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FANOTIFY(7) Linux Programmer's 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. Currently, only a limited set of events is supported. In
particular, there is no support for create, delete, and move events.
(See inotify(7) for details of an API that does notify those events.)
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, and mount points 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
activities for which no event shall be generated. Having these two
types of masks permits a mount point or directory to be marked for
receiving events, while at the same time ignoring events for specific
objects under that mount point or directory.
The fanotify_mark(2) system call adds a file, directory, 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 point 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 invali-
dated. 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, the respective entries
survive. If files or directories are deleted 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 except for closing the
file descriptor passed in the event (see below). 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 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;
};
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 buf-
fer, 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. In the current imple-
mentation, the value of event_len is always FAN_EVENT_META-
DATA_LEN. However, the API is designed to allow variable-length
structures to be returned in the future.
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 runtime 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. 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 cre-
ated.
pid This is the ID 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_MODIFY
A file was modified.
FAN_CLOSE_WRITE
A file that was opened for writing (O_WRONLY or O_RDWR) was
closed.
FAN_CLOSE_NOWRITE
A file or directory that was opened read-only (O_RDONLY) was
closed.
FAN_Q_OVERFLOW
The event queue exceeded the limit of 16384 entries. This limit
can be overridden by specifying the FAN_UNLIMITED_QUEUE flag
when calling fanotify_init(2).
FAN_ACCESS_PERM
An application wants to read a file or directory, for example
using read(2) or readdir(2). The reader must write a response
(as described below) that determines whether the permission to
access the filesystem object shall be granted.
FAN_OPEN_PERM
An application wants to open a file or directory. The reader
must write a response that determines whether the permission to
open the filesystem object shall be granted.
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
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 struc-
ture that follows meta, and reduces len by the number of bytes
in the 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.
Closing the fanotify file descriptor
When all file descriptors referring to the fanotify notification group
are closed, the fanotify group is released and its resources are freed
for reuse by the kernel. Upon close(2), outstanding permission events
will be set to allowed.
/proc/[pid]/fdinfo
The file /proc/[pid]/fdinfo/[fd] contains information about fanotify
marks for file descriptor fd of process pid. See the kernel source
file Documentation/filesystems/proc.txt for details.
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 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
already been written.
VERSIONS
The fanotify API was introduced in version 2.6.36 of the Linux kernel
and enabled in version 2.6.37. Fdinfo support was added in version
3.8.
CONFORMING TO
The fanotify API is Linux-specific.
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_FAN-
OTIFY_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 subdi-
rectories under a directory, additional marks must be created. (But
note that the fanotify API provides no way of detecting when a subdi-
rectory has been created under a marked directory, which makes recur-
sive monitoring difficult.) Monitoring mounts offers the capability to
monitor a whole directory tree.
The event queue can overflow. In this case, events are lost.
BUGS
As of Linux 3.15, the following bugs exist:
* 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.
EXAMPLE
The following program demonstrates the usage of the fanotify API. It
marks the mount point passed as a command-line argument and waits for
events of type FAN_PERM_OPEN and FAN_CLOSE_WRITE. When a permission
event occurs, a FAN_ALLOW response is given.
The following output was recorded while 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.
Example output
# ./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
#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, (void *) &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(struct fanotify_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, -1,
argv[1]) == -1) {
perror("fanotify_mark");
exit(EXIT_FAILURE);
}
/* Prepare for polling */
nfds = 2;
/* Console input */
fds[0].fd = STDIN_FILENO;
fds[0].events = POLLIN;
/* Fanotify input */
fds[1].fd = fd;
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);
}
SEE ALSO
fanotify_init(2), fanotify_mark(2), inotify(7)
COLOPHON
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