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| .TH FANOTIFY 7 2017-09-15 "Linux" "Linux Programmer's Manual" |
| .SH NAME |
| fanotify \- monitoring filesystem events |
| .SH DESCRIPTION |
| The fanotify API provides notification and interception of |
| filesystem events. |
| Use cases include virus scanning and hierarchical storage management. |
| Currently, only a limited set of events is supported. |
| In particular, there is no support for create, delete, and move events. |
| (See |
| .BR inotify (7) |
| for details of an API that does notify those events.) |
| .PP |
| Additional capabilities compared to the |
| .BR 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. |
| .PP |
| The following system calls are used with this API: |
| .BR fanotify_init (2), |
| .BR fanotify_mark (2), |
| .BR read (2), |
| .BR write (2), |
| and |
| .BR close (2). |
| .SS fanotify_init(), fanotify_mark(), and notification groups |
| The |
| .BR fanotify_init (2) |
| system call creates and initializes an fanotify notification group |
| and returns a file descriptor referring to it. |
| .PP |
| 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. |
| .PP |
| For each entry in an fanotify notification group, two bit masks exist: the |
| .I mark |
| mask and the |
| .I 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. |
| .PP |
| The |
| .BR 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. |
| .PP |
| 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. |
| .PP |
| 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. |
| .SS 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 |
| .BR read (2) |
| or similar) |
| from the fanotify file descriptor |
| returned by |
| .BR fanotify_init (2). |
| .PP |
| Two types of events are generated: |
| .I notification |
| events and |
| .I 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. |
| .PP |
| 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. |
| .SS Reading fanotify events |
| Calling |
| .BR read (2) |
| for the file descriptor returned by |
| .BR fanotify_init (2) |
| blocks (if the flag |
| .B FAN_NONBLOCK |
| is not specified in the call to |
| .BR fanotify_init (2)) |
| until either a file event occurs or the call is interrupted by a signal |
| (see |
| .BR signal (7)). |
| .PP |
| After a successful |
| .BR read (2), |
| the read buffer contains one or more of the following structures: |
| .PP |
| .in +4n |
| .EX |
| struct fanotify_event_metadata { |
| __u32 event_len; |
| __u8 vers; |
| __u8 reserved; |
| __u16 metadata_len; |
| __aligned_u64 mask; |
| __s32 fd; |
| __s32 pid; |
| }; |
| .EE |
| .in |
| .PP |
| 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 |
| .BR read (2). |
| .PP |
| The return value of |
| .BR read (2) |
| is the number of bytes placed in the buffer, |
| or \-1 in case of an error (but see BUGS). |
| .PP |
| The fields of the |
| .I fanotify_event_metadata |
| structure are as follows: |
| .TP |
| .I 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 |
| .I event_len |
| is always |
| .BR FAN_EVENT_METADATA_LEN . |
| However, the API is designed to allow |
| variable-length structures to be returned in the future. |
| .TP |
| .I vers |
| This field holds a version number for the structure. |
| It must be compared to |
| .B 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. |
| .TP |
| .I reserved |
| This field is not used. |
| .TP |
| .I 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. |
| .TP |
| .I mask |
| This is a bit mask describing the event (see below). |
| .TP |
| .I fd |
| This is an open file descriptor for the object being accessed, or |
| .B 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. |
| .IP |
| When calling |
| .BR fanotify_init (2), |
| the caller may specify (via the |
| .I 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) |
| .B 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. |
| .TP |
| .I 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 |
| .BR getpid (2), |
| to determine whether the event is caused by the listener itself, |
| or is due to a file access by another process. |
| .PP |
| The bit mask in |
| .I 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. |
| .PP |
| The bits that may appear in |
| .I mask |
| are as follows: |
| .TP |
| .B FAN_ACCESS |
| A file or a directory (but see BUGS) was accessed (read). |
| .TP |
| .B FAN_OPEN |
| A file or a directory was opened. |
| .TP |
| .B FAN_MODIFY |
| A file was modified. |
| .TP |
| .B FAN_CLOSE_WRITE |
| A file that was opened for writing |
| .RB ( O_WRONLY |
| or |
| .BR O_RDWR ) |
| was closed. |
| .TP |
| .B FAN_CLOSE_NOWRITE |
| A file or directory that was opened read-only |
| .RB ( O_RDONLY ) |
| was closed. |
| .TP |
| .B FAN_Q_OVERFLOW |
| The event queue exceeded the limit of 16384 entries. |
| This limit can be overridden by specifying the |
| .BR FAN_UNLIMITED_QUEUE |
| flag when calling |
| .BR fanotify_init (2). |
| .TP |
| .B FAN_ACCESS_PERM |
| An application wants to read a file or directory, for example using |
| .BR read (2) |
| or |
| .BR readdir (2). |
| The reader must write a response (as described below) |
| that determines whether the permission to |
| access the filesystem object shall be granted. |
| .TP |
| .B 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. |
| .PP |
| To check for any close event, the following bit mask may be used: |
| .TP |
| .B FAN_CLOSE |
| A file was closed. |
| This is a synonym for: |
| .IP |
| FAN_CLOSE_WRITE | FAN_CLOSE_NOWRITE |
| .PP |
| The following macros are provided to iterate over a buffer containing |
| fanotify event metadata returned by a |
| .BR read (2) |
| from an fanotify file descriptor: |
| .TP |
| .B FAN_EVENT_OK(meta, len) |
| This macro checks the remaining length |
| .I len |
| of the buffer |
| .I meta |
| against the length of the metadata structure and the |
| .I event_len |
| field of the first metadata structure in the buffer. |
| .TP |
| .B FAN_EVENT_NEXT(meta, len) |
| This macro uses the length indicated in the |
| .I event_len |
| field of the metadata structure pointed to by |
| .IR meta |
| to calculate the address of the next metadata structure that follows |
| .IR meta . |
| .I 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 |
| .IR meta , |
| and reduces |
| .I len |
| by the number of bytes in the metadata structure that |
| has been skipped over (i.e., it subtracts |
| .IR meta\->event_len |
| from |
| .IR len ). |
| .PP |
| In addition, there is: |
| .TP |
| .B FAN_EVENT_METADATA_LEN |
| This macro returns the size (in bytes) of the structure |
| .IR fanotify_event_metadata . |
| This is the minimum size (and currently the only size) of any event metadata. |
| .\" |
| .SS Monitoring an fanotify file descriptor for events |
| When an fanotify event occurs, the fanotify file descriptor indicates as |
| readable when passed to |
| .BR epoll (7), |
| .BR poll (2), |
| or |
| .BR select (2). |
| .SS Dealing with permission events |
| For permission events, the application must |
| .BR write (2) |
| a structure of the following form to the |
| fanotify file descriptor: |
| .PP |
| .in +4n |
| .EX |
| struct fanotify_response { |
| __s32 fd; |
| __u32 response; |
| }; |
| .EE |
| .in |
| .PP |
| The fields of this structure are as follows: |
| .TP |
| .I fd |
| This is the file descriptor from the structure |
| .IR fanotify_event_metadata . |
| .TP |
| .I response |
| This field indicates whether or not the permission is to be granted. |
| Its value must be either |
| .B FAN_ALLOW |
| to allow the file operation or |
| .B FAN_DENY |
| to deny the file operation. |
| .PP |
| If access is denied, the requesting application call will receive an |
| .BR EPERM |
| error. |
| .SS Closing the fanotify file descriptor |
| .PP |
| 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 |
| .BR close (2), |
| outstanding permission events will be set to allowed. |
| .SS /proc/[pid]/fdinfo |
| The file |
| .I /proc/[pid]/fdinfo/[fd] |
| contains information about fanotify marks for file descriptor |
| .I fd |
| of process |
| .IR pid . |
| See |
| .BR proc (5) |
| for details. |
| .SH ERRORS |
| In addition to the usual errors for |
| .BR read (2), |
| the following errors can occur when reading from the |
| fanotify file descriptor: |
| .TP |
| .B EINVAL |
| The buffer is too small to hold the event. |
| .TP |
| .B EMFILE |
| The per-process limit on the number of open files has been reached. |
| See the description of |
| .B RLIMIT_NOFILE |
| in |
| .BR getrlimit (2). |
| .TP |
| .B ENFILE |
| The system-wide limit on the total number of open files has been reached. |
| See |
| .I /proc/sys/fs/file-max |
| in |
| .BR proc (5). |
| .TP |
| .B ETXTBSY |
| This error is returned by |
| .BR read (2) |
| if |
| .B O_RDWR |
| or |
| .B O_WRONLY |
| was specified in the |
| .I event_f_flags |
| argument when calling |
| .BR fanotify_init (2) |
| and an event occurred for a monitored file that is currently being executed. |
| .PP |
| In addition to the usual errors for |
| .BR write (2), |
| the following errors can occur when writing to the fanotify file descriptor: |
| .TP |
| .B EINVAL |
| Fanotify access permissions are not enabled in the kernel configuration |
| or the value of |
| .I response |
| in the response structure is not valid. |
| .TP |
| .B ENOENT |
| The file descriptor |
| .I fd |
| in the response structure is not valid. |
| This may occur when a response for the permission event has already been |
| written. |
| .SH 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. |
| .SH CONFORMING TO |
| The fanotify API is Linux-specific. |
| .SH NOTES |
| The fanotify API is available only if the kernel was built with the |
| .B CONFIG_FANOTIFY |
| configuration option enabled. |
| In addition, fanotify permission handling is available only if the |
| .B CONFIG_FANOTIFY_ACCESS_PERMISSIONS |
| configuration option is enabled. |
| .SS 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. |
| .PP |
| The fanotify API does not report file accesses and modifications that |
| may occur because of |
| .BR mmap (2), |
| .BR msync (2), |
| and |
| .BR munmap (2). |
| .PP |
| 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. |
| .PP |
| 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. |
| .PP |
| The event queue can overflow. |
| In this case, events are lost. |
| .SH BUGS |
| Before Linux 3.19, |
| .BR fallocate (2) |
| did not generate fanotify events. |
| Since Linux 3.19, |
| .\" commit 820c12d5d6c0890bc93dd63893924a13041fdc35 |
| calls to |
| .BR fallocate (2) |
| generate |
| .B FAN_MODIFY |
| events. |
| .PP |
| As of Linux 3.17, |
| the following bugs exist: |
| .IP * 3 |
| On Linux, a filesystem object may be accessible through multiple paths, |
| for example, a part of a filesystem may be remounted using the |
| .IR \-\-bind |
| option of |
| .BR 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. |
| .IP * |
| .\" FIXME . A patch was proposed. |
| 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 |
| .B CAP_SYS_ADMIN |
| capability is set for programs executed by unprivileged users. |
| .IP * |
| If a call to |
| .BR 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 |
| .I errno |
| will not be set. |
| Thus, the reading application has no way to detect the error. |
| .SH 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 |
| .B FAN_PERM_OPEN |
| and |
| .BR FAN_CLOSE_WRITE . |
| When a permission event occurs, a |
| .B FAN_ALLOW |
| response is given. |
| .PP |
| The following output was recorded while editing the file |
| .IR /home/user/temp/notes . |
| Before the file was opened, a |
| .B FAN_OPEN_PERM |
| event occurred. |
| After the file was closed, a |
| .B FAN_CLOSE_WRITE |
| event occurred. |
| Execution of the program ends when the user presses the ENTER key. |
| .SS Example output |
| .in +4n |
| .EX |
| # ./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. |
| .EE |
| .in |
| .SS Program source |
| \& |
| .EX |
| #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); |
| } |
| .EE |
| .SH SEE ALSO |
| .ad l |
| .BR fanotify_init (2), |
| .BR fanotify_mark (2), |
| .BR inotify (7) |