fanotify(7) Miscellaneous Information Manual fanotify(7)
NAME
fanotify - monitoring filesystem events
DESCRIPTION
The fanotify API provides notification and interception of filesystem
events. Use cases include virus scanning and hierarchical storage man-
agement. 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 activi-
ties for which an event shall be created. The ignore mask defines ac-
tivities for which no event shall be generated. Having these two types
of masks permits a filesystem, mount, or directory to be marked for re-
ceiving events, while at the same time ignoring events for specific ob-
jects 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 re-
ported (or ignored), or removes or modifies such an entry.
A possible usage of the ignore mask is for a file cache. Events of in-
terest 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 di-
rectories 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 ac-
tion to be taken by the receiving application with one exception: if a
valid file descriptor is provided within a generic event, the file de-
scriptor must be closed. Permission events are requests to the receiv-
ing 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 fan-
otify 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 fan-
otify_init(2)) until either a file event occurs or the call is inter-
rupted 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 infor-
mation records that may be returned for an event. For example, if a
notification group is initialized with FAN_REPORT_FID or FAN_RE-
PORT_DIR_FID, then event listeners should also expect to receive a fan-
otify_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_meta-
data 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 fan-
otify_event_info_header. It is imperative for event listeners to in-
spect 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 fan-
otify_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 de-
scribing the error that occurred is returned alongside the generic fan-
otify_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 fan-
otify_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 iden-
tifies 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 struc-
tures returned at run time match the structures defined at com-
pile time. In case of a mismatch, the application should aban-
don 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 implementa-
tion.
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, appli-
cations should expect this value to be set to FAN_NOFD for each
event that is received. The file descriptor can be used to ac-
cess the contents of the monitored file or directory. The read-
ing 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 cre-
ated.
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 direc-
tory.
FAN_MOVED_TO
A file or directory has been moved to a watched parent direc-
tory.
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 de-
tails. The FAN_ONDIR flag is reported in an event mask only if
the fanotify group identifies filesystem objects by file han-
dles.
Information records that are supplied alongside the generic fan-
otify_event_metadata structure will always contain a nested structure
of type fanotify_event_info_header. The fields of the fan-
otify_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 de-
pendent on the flags that have been supplied to fan-
otify_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 ob-
ject 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 ex-
ample, when an fanotify file descriptor is created using FAN_RE-
PORT_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_RE-
PORT_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 cre-
ated/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 ob-
ject associated with the event. It is a structure of type
__kernel_fsid_t and contains the same value as f_fsid when call-
ing statfs(2).
handle This field contains a variable-length structure of type struct
file_handle. It is an opaque handle that corresponds to a spec-
ified object on a filesystem as returned by name_to_han-
dle_at(2). It can be used to uniquely identify a file on a
filesystem and can be passed as an argument to open_by_han-
dle_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 identi-
fies the object correlated to the event. If the value of
info_type field is FAN_EVENT_INFO_TYPE_DFID, the handle identi-
fies 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 di-
rectory 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 re-
sponsible for generating the event. The returned process file
descriptor is no different from one which could be obtained man-
ually if pidfd_open(2) were to be called on fanotify_event_meta-
data.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 re-
quired, 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 de-
scriptor:
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 ad-
dress 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 struc-
ture 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 fan-
otify_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 fan-
otify_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 infor-
mation 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 fan-
otify_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 fan-
otify_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 con-
figuration 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 al-
ready 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_AC-
CESS_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 it-
self.
Fanotify monitoring of directories is not recursive: to monitor subdi-
rectories 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 ob-
ject 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 pro-
gram 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 indi-
cates 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 spe-
cific action results in a FAN_CREATE event being generated and is re-
ported 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)
Linux man-pages 6.7 2023-10-31 fanotify(7)
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