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NAME | DESCRIPTION | ERRORS | STANDARDS | HISTORY | NOTES | BUGS | EXAMPLES | SEE ALSO | COLOPHON |
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fanotify(7) Miscellaneous Information Manual fanotify(7)
fanotify - monitoring filesystem events
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 cases where an fanotify group is initialized with
FAN_REPORT_MNT, event listeners should expect to receive the below
information record object alongside the generic
fanotify_event_metadata structure within the read buffer. This
structure is defined as follows:
struct fanotify_event_info_mnt {
struct fanotify_event_info_header hdr;
__u64 mnt_id;
};
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;
};
In case of FAN_PRE_ACCESS events, an additional information record
describing the access range is returned alongside the generic
fanotify_event_metadata structure within the read buffer. This
structure is defined as follows:
struct fanotify_event_info_range {
struct fanotify_event_info_header hdr;
__u32 pad;
__u64 offset;
__u64 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 size 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 size 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 size 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.
When an fanotify group is initialized using
FAN_REPORT_FD_ERROR, this field will contain a negative
error value in case a file descriptor could not be opened
and in case of a queue overflow, the value will be -EBADF.
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_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_MNT_ATTACH
A mount was attached to mount namespace.
FAN_MNT_DETACH
A mount was detached from mount namespace.
FAN_FS_ERROR
A filesystem error was detected.
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
FAN_EVENT_INFO_TYPE_PIDFD
FAN_EVENT_INFO_TYPE_ERROR
FAN_EVENT_INFO_TYPE_RANGE
FAN_EVENT_INFO_TYPE_MNT
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). Note that some filesystems (e.g.,
fuse(4)) report zero fsid. In these cases, it is not
possible to use fsid to associate the event with a specific
filesystem instance, so monitoring different filesystem
instances that report zero fsid with the same fanotify
group is not supported.
handle This field contains a variable-size 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).
When an fanotify group is initialized using
FAN_REPORT_FD_ERROR, this field will contain a negative
error value in case a pidfd creation failure and in case of
a terminated process, the value will be -ESRCH.
The fields of the fanotify_event_info_mnt 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_MNT.
.mnt_id
Identifies the mount associated with the event. It is a
64-bit unique mount id as the one returned by statx(2) with
the STATX_MNT_ID_UNIQUE flag.
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 fields of the fanotify_event_info_range 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_RANGE.
count The number of bytes being read or written to the file.
offset The offset from which bytes are read or written to the
file.
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, size)
This macro checks the remaining size size of the buffer
meta against the size of the metadata structure and the
event_len field of the first metadata structure in the
buffer.
FAN_EVENT_NEXT(meta, size)
This macro uses the size 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. size 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 size by the number of bytes in the metadata
structure that has been skipped over (i.e., it subtracts
meta->event_len from size).
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 contain either the flag 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. Since Linux 6.13, if a notification group is
initialized with class FAN_CLASS_PRE_CONTENT, the file operation
can be denied with errors other than EPERM. For example, for the
requesting application to get the EIO error, the event listener
can write the response FAN_DENY_ERRNO(EIO). At the time of
writing, only the following error values could be returned to the
application with FAN_DENY_ERRNO(e) macro: EPERM, EIO, EBUSY,
ETXTBSY, EAGAIN, ENOSPC, EDQUOT.
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.
Since Linux 6.3, the FAN_INFO flag can be set in the .response
field. It indicates that an extra variable-length response record
follows the fanotify_response structure. Extra response records
start with a common header:
struct fanotify_response_info_header {
__u8 type;
__u8 pad;
__u16 len;
};
The value of .type determines the format of the extra response
record.
FAN_RESPONSE_INFO_AUDIT_RULE
The following response record is expected with extra
details for the audit log:
struct fanotify_response_info_audit_rule {
struct fanotify_response_info_header hdr;
__u32 rule_number;
__u32 subj_trust;
__u32 obj_trust;
};
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 (and 5.10.220), 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).
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.
Linux.
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.
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.
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 size 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.
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 size;
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. */
size = read(fd, buf, sizeof(buf));
if (size == -1 && errno != EAGAIN) {
perror("read");
exit(EXIT_FAILURE);
}
/* Check if end of available data reached. */
if (size <= 0)
break;
/* Point to the first event in the buffer. */
metadata = buf;
/* Loop over all events in the buffer. */
while (FAN_EVENT_OK(metadata, size)) {
/* 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, size);
}
}
}
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 size, 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. */
size = read(fd, events_buf, sizeof(events_buf));
if (size == -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, size);
metadata = FAN_EVENT_NEXT(metadata, size)) {
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);
}
fanotify_init(2), fanotify_mark(2), inotify(7)
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⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
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Linux man-pages 6.15 2025-05-17 fanotify(7)
Pages that refer to this page: inotifywait(1), inotifywatch(1), fanotify_init(2), fanotify_mark(2), open_by_handle_at(2), proc_pid_fdinfo(5), inotify(7)
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