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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 case of a FAN_FS_ERROR event, an additional information record
describing the error that occurred is returned alongside the
generic fanotify_event_metadata structure within the read buffer.
This structure is defined as follows:
struct fanotify_event_info_error {
struct fanotify_event_info_header hdr;
__s32 error;
__u32 error_count;
};
All information records contain a nested structure of type
fanotify_event_info_header. This structure holds meta-
information about the information record that may have been
returned alongside the generic fanotify_event_metadata structure.
This structure is defined as follows:
struct fanotify_event_info_header {
__u8 info_type;
__u8 pad;
__u16 len;
};
For performance reasons, it is recommended to use a large buffer
size (for example, 4096 bytes), so that multiple events can be
retrieved by a single read(2).
The return value of read(2) is the number of bytes placed in the
buffer, or -1 in case of an error (but see BUGS).
The fields of the fanotify_event_metadata structure are as
follows:
event_len
This is the length of the data for the current event and
the offset to the next event in the buffer. Unless the
group identifies filesystem objects by file handles, the
value of event_len is always FAN_EVENT_METADATA_LEN. For
a group that identifies filesystem objects by file
handles, event_len also includes the variable length file
identifier records.
vers This field holds a version number for the structure. It
must be compared to FANOTIFY_METADATA_VERSION to verify
that the structures returned at run time match the
structures defined at compile time. In case of a
mismatch, the application should abandon trying to use the
fanotify file descriptor.
reserved
This field is not used.
metadata_len
This is the length of the structure. The field was
introduced to facilitate the implementation of optional
headers per event type. No such optional headers exist in
the current implementation.
mask This is a bit mask describing the event (see below).
fd This is an open file descriptor for the object being
accessed, or FAN_NOFD if a queue overflow occurred. With
an fanotify group that identifies filesystem objects by
file handles, applications should expect this value to be
set to FAN_NOFD for each event that is received. The file
descriptor can be used to access the contents of the
monitored file or directory. The reading application is
responsible for closing this file descriptor.
When calling fanotify_init(2), the caller may specify (via
the event_f_flags argument) various file status flags that
are to be set on the open file description that
corresponds to this file descriptor. In addition, the
(kernel-internal) FMODE_NONOTIFY file status flag is set
on the open file description. This flag suppresses
fanotify event generation. Hence, when the receiver of
the fanotify event accesses the notified file or directory
using this file descriptor, no additional events will be
created.
pid If flag FAN_REPORT_TID was set in fanotify_init(2), this
is the TID of the thread that caused the event.
Otherwise, this the PID of the process that caused the
event.
A program listening to fanotify events can compare this PID to
the PID returned by getpid(2), to determine whether the event is
caused by the listener itself, or is due to a file access by
another process.
The bit mask in mask indicates which events have occurred for a
single filesystem object. Multiple bits may be set in this mask,
if more than one event occurred for the monitored filesystem
object. In particular, consecutive events for the same
filesystem object and originating from the same process may be
merged into a single event, with the exception that two
permission events are never merged into one queue entry.
The bits that may appear in mask are as follows:
FAN_ACCESS
A file or a directory (but see BUGS) was accessed (read).
FAN_OPEN
A file or a directory was opened.
FAN_OPEN_EXEC
A file was opened with the intent to be executed. See
NOTES in fanotify_mark(2) for additional details.
FAN_ATTRIB
A file or directory metadata was changed.
FAN_CREATE
A child file or directory was created in a watched parent.
FAN_DELETE
A child file or directory was deleted in a watched parent.
FAN_DELETE_SELF
A watched file or directory was deleted.
FAN_FS_ERROR
A filesystem error was detected.
FAN_RENAME
A file or directory has been moved to or from a watched
parent directory.
FAN_MOVED_FROM
A file or directory has been moved from a watched parent
directory.
FAN_MOVED_TO
A file or directory has been moved to a watched parent
directory.
FAN_MOVE_SELF
A watched file or directory was moved.
FAN_MODIFY
A file was modified.
FAN_CLOSE_WRITE
A file that was opened for writing (O_WRONLY or O_RDWR)
was closed.
FAN_CLOSE_NOWRITE
A file or directory that was opened read-only (O_RDONLY)
was closed.
FAN_Q_OVERFLOW
The event queue exceeded the limit on number of events.
This limit can be overridden by specifying the
FAN_UNLIMITED_QUEUE flag when calling fanotify_init(2).
FAN_ACCESS_PERM
An application wants to read a file or directory, for
example using read(2) or readdir(2). The reader must
write a response (as described below) that determines
whether the permission to access the filesystem object
shall be granted.
FAN_OPEN_PERM
An application wants to open a file or directory. The
reader must write a response that determines whether the
permission to open the filesystem object shall be granted.
FAN_OPEN_EXEC_PERM
An application wants to open a file for execution. The
reader must write a response that determines whether the
permission to open the filesystem object for execution
shall be granted. See NOTES in fanotify_mark(2) for
additional details.
To check for any close event, the following bit mask may be used:
FAN_CLOSE
A file was closed. This is a synonym for:
FAN_CLOSE_WRITE | FAN_CLOSE_NOWRITE
To check for any move event, the following bit mask may be used:
FAN_MOVE
A file or directory was moved. This is a synonym for:
FAN_MOVED_FROM | FAN_MOVED_TO
The following bits may appear in mask only in conjunction with
other event type bits:
FAN_ONDIR
The events described in the mask have occurred on a
directory object. Reporting events on directories
requires setting this flag in the mark mask. See
fanotify_mark(2) for additional details. The FAN_ONDIR
flag is reported in an event mask only if the fanotify
group identifies filesystem objects by file handles.
Information records that are supplied alongside the generic
fanotify_event_metadata structure will always contain a nested
structure of type fanotify_event_info_header. The fields of the
fanotify_event_info_header are as follows:
info_type
A unique integer value representing the type of
information record object received for an event. The
value of this field can be set to one of the following:
FAN_EVENT_INFO_TYPE_FID, FAN_EVENT_INFO_TYPE_DFID,
FAN_EVENT_INFO_TYPE_DFID_NAME, or
FAN_EVENT_INFO_TYPE_PIDFD. The value set for this field
is dependent on the flags that have been supplied to
fanotify_init(2). Refer to the field details of each
information record object type below to understand the
different cases in which the info_type values can be set.
pad This field is currently not used by any information record
object type and therefore is set to zero.
len The value of len is set to the size of the information
record object, including the fanotify_event_info_header.
The total size of all additional information records is
not expected to be larger than (event_len - metadata_len).
The fields of the fanotify_event_info_fid structure are as
follows:
hdr This is a structure of type fanotify_event_info_header.
For example, when an fanotify file descriptor is created
using FAN_REPORT_FID, a single information record is
expected to be attached to the event with info_type field
value of FAN_EVENT_INFO_TYPE_FID. When an fanotify file
descriptor is created using the combination of
FAN_REPORT_FID and FAN_REPORT_DIR_FID, there may be two
information records attached to the event: one with
info_type field value of FAN_EVENT_INFO_TYPE_DFID,
identifying a parent directory object, and one with
info_type field value of FAN_EVENT_INFO_TYPE_FID,
identifying a child object. Note that for the directory
entry modification events FAN_CREATE, FAN_DELETE,
FAN_MOVE, and FAN_RENAME, an information record
identifying the created/deleted/moved child object is
reported only if an fanotify group was initialized with
the flag FAN_REPORT_TARGET_FID.
fsid This is a unique identifier of the filesystem containing
the object associated with the event. It is a structure
of type __kernel_fsid_t and contains the same value as
f_fsid when calling statfs(2).
handle This field contains a variable-length structure of type
struct file_handle. It is an opaque handle that
corresponds to a specified object on a filesystem as
returned by name_to_handle_at(2). It can be used to
uniquely identify a file on a filesystem and can be passed
as an argument to open_by_handle_at(2). If the value of
info_type field is FAN_EVENT_INFO_TYPE_DFID_NAME, the file
handle is followed by a null terminated string that
identifies the created/deleted/moved directory entry name.
For other events such as FAN_OPEN, FAN_ATTRIB,
FAN_DELETE_SELF, and FAN_MOVE_SELF, if the value of
info_type field is FAN_EVENT_INFO_TYPE_FID, the handle
identifies the object correlated to the event. If the
value of info_type field is FAN_EVENT_INFO_TYPE_DFID, the
handle identifies the directory object correlated to the
event or the parent directory of a non-directory object
correlated to the event. If the value of info_type field
is FAN_EVENT_INFO_TYPE_DFID_NAME, the handle identifies
the same directory object that would be reported with
FAN_EVENT_INFO_TYPE_DFID and the file handle is followed
by a null terminated string that identifies the name of a
directory entry in that directory, or '.' to identify the
directory object itself.
The fields of the fanotify_event_info_pidfd structure are as
follows:
hdr This is a structure of type fanotify_event_info_header.
When an fanotify group is initialized using
FAN_REPORT_PIDFD, the info_type field value of the
fanotify_event_info_header is set to
FAN_EVENT_INFO_TYPE_PIDFD.
pidfd This is a process file descriptor that refers to the
process responsible for generating the event. The
returned process file descriptor is no different from one
which could be obtained manually if pidfd_open(2) were to
be called on fanotify_event_metadata.pid. In the instance
that an error is encountered during pidfd creation, one of
two possible error types represented by a negative integer
value may be returned in this pidfd field. In cases where
the process responsible for generating the event has
terminated prior to the event listener being able to read
events from the notification queue, FAN_NOPIDFD is
returned. The pidfd creation for an event is only
performed at the time the events are read from the
notification queue. All other possible pidfd creation
failures are represented by FAN_EPIDFD. Once the event
listener has dealt with an event and the pidfd is no
longer required, the pidfd should be closed via close(2).
The fields of the fanotify_event_info_error structure are as
follows:
hdr This is a structure of type fanotify_event_info_header.
The info_type field is set to FAN_EVENT_INFO_TYPE_ERROR.
error Identifies the type of error that occurred.
error_count
This is a counter of the number of errors suppressed since
the last error was read.
The following macros are provided to iterate over a buffer
containing fanotify event metadata returned by a read(2) from an
fanotify file descriptor:
FAN_EVENT_OK(meta, len)
This macro checks the remaining length len of the buffer
meta against the length of the metadata structure and the
event_len field of the first metadata structure in the
buffer.
FAN_EVENT_NEXT(meta, len)
This macro uses the length indicated in the event_len
field of the metadata structure pointed to by meta to
calculate the address of the next metadata structure that
follows meta. len is the number of bytes of metadata that
currently remain in the buffer. The macro returns a
pointer to the next metadata structure that follows meta,
and reduces len by the number of bytes in the metadata
structure that has been skipped over (i.e., it subtracts
meta->event_len from len).
In addition, there is:
FAN_EVENT_METADATA_LEN
This macro returns the size (in bytes) of the structure
fanotify_event_metadata. This is the minimum size (and
currently the only size) of any event metadata.
Monitoring an fanotify file descriptor for events
When an fanotify event occurs, the fanotify file descriptor
indicates as readable when passed to epoll(7), poll(2), or
select(2).
Dealing with permission events
For permission events, the application must write(2) a structure
of the following form to the fanotify file descriptor:
struct fanotify_response {
__s32 fd;
__u32 response;
};
The fields of this structure are as follows:
fd This is the file descriptor from the structure
fanotify_event_metadata.
response
This field indicates whether or not the permission is to
be granted. Its value must be either FAN_ALLOW to allow
the file operation or FAN_DENY to deny the file operation.
If access is denied, the requesting application call will receive
an EPERM error. Additionally, if the notification group has been
created with the FAN_ENABLE_AUDIT flag, then the FAN_AUDIT flag
can be set in the response field. In that case, the audit
subsystem will log information about the access decision to the
audit logs.
Monitoring filesystems for errors
A single FAN_FS_ERROR event is stored per filesystem at once.
Extra error messages are suppressed and accounted for in the
error_count field of the existing FAN_FS_ERROR event record, but
details about the errors are lost.
Errors reported by FAN_FS_ERROR are generic errno values, but not
all kinds of error types are reported by all filesystems.
Errors not directly related to a file (i.e. super block
corruption) are reported with an invalid handle. For these
errors, the handle will have the field handle_type set to
FILEID_INVALID, and the handle buffer size set to 0.
Closing the fanotify file descriptor
When all file descriptors referring to the fanotify notification
group are closed, the fanotify group is released and its
resources are freed for reuse by the kernel. Upon close(2),
outstanding permission events will be set to allowed.
/proc interfaces
The file /proc/pid/fdinfo/fd contains information about fanotify
marks for file descriptor fd of process pid. See proc(5) for
details.
Since Linux 5.13, the following interfaces can be used to control
the amount of kernel resources consumed by fanotify:
/proc/sys/fs/fanotify/max_queued_events
The value in this file is used when an application calls
fanotify_init(2) to set an upper limit on the number of
events that can be queued to the corresponding fanotify
group. Events in excess of this limit are dropped, but an
FAN_Q_OVERFLOW event is always generated. Prior to Linux
kernel 5.13, the hardcoded limit was 16384 events.
/proc/sys/fs/fanotify/max_user_group
This specifies an upper limit on the number of fanotify
groups that can be created per real user ID. Prior to
Linux kernel 5.13, the hardcoded limit was 128 groups per
user.
/proc/sys/fs/fanotify/max_user_marks
This specifies an upper limit on the number of fanotify
marks that can be created per real user ID. Prior to
Linux kernel 5.13, the hardcoded limit was 8192 marks per
group (not per user).
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 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.
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);
}
fanotify_init(2), fanotify_mark(2), inotify(7)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
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⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
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2024-06-26. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
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Linux man-pages 6.9.1 2024-06-15 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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