fanotify_init(), fanotify_mark(), and notification groups¶

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 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 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 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 mounts are unmounted, the corresponding entries are deleted.

The event queue¶

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 fanotify file descriptor is closed.

Reading fanotify events¶

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 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. In the current implementation, the value of event_len is always FAN_EVENT_METADATA_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 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. 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

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 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¶

Dealing with permission events¶

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.

Closing the fanotify file descriptor¶

/proc/[pid]/fdinfo¶

Limitations and caveats¶

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. (But note that the fanotify API provides no way of detecting when a subdirectory has been created under a marked directory, which makes recursive monitoring difficult.) Monitoring mounts offers the capability to monitor a whole directory tree.

The event queue can overflow. In this case, events are lost.

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, AT_FDCWD,
                      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);
}