not directly related to a file (i.e. super block corruption) are reported  with  an
       invalid  handle.   For  these  errors,  the  handle will have the field handle_type set to
       FILEID_INVALID, and the handle buffer size set to 0.

   Closing the fanotify file descriptor
       When all file descriptors referring to the fanotify notification  group  are  closed,  the
       fanotify  group  is  released  and  its resources are freed for reuse by the kernel.  Upon
       close(2), outstanding permission events will be set to allowed.

   /proc interfaces
       The file /proc/pid/fdinfo/fd contains information about fanotify marks for file descriptor
       fd of process pid.  See proc(5) for details.

       Since  Linux  5.13,  the  following interfaces can be used to control the amount of kernel
       resources consumed by fanotify:

       /proc/sys/fs/fanotify/max_queued_events
              The value in this file is used when an application calls fanotify_init(2) to set an
              upper  limit  on  the  number  of  events  that  can be queued to the corresponding
              fanotify group.  Events in excess of this limit are dropped, but an  FAN_Q_OVERFLOW
              event  is  always  generated.   Prior to Linux kernel 5.13, the hardcoded limit was
              16384 events.

       /proc/sys/fs/fanotify/max_user_group
              This specifies an upper limit on the number of fanotify groups that can be  created
              per  real  user ID.  Prior to Linux kernel 5.13, the hardcoded limit was 128 groups
              per user.

       /proc/sys/fs/fanotify/max_user_marks
              This specifies an upper limit on the number of fanotify marks that can  be  created
              per  real  user ID.  Prior to Linux kernel 5.13, the hardcoded limit was 8192 marks
              per group (not per user).

ERRORS

       In addition to the usual errors for read(2), the following errors can occur  when  reading
       from the fanotify file descriptor:

       EINVAL The buffer is too small to hold the event.

       EMFILE The  per-process  limit  on  the  number  of  open files has been reached.  See the
              description of RLIMIT_NOFILE in getrlimit(2).

       ENFILE The system-wide limit on the total number of open  files  has  been  reached.   See
              /proc/sys/fs/file-max in proc(5).

       ETXTBSY
              This  error  is  returned  by  read(2)  if  O_RDWR or O_WRONLY was specified in the
              event_f_flags argument when calling fanotify_init(2) and an event  occurred  for  a
              monitored file that is currently being executed.

       In  addition to the usual errors for write(2), the following errors can occur when writing
       to the fanotify file descriptor:

       EINVAL Fanotify access permissions are not enabled in  the  kernel  configuration  or  the
              value of response in the response structure is not valid.

       ENOENT The file descriptor fd in the response structure is not valid.  This may occur when
              a response for the permission event has already been written.

STANDARDS

       Linux.

HISTORY

       The fanotify API was introduced in Linux 2.6.36  and  enabled  in  Linux  2.6.37.   fdinfo
       support was added in Linux 3.8.

NOTES

       The  fanotify  API  is  available  only  if  the kernel was built with the CONFIG_FANOTIFY
       configuration option enabled.  In addition, fanotify permission handling is available only
       if the CONFIG_FANOTIFY_ACCESS_PERMISSIONS configuration option is enabled.

   Limitations and caveats
       Fanotify  reports  only  events  that a user-space program triggers through the filesystem
       API.  As a result, it does not catch remote events that occur on network filesystems.

       The fanotify API does not report file accesses and modifications that may occur because of
       mmap(2), msync(2), and munmap(2).

       Events  for  directories  are  created  only  if the directory itself is opened, read, and
       closed.  Adding, removing, or changing children of a  marked  directory  does  not  create
       events for the monitored directory itself.

       Fanotify  monitoring  of  directories  is not recursive: to monitor subdirectories under a
       directory, additional marks must be  created.   The  FAN_CREATE  event  can  be  used  for
       detecting  when  a  subdirectory has been created under a marked directory.  An additional
       mark must then be set on the newly created subdirectory.  This approach is  racy,  because
       it  can  lose events that occurred inside the newly created subdirectory, before a mark is
       added on that subdirectory.  Monitoring mounts offers the capability to  monitor  a  whole
       directory  tree  in  a  race-free manner.  Monitoring filesystems offers the capability to
       monitor changes made from any mount of a filesystem instance in a race-free manner.

       The event queue can overflow.  In this case, events are lost.

BUGS

       Before Linux 3.19, fallocate(2) did not generate fanotify events.  Since Linux 3.19, calls
       to fallocate(2) generate FAN_MODIFY events.

       As of Linux 3.17, the following bugs exist:

       •  On  Linux, a filesystem object may be accessible through multiple paths, for example, a
          part of a filesystem may be remounted using the --bind option of mount(8).  A  listener
          that  marked  a  mount  will  be  notified  only  of  events  that were triggered for a
          filesystem object using the same mount.  Any other event will pass unnoticed.

       •  When an event is generated, no check is  made  to  see  whether  the  user  ID  of  the
          receiving  process  has  authorization  to read or write the file before passing a file
          descriptor for  that  file.   This  poses  a  security  risk,  when  the  CAP_SYS_ADMIN
          capability is set for programs executed by unprivileged users.

       •  If  a  call  to  read(2) processes multiple events from the fanotify queue and an error
          occurs, the return value will be the total length of the events successfully copied  to
          the  user-space buffer before the error occurred.  The return value will not be -1, and
          errno will not be set.  Thus, the reading application has no way to detect the error.

EXAMPLES

       The two example programs below demonstrate the usage of the fanotify API.

   Example program: fanotify_example.c
       The first program is an example of fanotify being used with its event  object  information
       passed in the form of a file descriptor.  The program marks the mount passed as a command-
       line argument and waits for events of type  FAN_OPEN_PERM  and  FAN_CLOSE_WRITE.   When  a
       permission event occurs, a FAN_ALLOW response is given.

       The  following  shell  session  shows  an  example  of running this program.  This session
       involved  editing  the  file  /home/user/temp/notes.   Before  the  file  was  opened,   a
       FAN_OPEN_PERM  event  occurred.   After  the  file  was  closed,  a  FAN_CLOSE_WRITE event
       occurred.  Execution of the program ends when the user presses the ENTER key.

           # ./fanotify_example /home
           Press enter key to terminate.
           Listening for events.
           FAN_OPEN_PERM: File /home/user/temp/notes
           FAN_CLOSE_WRITE: File /home/user/temp/notes

           Listening for events stopped.

   Program source: fanotify_example.c

       #define _GNU_SOURCE     /* Needed to get O_LARGEFILE definition */
       #include <errno.h>
       #include <fcntl.h>
       #include <limits.h>
       #include <poll.h>
       #include <stdio.h>
       #include <stdlib.h>
       #include <sys/fanotify.h>
       #include <unistd.h>

       /* Read all available fanotify events from the file descriptor 'fd'. */

       static void
       handle_events(int fd)
       {
           const struct fanotify_event_metadata *metadata;
           struct fanotify_event_metadata buf[200];
           ssize_t len;
           char path[PATH_MAX];
           ssize_t path_len;
           char procfd_path[PATH_MAX];
           struct fanotify_response response;

           /* Loop while events can be read from fanotify file descriptor. */

           for (;;) {

               /* Read some events. */

               len = read(fd, buf, sizeof(buf));
               if (len == -1 && errno != EAGAIN) {
                   perror("read");
                   exit(EXIT_FAILURE);
               }

               /* Check if end of available data reached. */

               if (len <= 0)
                   break;

               /* Point to the first event in the buffer. */

               metadata = buf;

               /* Loop over all events in the buffer. */

               while (FAN_EVENT_OK(metadata, len)) {

                   /* Check that run-time and compile-time structures match. */

                   if (metadata->vers != FANOTIFY_METADATA_VERSION) {
                       fprintf(stderr,
                               "Mismatch of fanotify metadata version.\n");
                       exit(EXIT_FAILURE);
                   }

                   /* metadata->fd contains either FAN_NOFD, indicating a
                      queue overflow, or a file descriptor (a nonnegative
                      integer). Here, we simply ignore queue overflow. */

                   if (metadata->fd >= 0) {

                       /* Handle open permission event. */

                       if (metadata->mask & FAN_OPEN_PERM) {
                           printf("FAN_OPEN_PERM: ");

                           /* Allow file to be opened. */

                           response.fd = metadata->fd;
                           response.response = FAN_ALLOW;
                           write(fd, &response, sizeof(response));
                       }

                       /* Handle closing of writable file event. */

                       if (metadata->mask & FAN_CLOSE_WRITE)
                           printf("FAN_CLOSE_WRITE: ");

                       /* Retrieve and print pathname of the accessed file. */

                       snprintf(procfd_path, sizeof(procfd_path),
                                "/proc/self/fd/%d", metadata->fd);
                       path_len = readlink(procfd_path, path,
                                           sizeof(path) - 1);
                       if (path_len == -1) {
                           perror("readlink");
                           exit(EXIT_FAILURE);
                       }

                       path[path_len] = '\0';
                       printf("File %s\n", path);

                       /* Close the file descriptor of the event. */

                       close(metadata->fd);
                   }

                   /* Advance to next event. */

                   metadata = FAN_EVENT_NEXT(metadata, len);
               }
           }
       }

       int
       main(int argc, char *argv[])
       {
           char buf;
           int fd, poll_num;
           nfds_t nfds;
           struct pollfd fds[2];

           /* Check mount point is supplied. */

           if (argc != 2) {
               fprintf(stderr, "Usage: %s MOUNT\n", argv[0]);
               exit(EXIT_FAILURE);
           }

           printf("Press enter key to terminate.\n");

           /* Create the file descriptor for accessing the fanotify API. */

           fd = fanotify_init(FAN_CLOEXEC | FAN_CLASS_CONTENT | FAN_NONBLOCK,
                              O_RDONLY | O_LARGEFILE);
           if (fd == -1) {
               perror("fanotify_init");
               exit(EXIT_FAILURE);
           }

           /* Mark the mount for:
              - permission events before opening files
              - notification events after closing a write-enabled
                file descriptor. */

           if (fanotify_mark(fd, FAN_MARK_ADD | FAN_MARK_MOUNT,
                             FAN_OPEN_PERM | FAN_CLOSE_WRITE, AT_FDCWD,
                             argv[1]) == -1) {
               perror("fanotify_mark");
               exit(EXIT_FAILURE);
           }

           /* Prepare for polling. */

           nfds = 2;

           fds[0].fd = STDIN_FILENO;       /* Console input */
           fds[0].events = POLLIN;

           fds[1].fd = fd;                 /* Fanotify input */
           fds[1].events = POLLIN;

           /* This is the loop to wait for incoming events. */

           printf("Listening for events.\n");

           while (1) {
               poll_num = poll(fds, nfds, -1);
               if (poll_num == -1) {
                   if (errno == EINTR)     /* Interrupted by a signal */
                       continue;           /* Restart poll() */

                   perror("poll");         /* Unexpected error */
                   exit(EXIT_FAILURE);
               }

               if (poll_num > 0) {
                   if (fds[0].revents & POLLIN) {

                       /* Console input is available: empty stdin and quit. */

                       while (read(STDIN_FILENO, &buf, 1) > 0 && buf != '\n')
                           continue;
                       break;
                   }

                   if (fds[1].revents & POLLIN) {

                       /* Fanotify events are available. */

                       handle_events(fd);
                   }
               }
           }

           printf("Listening for events stopped.\n");
           exit(EXIT_SUCCESS);
       }

   Example program: fanotify_fid.c
       The second program is an example of fanotify being  used  with  a  group  that  identifies
       objects  by  file  handles.   The  program marks the filesystem object that is passed as a
       command-line argument and waits until an event of type FAN_CREATE has occurred.  The event
       mask  indicates  which type of filesystem object—either a file or a directory—was created.
       Once all events have been read from the buffer  and  processed  accordingly,  the  program
       simply terminates.

       The  following  shell  sessions  show  two  different  invocations  of  this program, with
       different actions performed on a watched object.

       The first session shows a mark being placed  on  /home/user.   This  is  followed  by  the
       creation  of  a regular file, /home/user/testfile.txt.  This results in a FAN_CREATE event
       being generated and reported against the file's parent watched directory object  and  with
       the  created file name.  Program execution ends once all events captured within the buffer
       have been processed.

           # ./fanotify_fid /home/user
           Listening for events.
           FAN_CREATE (file created):
                   Directory /home/user has been modified.
                   Entry 'testfile.txt' is not a subdirectory.
           All events processed successfully. Program exiting.

           $ touch /home/user/testfile.txt              # In another terminal

       The second session shows a mark being placed on  /home/user.   This  is  followed  by  the
       creation of a directory, /home/user/testdir.  This specific action results in a FAN_CREATE
       event being generated and is reported with the FAN_ONDIR flag set  and  with  the  created
       directory name.

           # ./fanotify_fid /home/user
           Listening for events.
           FAN_CREATE | FAN_ONDIR (subdirectory created):
                   Directory /home/user has been modified.
                   Entry 'testdir' is a subdirectory.
           All events processed successfully. Program exiting.

           $ mkdir -p /home/user/testdir          # In another terminal

   Program source: fanotify_fid.c

       #define _GNU_SOURCE
       #include <errno.h>
       #include <fcntl.h>
       #include <limits.h>
       #include <stdio.h>
       #include <stdlib.h>
       #include <sys/types.h>
       #include <sys/stat.h>
       #include <sys/fanotify.h>
       #include <unistd.h>

       #define BUF_SIZE 256

       int
       main(int argc, char *argv[])
       {
           int fd, ret, event_fd, mount_fd;
           ssize_t len, path_len;
           char path[PATH_MAX];
           char procfd_path[PATH_MAX];
           char events_buf[BUF_SIZE];
           struct file_handle *file_handle;
           struct fanotify_event_metadata *metadata;
           struct fanotify_event_info_fid *fid;
           const char *file_name;
           struct stat sb;

           if (argc != 2) {
               fprintf(stderr, "Invalid number of command line arguments.\n");
               exit(EXIT_FAILURE);
           }

           mount_fd = open(argv[1], O_DIRECTORY | O_RDONLY);
           if (mount_fd == -1) {
               perror(argv[1]);
               exit(EXIT_FAILURE);
           }

           /* Create an fanotify file descriptor with FAN_REPORT_DFID_NAME as
              a flag so that program can receive fid events with directory
              entry name. */

           fd = fanotify_init(FAN_CLASS_NOTIF | FAN_REPORT_DFID_NAME, 0);
           if (fd == -1) {
               perror("fanotify_init");
               exit(EXIT_FAILURE);
           }

           /* Place a mark on the filesystem object supplied in argv[1]. */

           ret = fanotify_mark(fd, FAN_MARK_ADD | FAN_MARK_ONLYDIR,
                               FAN_CREATE | FAN_ONDIR,
                               AT_FDCWD, argv[1]);
           if (ret == -1) {
               perror("fanotify_mark");
               exit(EXIT_FAILURE);
           }

           printf("Listening for events.\n");

           /* Read events from the event queue into a buffer. */

           len = read(fd, events_buf, sizeof(events_buf));
           if (len == -1 && errno != EAGAIN) {
               perror("read");
               exit(EXIT_FAILURE);
           }

           /* Process all events within the buffer. */

           for (metadata = (struct fanotify_event_metadata *) events_buf;
                   FAN_EVENT_OK(metadata, len);
                   metadata = FAN_EVENT_NEXT(metadata, len)) {
               fid = (struct fanotify_event_info_fid *) (metadata + 1);
               file_handle = (struct file_handle *) fid->handle;

               /* Ensure that the event info is of the correct type. */

               if (fid->hdr.info_type == FAN_EVENT_INFO_TYPE_FID ||
                   fid->hdr.info_type == FAN_EVENT_INFO_TYPE_DFID) {
                   file_name = NULL;
               } else if (fid->hdr.info_type == FAN_EVENT_INFO_TYPE_DFID_NAME) {
                   file_name = file_handle->f_handle +
                               file_handle->handle_bytes;
               } else {
                   fprintf(stderr, "Received unexpected event info type.\n");
                   exit(EXIT_FAILURE);
               }

               if (metadata->mask == FAN_CREATE)
                   printf("FAN_CREATE (file created):\n");

               if (metadata->mask == (FAN_CREATE | FAN_ONDIR))
                   printf("FAN_CREATE | FAN_ONDIR (subdirectory created):\n");

            /* metadata->fd is set to FAN_NOFD when the group identifies
               objects by file handles.  To obtain a file descriptor for
               the file object corresponding to an event you can use the
               struct file_handle that's provided within the
               fanotify_event_info_fid in conjunction with the
               open_by_handle_at(2) system call.  A check for ESTALE is
               done to accommodate for the situation where the file handle
               for the object was deleted prior to this system call. */

               event_fd = open_by_handle_at(mount_fd, file_handle, O_RDONLY);
               if (event_fd == -1) {
                   if (errno == ESTALE) {
                       printf("File handle is no longer valid. "
                               "File has been deleted\n");
                       continue;
                   } else {
                       perror("open_by_handle_at");
                       exit(EXIT_FAILURE);
                   }
               }

               snprintf(procfd_path, sizeof(procfd_path), "/proc/self/fd/%d",
                       event_fd);

               /* Retrieve and print the path of the modified dentry. */

               path_len = readlink(procfd_path, path, sizeof(path) - 1);
               if (path_len == -1) {
                   perror("readlink");
                   exit(EXIT_FAILURE);
               }

               path[path_len] = '\0';
               printf("\tDirectory '%s' has been modified.\n", path);

               if (file_name) {
                   ret = fstatat(event_fd, file_name, &sb, 0);
                   if (ret == -1) {
                       if (errno != ENOENT) {
                           perror("fstatat");
                           exit(EXIT_FAILURE);
                       }
                       printf("\tEntry '%s' does not exist.\n", file_name);
                   } else if ((sb.st_mode & S_IFMT) == S_IFDIR) {
                       printf("\tEntry '%s' is a subdirectory.\n", file_name);
                   } else {
                       printf("\tEntry '%s' is not a subdirectory.\n",
                               file_name);
                   }
               }

               /* Close associated file descriptor for this event. */

               close(event_fd);
           }

           printf("All events processed successfully. Program exiting.\n");
           exit(EXIT_SUCCESS);
       }

SEE ALSO

       fanotify_init(2), fanotify_mark(2), inotify(7)