fanotify(7) — Linux manual page

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FANOTIFY(7)             Linux Programmer's Manual            FANOTIFY(7)

NAME         top

       fanotify - monitoring filesystem events

DESCRIPTION         top

       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)).

       The use of one of the flags FAN_REPORT_FID, FAN_REPORT_DIR_FID in
       fanotify_init(2) influences what data structures are returned to
       the event listener for each event.  Events reported to a group
       initialized with one of these flags will use file handles to
       identify filesystem objects instead of file descriptors.

       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;
           };

       In case of an fanotify group that identifies filesystem objects
       by file handles, you should also expect to receive one or more
       additional information records of the structure detailed below
       following the generic fanotify_event_metadata structure within
       the read buffer:

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

           struct fanotify_event_info_fid {
               struct fanotify_event_info_header hdr;
               __kernel_fsid_t fsid;
               unsigned char file_handle[0];
           };

       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_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 of 16384 entries.  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.

       The fields of the fanotify_event_info_fid structure are as
       follows:

       hdr    This is a structure of type fanotify_event_info_header.
              It is a generic header that contains information used to
              describe an additional information record attached to the
              event.  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 non-directory object.  The
              fanotify_event_info_header contains a len field.  The
              value of len is the size of the additional information
              record including the fanotify_event_info_header itself.
              The total size of all additional information records is
              not expected to be bigger than ( event_len - metadata_len
              ).

       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).

       file_handle
              This is 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).  Note that for the directory entry
              modification events FAN_CREATE, FAN_DELETE, and FAN_MOVE,
              the file_handle identifies the modified directory and not
              the created/deleted/moved child object.  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
              file_handle identifies the object correlated to the event.
              If the value of info_type field is
              FAN_EVENT_INFO_TYPE_DFID, the file_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 file_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 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.

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

ERRORS         top

       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.

VERSIONS         top

       The fanotify API was introduced in version 2.6.36 of the Linux
       kernel and enabled in version 2.6.37.  Fdinfo support was added
       in version 3.8.

CONFORMING TO         top

       The fanotify API is Linux-specific.

NOTES         top

       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         top

       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         top

       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         top

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

COLOPHON         top

       This page is part of release 5.13 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                          2021-08-27                    FANOTIFY(7)

Pages that refer to this page: inotifywait(1)inotifywatch(1)fanotify_init(2)fanotify_mark(2)proc(5)inotify(7)