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25 | .TH FANOTIFY 7 2019-03-06 "Linux" "Linux Programmer's Manual" | |
26 | .SH NAME | |
27 | fanotify \- monitoring filesystem events | |
28 | .SH DESCRIPTION | |
29 | The fanotify API provides notification and interception of | |
30 | filesystem events. | |
31 | Use cases include virus scanning and hierarchical storage management. | |
32 | Currently, only a limited set of events is supported. | |
33 | In particular, there is no support for create, delete, and move events. | |
34 | (See | |
35 | .BR inotify (7) | |
36 | for details of an API that does notify those events.) | |
37 | .PP | |
38 | Additional capabilities compared to the | |
39 | .BR inotify (7) | |
40 | API include the ability to monitor all of the objects | |
41 | in a mounted filesystem, | |
42 | the ability to make access permission decisions, and the | |
43 | possibility to read or modify files before access by other applications. | |
44 | .PP | |
45 | The following system calls are used with this API: | |
46 | .BR fanotify_init (2), | |
47 | .BR fanotify_mark (2), | |
48 | .BR read (2), | |
49 | .BR write (2), | |
50 | and | |
51 | .BR close (2). | |
52 | .SS fanotify_init(), fanotify_mark(), and notification groups | |
53 | The | |
54 | .BR fanotify_init (2) | |
55 | system call creates and initializes an fanotify notification group | |
56 | and returns a file descriptor referring to it. | |
57 | .PP | |
58 | An fanotify notification group is a kernel-internal object that holds | |
59 | a list of files, directories, filesystems, and mount points for which | |
60 | events shall be created. | |
61 | .PP | |
62 | For each entry in an fanotify notification group, two bit masks exist: the | |
63 | .I mark | |
64 | mask and the | |
65 | .I ignore | |
66 | mask. | |
67 | The mark mask defines file activities for which an event shall be created. | |
68 | The ignore mask defines activities for which no event shall be generated. | |
69 | Having these two types of masks permits a filesystem, mount point, or | |
70 | directory to be marked for receiving events, while at the same time | |
71 | ignoring events for specific objects under a mount point or directory. | |
72 | .PP | |
73 | The | |
74 | .BR fanotify_mark (2) | |
75 | system call adds a file, directory, filesystem or mount point to a | |
76 | notification group and specifies which events | |
77 | shall be reported (or ignored), or removes or modifies such an entry. | |
78 | .PP | |
79 | A possible usage of the ignore mask is for a file cache. | |
80 | Events of interest for a file cache are modification of a file and closing | |
81 | of the same. | |
82 | Hence, the cached directory or mount point is to be marked to receive these | |
83 | events. | |
84 | After receiving the first event informing that a file has been modified, | |
85 | the corresponding cache entry will be invalidated. | |
86 | No further modification events for this file are of interest until the file | |
87 | is closed. | |
88 | Hence, the modify event can be added to the ignore mask. | |
89 | Upon receiving the close event, the modify event can be removed from the | |
90 | ignore mask and the file cache entry can be updated. | |
91 | .PP | |
92 | The entries in the fanotify notification groups refer to files and | |
93 | directories via their inode number and to mounts via their mount ID. | |
94 | If files or directories are renamed or moved within the same mount, | |
95 | the respective entries survive. | |
96 | If files or directories are deleted or moved to another mount or if | |
97 | filesystems or mounts are unmounted, the corresponding entries are deleted. | |
98 | .SS The event queue | |
99 | As events occur on the filesystem objects monitored by a notification group, | |
100 | the fanotify system generates events that are collected in a queue. | |
101 | These events can then be read (using | |
102 | .BR read (2) | |
103 | or similar) | |
104 | from the fanotify file descriptor | |
105 | returned by | |
106 | .BR fanotify_init (2). | |
107 | .PP | |
108 | Two types of events are generated: | |
109 | .I notification | |
110 | events and | |
111 | .I permission | |
112 | events. | |
113 | Notification events are merely informative | |
114 | and require no action to be taken by | |
115 | the receiving application with the exception being that the file | |
116 | descriptor provided within a generic event must be closed. | |
117 | The closing of file descriptors for each event applies only to | |
118 | applications that have initialized fanotify without using | |
119 | .BR FAN_REPORT_FID | |
120 | (see below). | |
121 | Permission events are requests to the receiving application to decide | |
122 | whether permission for a file access shall be granted. | |
123 | For these events, the recipient must write a response which decides whether | |
124 | access is granted or not. | |
125 | .PP | |
126 | An event is removed from the event queue of the fanotify group | |
127 | when it has been read. | |
128 | Permission events that have been read are kept in an internal list of the | |
129 | fanotify group until either a permission decision has been taken by | |
130 | writing to the fanotify file descriptor or the fanotify file descriptor | |
131 | is closed. | |
132 | .SS Reading fanotify events | |
133 | Calling | |
134 | .BR read (2) | |
135 | for the file descriptor returned by | |
136 | .BR fanotify_init (2) | |
137 | blocks (if the flag | |
138 | .B FAN_NONBLOCK | |
139 | is not specified in the call to | |
140 | .BR fanotify_init (2)) | |
141 | until either a file event occurs or the call is interrupted by a signal | |
142 | (see | |
143 | .BR signal (7)). | |
144 | .PP | |
145 | The use of the | |
146 | .BR FAN_REPORT_FID | |
147 | flag in | |
148 | .BR fanotify_init (2) | |
149 | influences what data structures are returned to the event listener for each | |
150 | event. | |
151 | After a successful | |
152 | .BR read (2), | |
153 | the read buffer contains one or more of the following structures: | |
154 | .PP | |
155 | .in +4n | |
156 | .EX | |
157 | struct fanotify_event_metadata { | |
158 | __u32 event_len; | |
159 | __u8 vers; | |
160 | __u8 reserved; | |
161 | __u16 metadata_len; | |
162 | __aligned_u64 mask; | |
163 | __s32 fd; | |
164 | __s32 pid; | |
165 | }; | |
166 | .EE | |
167 | .in | |
168 | .PP | |
169 | In the case where | |
170 | .BR FAN_REPORT_FID | |
171 | is supplied as one of the flags to | |
172 | .BR fanotify_init (2), | |
173 | you should also expect to receive the structure detailed below following | |
174 | the generic | |
175 | .I fanotify_event_metadata | |
176 | structure within the read buffer: | |
177 | .PP | |
178 | .in +4n | |
179 | .EX | |
180 | struct fanotify_event_info_fid { | |
181 | struct fanotify_event_info_header hdr; | |
182 | __kernel_fsid_t fsid; | |
183 | unsigned char file_handle[0]; | |
184 | }; | |
185 | .EE | |
186 | .in | |
187 | .PP | |
188 | For performance reasons, it is recommended to use a large | |
189 | buffer size (for example, 4096 bytes), | |
190 | so that multiple events can be retrieved by a single | |
191 | .BR read (2). | |
192 | .PP | |
193 | The return value of | |
194 | .BR read (2) | |
195 | is the number of bytes placed in the buffer, | |
196 | or \-1 in case of an error (but see BUGS). | |
197 | .PP | |
198 | The fields of the | |
199 | .I fanotify_event_metadata | |
200 | structure are as follows: | |
201 | .TP | |
202 | .I event_len | |
203 | This is the length of the data for the current event and the offset | |
204 | to the next event in the buffer. | |
205 | Without | |
206 | .BR FAN_REPORT_FID , | |
207 | the value of | |
208 | .I event_len | |
209 | is always | |
210 | .BR FAN_EVENT_METADATA_LEN . | |
211 | With | |
212 | .BR FAN_REPORT_FID , | |
213 | .I event_len | |
214 | also includes the variable length file identifier. | |
215 | .TP | |
216 | .I vers | |
217 | This field holds a version number for the structure. | |
218 | It must be compared to | |
219 | .B FANOTIFY_METADATA_VERSION | |
220 | to verify that the structures returned at run time match | |
221 | the structures defined at compile time. | |
222 | In case of a mismatch, the application should abandon trying to use the | |
223 | fanotify file descriptor. | |
224 | .TP | |
225 | .I reserved | |
226 | This field is not used. | |
227 | .TP | |
228 | .I metadata_len | |
229 | This is the length of the structure. | |
230 | The field was introduced to facilitate the implementation of | |
231 | optional headers per event type. | |
232 | No such optional headers exist in the current implementation. | |
233 | .TP | |
234 | .I mask | |
235 | This is a bit mask describing the event (see below). | |
236 | .TP | |
237 | .I fd | |
238 | This is an open file descriptor for the object being accessed, or | |
239 | .B FAN_NOFD | |
240 | if a queue overflow occurred. | |
241 | If the fanotify file descriptor has been initialized using | |
242 | .BR FAN_REPORT_FID , | |
243 | applications should expect this value to be set to | |
244 | .B FAN_NOFD | |
245 | for each event that is received. | |
246 | The file descriptor can be used to access the contents | |
247 | of the monitored file or directory. | |
248 | The reading application is responsible for closing this file descriptor. | |
249 | .IP | |
250 | When calling | |
251 | .BR fanotify_init (2), | |
252 | the caller may specify (via the | |
253 | .I event_f_flags | |
254 | argument) various file status flags that are to be set | |
255 | on the open file description that corresponds to this file descriptor. | |
256 | In addition, the (kernel-internal) | |
257 | .B FMODE_NONOTIFY | |
258 | file status flag is set on the open file description. | |
259 | This flag suppresses fanotify event generation. | |
260 | Hence, when the receiver of the fanotify event accesses the notified file or | |
261 | directory using this file descriptor, no additional events will be created. | |
262 | .TP | |
263 | .I pid | |
264 | If flag | |
265 | .B FAN_REPORT_TID | |
266 | was set in | |
267 | .BR fanotify_init (2), | |
268 | this is the TID of the thread that caused the event. | |
269 | Otherwise, this the PID of the process that caused the event. | |
270 | .PP | |
271 | A program listening to fanotify events can compare this PID | |
272 | to the PID returned by | |
273 | .BR getpid (2), | |
274 | to determine whether the event is caused by the listener itself, | |
275 | or is due to a file access by another process. | |
276 | .PP | |
277 | The bit mask in | |
278 | .I mask | |
279 | indicates which events have occurred for a single filesystem object. | |
280 | Multiple bits may be set in this mask, | |
281 | if more than one event occurred for the monitored filesystem object. | |
282 | In particular, | |
283 | consecutive events for the same filesystem object and originating from the | |
284 | same process may be merged into a single event, with the exception that two | |
285 | permission events are never merged into one queue entry. | |
286 | .PP | |
287 | The bits that may appear in | |
288 | .I mask | |
289 | are as follows: | |
290 | .TP | |
291 | .B FAN_ACCESS | |
292 | A file or a directory (but see BUGS) was accessed (read). | |
293 | .TP | |
294 | .B FAN_OPEN | |
295 | A file or a directory was opened. | |
296 | .TP | |
297 | .B FAN_OPEN_EXEC | |
298 | A file was opened with the intent to be executed. | |
299 | See NOTES in | |
300 | .BR fanotify_mark (2) | |
301 | for additional details. | |
302 | .TP | |
303 | .B FAN_ATTRIB | |
304 | A file or directory metadata was changed. | |
305 | .TP | |
306 | .B FAN_CREATE | |
307 | A child file or directory was created in a watched parent. | |
308 | .TP | |
309 | .B FAN_DELETE | |
310 | A child file or directory was deleted in a watched parent. | |
311 | .TP | |
312 | .B FAN_DELETE_SELF | |
313 | A watched file or directory was deleted. | |
314 | .TP | |
315 | .B FAN_MOVED_FROM | |
316 | A file or directory has been moved from a watched parent directory. | |
317 | .TP | |
318 | .B FAN_MOVED_TO | |
319 | A file or directory has been moved to a watched parent directory. | |
320 | .TP | |
321 | .B FAN_MOVE_SELF | |
322 | A watched file or directory was moved. | |
323 | .TP | |
324 | .B FAN_MODIFY | |
325 | A file was modified. | |
326 | .TP | |
327 | .B FAN_CLOSE_WRITE | |
328 | A file that was opened for writing | |
329 | .RB ( O_WRONLY | |
330 | or | |
331 | .BR O_RDWR ) | |
332 | was closed. | |
333 | .TP | |
334 | .B FAN_CLOSE_NOWRITE | |
335 | A file or directory that was opened read-only | |
336 | .RB ( O_RDONLY ) | |
337 | was closed. | |
338 | .TP | |
339 | .B FAN_Q_OVERFLOW | |
340 | The event queue exceeded the limit of 16384 entries. | |
341 | This limit can be overridden by specifying the | |
342 | .BR FAN_UNLIMITED_QUEUE | |
343 | flag when calling | |
344 | .BR fanotify_init (2). | |
345 | .TP | |
346 | .B FAN_ACCESS_PERM | |
347 | An application wants to read a file or directory, for example using | |
348 | .BR read (2) | |
349 | or | |
350 | .BR readdir (2). | |
351 | The reader must write a response (as described below) | |
352 | that determines whether the permission to | |
353 | access the filesystem object shall be granted. | |
354 | .TP | |
355 | .B FAN_OPEN_PERM | |
356 | An application wants to open a file or directory. | |
357 | The reader must write a response that determines whether the permission to | |
358 | open the filesystem object shall be granted. | |
359 | .TP | |
360 | .B FAN_OPEN_EXEC_PERM | |
361 | An application wants to open a file for execution. | |
362 | The reader must write a response that determines whether the permission to | |
363 | open the filesystem object for execution shall be granted. | |
364 | See NOTES in | |
365 | .BR fanotify_mark (2) | |
366 | for additional details. | |
367 | .PP | |
368 | To check for any close event, the following bit mask may be used: | |
369 | .TP | |
370 | .B FAN_CLOSE | |
371 | A file was closed. | |
372 | This is a synonym for: | |
373 | .IP | |
374 | FAN_CLOSE_WRITE | FAN_CLOSE_NOWRITE | |
375 | .PP | |
376 | To check for any move event, the following bit mask may be used: | |
377 | .TP | |
378 | .B FAN_MOVE | |
379 | A file or directory was moved. | |
380 | This is a synonym for: | |
381 | .IP | |
382 | FAN_MOVED_FROM | FAN_MOVED_TO | |
383 | .PP | |
384 | The fields of the | |
385 | .I fanotify_event_info_fid | |
386 | structure are as follows: | |
387 | .TP | |
388 | .I hdr | |
389 | This is a structure of type | |
390 | .IR fanotify_event_info_header . | |
391 | It is a generic header that contains information used to describe | |
392 | additional information attached to the event. | |
393 | For example, when an fanotify file descriptor is created using | |
394 | .BR FAN_REPORT_FID , | |
395 | the | |
396 | .I info_type | |
397 | field of this header is set to | |
398 | .BR FAN_EVENT_INFO_TYPE_FID . | |
399 | Event listeners can use this field to check that the additional | |
400 | information received for an event is of the correct type. | |
401 | Additionally, the | |
402 | .I fanotify_event_info_header | |
403 | also contains a | |
404 | .I len | |
405 | field. | |
406 | In the current implementation, the value of | |
407 | .I len | |
408 | is always (event_len \- FAN_EVENT_METADATA_LEN). | |
409 | .TP | |
410 | .I fsid | |
411 | This is a unique identifier of the filesystem containing the object | |
412 | associated with the event. | |
413 | It is a structure of type | |
414 | .I __kernel_fsid_t | |
415 | and contains the same value as | |
416 | .I f_fsid | |
417 | when calling | |
418 | .BR statfs (2). | |
419 | .TP | |
420 | .I file_handle | |
421 | This is a variable length structure of type | |
422 | .IR file_handle . | |
423 | It is an opaque handle that corresponds to a specified object on a | |
424 | filesystem as returned by | |
425 | .BR name_to_handle_at (2). | |
426 | It can be used to uniquely identify a file on a filesystem and can be | |
427 | passed as an argument to | |
428 | .BR open_by_handle_at (2). | |
429 | Note that for directory entry events, such as | |
430 | .BR FAN_CREATE , | |
431 | .BR FAN_DELETE , | |
432 | and | |
433 | .BR FAN_MOVE , | |
434 | the | |
435 | .IR file_handle | |
436 | describes the modified directory and not the created/deleted/moved child | |
437 | object. | |
438 | The events | |
439 | .BR FAN_ATTRIB , | |
440 | .BR FAN_DELETE_SELF , | |
441 | and | |
442 | .BR FAN_MOVE_SELF | |
443 | will carry the | |
444 | .IR file_handle | |
445 | information for the child object if the child object is being watched. | |
446 | .PP | |
447 | The following macros are provided to iterate over a buffer containing | |
448 | fanotify event metadata returned by a | |
449 | .BR read (2) | |
450 | from an fanotify file descriptor: | |
451 | .TP | |
452 | .B FAN_EVENT_OK(meta, len) | |
453 | This macro checks the remaining length | |
454 | .I len | |
455 | of the buffer | |
456 | .I meta | |
457 | against the length of the metadata structure and the | |
458 | .I event_len | |
459 | field of the first metadata structure in the buffer. | |
460 | .TP | |
461 | .B FAN_EVENT_NEXT(meta, len) | |
462 | This macro uses the length indicated in the | |
463 | .I event_len | |
464 | field of the metadata structure pointed to by | |
465 | .IR meta | |
466 | to calculate the address of the next metadata structure that follows | |
467 | .IR meta . | |
468 | .I len | |
469 | is the number of bytes of metadata that currently remain in the buffer. | |
470 | The macro returns a pointer to the next metadata structure that follows | |
471 | .IR meta , | |
472 | and reduces | |
473 | .I len | |
474 | by the number of bytes in the metadata structure that | |
475 | has been skipped over (i.e., it subtracts | |
476 | .IR meta\->event_len | |
477 | from | |
478 | .IR len ). | |
479 | .PP | |
480 | In addition, there is: | |
481 | .TP | |
482 | .B FAN_EVENT_METADATA_LEN | |
483 | This macro returns the size (in bytes) of the structure | |
484 | .IR fanotify_event_metadata . | |
485 | This is the minimum size (and currently the only size) of any event metadata. | |
486 | .\" | |
487 | .SS Monitoring an fanotify file descriptor for events | |
488 | When an fanotify event occurs, the fanotify file descriptor indicates as | |
489 | readable when passed to | |
490 | .BR epoll (7), | |
491 | .BR poll (2), | |
492 | or | |
493 | .BR select (2). | |
494 | .SS Dealing with permission events | |
495 | For permission events, the application must | |
496 | .BR write (2) | |
497 | a structure of the following form to the | |
498 | fanotify file descriptor: | |
499 | .PP | |
500 | .in +4n | |
501 | .EX | |
502 | struct fanotify_response { | |
503 | __s32 fd; | |
504 | __u32 response; | |
505 | }; | |
506 | .EE | |
507 | .in | |
508 | .PP | |
509 | The fields of this structure are as follows: | |
510 | .TP | |
511 | .I fd | |
512 | This is the file descriptor from the structure | |
513 | .IR fanotify_event_metadata . | |
514 | .TP | |
515 | .I response | |
516 | This field indicates whether or not the permission is to be granted. | |
517 | Its value must be either | |
518 | .B FAN_ALLOW | |
519 | to allow the file operation or | |
520 | .B FAN_DENY | |
521 | to deny the file operation. | |
522 | .PP | |
523 | If access is denied, the requesting application call will receive an | |
524 | .BR EPERM | |
525 | error. | |
526 | .SS Closing the fanotify file descriptor | |
527 | .PP | |
528 | When all file descriptors referring to the fanotify notification group are | |
529 | closed, the fanotify group is released and its resources | |
530 | are freed for reuse by the kernel. | |
531 | Upon | |
532 | .BR close (2), | |
533 | outstanding permission events will be set to allowed. | |
534 | .SS /proc/[pid]/fdinfo | |
535 | The file | |
536 | .I /proc/[pid]/fdinfo/[fd] | |
537 | contains information about fanotify marks for file descriptor | |
538 | .I fd | |
539 | of process | |
540 | .IR pid . | |
541 | See | |
542 | .BR proc (5) | |
543 | for details. | |
544 | .SH ERRORS | |
545 | In addition to the usual errors for | |
546 | .BR read (2), | |
547 | the following errors can occur when reading from the | |
548 | fanotify file descriptor: | |
549 | .TP | |
550 | .B EINVAL | |
551 | The buffer is too small to hold the event. | |
552 | .TP | |
553 | .B EMFILE | |
554 | The per-process limit on the number of open files has been reached. | |
555 | See the description of | |
556 | .B RLIMIT_NOFILE | |
557 | in | |
558 | .BR getrlimit (2). | |
559 | .TP | |
560 | .B ENFILE | |
561 | The system-wide limit on the total number of open files has been reached. | |
562 | See | |
563 | .I /proc/sys/fs/file\-max | |
564 | in | |
565 | .BR proc (5). | |
566 | .TP | |
567 | .B ETXTBSY | |
568 | This error is returned by | |
569 | .BR read (2) | |
570 | if | |
571 | .B O_RDWR | |
572 | or | |
573 | .B O_WRONLY | |
574 | was specified in the | |
575 | .I event_f_flags | |
576 | argument when calling | |
577 | .BR fanotify_init (2) | |
578 | and an event occurred for a monitored file that is currently being executed. | |
579 | .PP | |
580 | In addition to the usual errors for | |
581 | .BR write (2), | |
582 | the following errors can occur when writing to the fanotify file descriptor: | |
583 | .TP | |
584 | .B EINVAL | |
585 | Fanotify access permissions are not enabled in the kernel configuration | |
586 | or the value of | |
587 | .I response | |
588 | in the response structure is not valid. | |
589 | .TP | |
590 | .B ENOENT | |
591 | The file descriptor | |
592 | .I fd | |
593 | in the response structure is not valid. | |
594 | This may occur when a response for the permission event has already been | |
595 | written. | |
596 | .SH VERSIONS | |
597 | The fanotify API was introduced in version 2.6.36 of the Linux kernel and | |
598 | enabled in version 2.6.37. | |
599 | Fdinfo support was added in version 3.8. | |
600 | .SH CONFORMING TO | |
601 | The fanotify API is Linux-specific. | |
602 | .SH NOTES | |
603 | The fanotify API is available only if the kernel was built with the | |
604 | .B CONFIG_FANOTIFY | |
605 | configuration option enabled. | |
606 | In addition, fanotify permission handling is available only if the | |
607 | .B CONFIG_FANOTIFY_ACCESS_PERMISSIONS | |
608 | configuration option is enabled. | |
609 | .SS Limitations and caveats | |
610 | Fanotify reports only events that a user-space program triggers through the | |
611 | filesystem API. | |
612 | As a result, | |
613 | it does not catch remote events that occur on network filesystems. | |
614 | .PP | |
615 | The fanotify API does not report file accesses and modifications that | |
616 | may occur because of | |
617 | .BR mmap (2), | |
618 | .BR msync (2), | |
619 | and | |
620 | .BR munmap (2). | |
621 | .PP | |
622 | Events for directories are created only if the directory itself is opened, | |
623 | read, and closed. | |
624 | Adding, removing, or changing children of a marked directory does not create | |
625 | events for the monitored directory itself. | |
626 | .PP | |
627 | Fanotify monitoring of directories is not recursive: | |
628 | to monitor subdirectories under a directory, | |
629 | additional marks must be created. | |
630 | (But note that the fanotify API provides no way of detecting when a | |
631 | subdirectory has been created under a marked directory, | |
632 | which makes recursive monitoring difficult.) | |
633 | Monitoring mounts offers the capability to monitor a whole directory tree. | |
634 | Monitoring filesystems offers the capability to monitor changes made from | |
635 | any mount of a filesystem instance. | |
636 | .PP | |
637 | The event queue can overflow. | |
638 | In this case, events are lost. | |
639 | .SH BUGS | |
640 | Before Linux 3.19, | |
641 | .BR fallocate (2) | |
642 | did not generate fanotify events. | |
643 | Since Linux 3.19, | |
644 | .\" commit 820c12d5d6c0890bc93dd63893924a13041fdc35 | |
645 | calls to | |
646 | .BR fallocate (2) | |
647 | generate | |
648 | .B FAN_MODIFY | |
649 | events. | |
650 | .PP | |
651 | As of Linux 3.17, | |
652 | the following bugs exist: | |
653 | .IP * 3 | |
654 | On Linux, a filesystem object may be accessible through multiple paths, | |
655 | for example, a part of a filesystem may be remounted using the | |
656 | .IR \-\-bind | |
657 | option of | |
658 | .BR mount (8). | |
659 | A listener that marked a mount will be notified only of events that were | |
660 | triggered for a filesystem object using the same mount. | |
661 | Any other event will pass unnoticed. | |
662 | .IP * | |
663 | .\" FIXME . A patch was proposed. | |
664 | When an event is generated, | |
665 | no check is made to see whether the user ID of the | |
666 | receiving process has authorization to read or write the file | |
667 | before passing a file descriptor for that file. | |
668 | This poses a security risk, when the | |
669 | .B CAP_SYS_ADMIN | |
670 | capability is set for programs executed by unprivileged users. | |
671 | .IP * | |
672 | If a call to | |
673 | .BR read (2) | |
674 | processes multiple events from the fanotify queue and an error occurs, | |
675 | the return value will be the total length of the events successfully | |
676 | copied to the user-space buffer before the error occurred. | |
677 | The return value will not be \-1, and | |
678 | .I errno | |
679 | will not be set. | |
680 | Thus, the reading application has no way to detect the error. | |
681 | .SH EXAMPLE | |
682 | The two example programs below demonstrate the usage of the fanotify API. | |
683 | .SS Example program: fanotify_example.c | |
684 | The first program is an example of fanotify being | |
685 | used with its event object information passed in the form of a file | |
686 | descriptor. | |
687 | The program marks the mount point passed as a command-line argument and | |
688 | waits for events of type | |
689 | .B FAN_OPEN_PERM | |
690 | and | |
691 | .BR FAN_CLOSE_WRITE . | |
692 | When a permission event occurs, a | |
693 | .B FAN_ALLOW | |
694 | response is given. | |
695 | .PP | |
696 | The following shell session shows an example of | |
697 | running this program. | |
698 | This session involved editing the file | |
699 | .IR /home/user/temp/notes . | |
700 | Before the file was opened, a | |
701 | .B FAN_OPEN_PERM | |
702 | event occurred. | |
703 | After the file was closed, a | |
704 | .B FAN_CLOSE_WRITE | |
705 | event occurred. | |
706 | Execution of the program ends when the user presses the ENTER key. | |
707 | .PP | |
708 | .in +4n | |
709 | .EX | |
710 | # \fB./fanotify_example /home\fP | |
711 | Press enter key to terminate. | |
712 | Listening for events. | |
713 | FAN_OPEN_PERM: File /home/user/temp/notes | |
714 | FAN_CLOSE_WRITE: File /home/user/temp/notes | |
715 | ||
716 | Listening for events stopped. | |
717 | .EE | |
718 | .in | |
719 | .EE | |
720 | .in | |
721 | .SS Program source: fanotify_example.c | |
722 | \& | |
723 | .EX | |
724 | #define _GNU_SOURCE /* Needed to get O_LARGEFILE definition */ | |
725 | #include <errno.h> | |
726 | #include <fcntl.h> | |
727 | #include <limits.h> | |
728 | #include <poll.h> | |
729 | #include <stdio.h> | |
730 | #include <stdlib.h> | |
731 | #include <sys/fanotify.h> | |
732 | #include <unistd.h> | |
733 | ||
734 | /* Read all available fanotify events from the file descriptor \(aqfd\(aq */ | |
735 | ||
736 | static void | |
737 | handle_events(int fd) | |
738 | { | |
739 | const struct fanotify_event_metadata *metadata; | |
740 | struct fanotify_event_metadata buf[200]; | |
741 | ssize_t len; | |
742 | char path[PATH_MAX]; | |
743 | ssize_t path_len; | |
744 | char procfd_path[PATH_MAX]; | |
745 | struct fanotify_response response; | |
746 | ||
747 | /* Loop while events can be read from fanotify file descriptor */ | |
748 | ||
749 | for (;;) { | |
750 | ||
751 | /* Read some events */ | |
752 | ||
753 | len = read(fd, (void *) &buf, sizeof(buf)); | |
754 | if (len == \-1 && errno != EAGAIN) { | |
755 | perror("read"); | |
756 | exit(EXIT_FAILURE); | |
757 | } | |
758 | ||
759 | /* Check if end of available data reached */ | |
760 | ||
761 | if (len <= 0) | |
762 | break; | |
763 | ||
764 | /* Point to the first event in the buffer */ | |
765 | ||
766 | metadata = buf; | |
767 | ||
768 | /* Loop over all events in the buffer */ | |
769 | ||
770 | while (FAN_EVENT_OK(metadata, len)) { | |
771 | ||
772 | /* Check that run\-time and compile\-time structures match */ | |
773 | ||
774 | if (metadata\->vers != FANOTIFY_METADATA_VERSION) { | |
775 | fprintf(stderr, | |
776 | "Mismatch of fanotify metadata version.\en"); | |
777 | exit(EXIT_FAILURE); | |
778 | } | |
779 | ||
780 | /* metadata\->fd contains either FAN_NOFD, indicating a | |
781 | queue overflow, or a file descriptor (a nonnegative | |
782 | integer). Here, we simply ignore queue overflow. */ | |
783 | ||
784 | if (metadata\->fd >= 0) { | |
785 | ||
786 | /* Handle open permission event */ | |
787 | ||
788 | if (metadata\->mask & FAN_OPEN_PERM) { | |
789 | printf("FAN_OPEN_PERM: "); | |
790 | ||
791 | /* Allow file to be opened */ | |
792 | ||
793 | response.fd = metadata\->fd; | |
794 | response.response = FAN_ALLOW; | |
795 | write(fd, &response, | |
796 | sizeof(struct fanotify_response)); | |
797 | } | |
798 | ||
799 | /* Handle closing of writable file event */ | |
800 | ||
801 | if (metadata\->mask & FAN_CLOSE_WRITE) | |
802 | printf("FAN_CLOSE_WRITE: "); | |
803 | ||
804 | /* Retrieve and print pathname of the accessed file */ | |
805 | ||
806 | snprintf(procfd_path, sizeof(procfd_path), | |
807 | "/proc/self/fd/%d", metadata\->fd); | |
808 | path_len = readlink(procfd_path, path, | |
809 | sizeof(path) \- 1); | |
810 | if (path_len == \-1) { | |
811 | perror("readlink"); | |
812 | exit(EXIT_FAILURE); | |
813 | } | |
814 | ||
815 | path[path_len] = \(aq\e0\(aq; | |
816 | printf("File %s\en", path); | |
817 | ||
818 | /* Close the file descriptor of the event */ | |
819 | ||
820 | close(metadata\->fd); | |
821 | } | |
822 | ||
823 | /* Advance to next event */ | |
824 | ||
825 | metadata = FAN_EVENT_NEXT(metadata, len); | |
826 | } | |
827 | } | |
828 | } | |
829 | ||
830 | int | |
831 | main(int argc, char *argv[]) | |
832 | { | |
833 | char buf; | |
834 | int fd, poll_num; | |
835 | nfds_t nfds; | |
836 | struct pollfd fds[2]; | |
837 | ||
838 | /* Check mount point is supplied */ | |
839 | ||
840 | if (argc != 2) { | |
841 | fprintf(stderr, "Usage: %s MOUNT\en", argv[0]); | |
842 | exit(EXIT_FAILURE); | |
843 | } | |
844 | ||
845 | printf("Press enter key to terminate.\en"); | |
846 | ||
847 | /* Create the file descriptor for accessing the fanotify API */ | |
848 | ||
849 | fd = fanotify_init(FAN_CLOEXEC | FAN_CLASS_CONTENT | FAN_NONBLOCK, | |
850 | O_RDONLY | O_LARGEFILE); | |
851 | if (fd == \-1) { | |
852 | perror("fanotify_init"); | |
853 | exit(EXIT_FAILURE); | |
854 | } | |
855 | ||
856 | /* Mark the mount for: | |
857 | \- permission events before opening files | |
858 | \- notification events after closing a write\-enabled | |
859 | file descriptor */ | |
860 | ||
861 | if (fanotify_mark(fd, FAN_MARK_ADD | FAN_MARK_MOUNT, | |
862 | FAN_OPEN_PERM | FAN_CLOSE_WRITE, AT_FDCWD, | |
863 | argv[1]) == \-1) { | |
864 | perror("fanotify_mark"); | |
865 | exit(EXIT_FAILURE); | |
866 | } | |
867 | ||
868 | /* Prepare for polling */ | |
869 | ||
870 | nfds = 2; | |
871 | ||
872 | /* Console input */ | |
873 | ||
874 | fds[0].fd = STDIN_FILENO; | |
875 | fds[0].events = POLLIN; | |
876 | ||
877 | /* Fanotify input */ | |
878 | ||
879 | fds[1].fd = fd; | |
880 | fds[1].events = POLLIN; | |
881 | ||
882 | /* This is the loop to wait for incoming events */ | |
883 | ||
884 | printf("Listening for events.\en"); | |
885 | ||
886 | while (1) { | |
887 | poll_num = poll(fds, nfds, \-1); | |
888 | if (poll_num == \-1) { | |
889 | if (errno == EINTR) /* Interrupted by a signal */ | |
890 | continue; /* Restart poll() */ | |
891 | ||
892 | perror("poll"); /* Unexpected error */ | |
893 | exit(EXIT_FAILURE); | |
894 | } | |
895 | ||
896 | if (poll_num > 0) { | |
897 | if (fds[0].revents & POLLIN) { | |
898 | ||
899 | /* Console input is available: empty stdin and quit */ | |
900 | ||
901 | while (read(STDIN_FILENO, &buf, 1) > 0 && buf != \(aq\en\(aq) | |
902 | continue; | |
903 | break; | |
904 | } | |
905 | ||
906 | if (fds[1].revents & POLLIN) { | |
907 | ||
908 | /* Fanotify events are available */ | |
909 | ||
910 | handle_events(fd); | |
911 | } | |
912 | } | |
913 | } | |
914 | ||
915 | printf("Listening for events stopped.\en"); | |
916 | exit(EXIT_SUCCESS); | |
917 | } | |
918 | .EE | |
919 | .\" | |
920 | .SS Example program: fanotify_fid.c | |
921 | The second program is an example of fanotify being used with | |
922 | .B FAN_REPORT_FID | |
923 | enabled. | |
924 | The program marks the filesystem object that is passed as | |
925 | a command-line argument | |
926 | and waits until an event of type | |
927 | .B FAN_CREATE | |
928 | has occurred. | |
929 | The event mask indicates which type of filesystem object\(emeither | |
930 | a file or a directory\(emwas created". | |
931 | Once all events have been read from the buffer and processed accordingly, | |
932 | the program simply terminates. | |
933 | .PP | |
934 | The following shell sessions show two different invocations of | |
935 | this program, with different actions performed on a watched object. | |
936 | .PP | |
937 | The first session shows a mark being placed on | |
938 | .IR /home/user . | |
939 | This is followed by the creation of a regular file, | |
940 | .IR /home/user/testfile.txt . | |
941 | This results in a | |
942 | .B FAN_CREATE | |
943 | event being created and reported against the file's parent watched | |
944 | directory object. | |
945 | Program execution ends once all events captured within the buffer have | |
946 | been processed. | |
947 | Program execution ends once all events captured within the buffer are | |
948 | processed. | |
949 | .PP | |
950 | .in +4n | |
951 | .EX | |
952 | # \fB./fanotify_fid /home/user\fP | |
953 | Listening for events. | |
954 | FAN_CREATE (file created): Directory /home/user has been modified. | |
955 | All events processed successfully. Program exiting. | |
956 | ||
957 | $ \fBtouch /home/user/testing\fP # In another terminal | |
958 | .EE | |
959 | .in | |
960 | .PP | |
961 | The second session shows a mark being placed on | |
962 | .IR /home/user . | |
963 | This is followed by the creation of a directory, | |
964 | .IR /home/user/testdir . | |
965 | This specific action results in the program producing a | |
966 | .B FAN_CREATE | |
967 | and | |
968 | .B FAN_ONDIR | |
969 | event. | |
970 | .PP | |
971 | .in +4n | |
972 | .EX | |
973 | # \fB./fanotify_fid /home/user\fP | |
974 | Listening for events. | |
975 | FAN_CREATE | FAN_ONDIR (subdirectory created): | |
976 | Directory /home/user has been modified. | |
977 | All events processed successfully. Program exiting. | |
978 | ||
979 | $ \fBmkdir \-p /home/user/testing\fP # In another terminal | |
980 | .EE | |
981 | .in | |
982 | .SS Program source: fanotify_fid.c | |
983 | \& | |
984 | .EX | |
985 | #define _GNU_SOURCE | |
986 | #include <errno.h> | |
987 | #include <fcntl.h> | |
988 | #include <limits.h> | |
989 | #include <stdio.h> | |
990 | #include <stdlib.h> | |
991 | #include <sys/types.h> | |
992 | #include <sys/stat.h> | |
993 | #include <sys/fanotify.h> | |
994 | #include <unistd.h> | |
995 | ||
996 | #define BUF_SIZE 256 | |
997 | ||
998 | int | |
999 | main(int argc, char **argv) | |
1000 | { | |
1001 | int fd, ret, event_fd; | |
1002 | ssize_t len, path_len; | |
1003 | char path[PATH_MAX]; | |
1004 | char procfd_path[PATH_MAX]; | |
1005 | char events_buf[BUF_SIZE]; | |
1006 | struct file_handle *file_handle; | |
1007 | struct fanotify_event_metadata *metadata; | |
1008 | struct fanotify_event_info_fid *fid; | |
1009 | ||
1010 | if (argc != 2) { | |
1011 | fprintf(stderr, "Invalid number of command line arguments.\e\n"); | |
1012 | exit(EXIT_FAILURE); | |
1013 | } | |
1014 | ||
1015 | /* Create an fanotify file descriptor with FAN_REPORT_FID as a flag | |
1016 | so that program can receive fid events. */ | |
1017 | ||
1018 | fd = fanotify_init(FAN_CLASS_NOTIF | FAN_REPORT_FID, 0); | |
1019 | if (fd == \-1) { | |
1020 | perror("fanotify_init"); | |
1021 | exit(EXIT_FAILURE); | |
1022 | } | |
1023 | ||
1024 | /* Place a mark on the filesystem object supplied in argv[1]. */ | |
1025 | ||
1026 | ret = fanotify_mark(fd, FAN_MARK_ADD | FAN_MARK_ONLYDIR, | |
1027 | FAN_CREATE | FAN_ONDIR, | |
1028 | AT_FDCWD, argv[1]); | |
1029 | if (ret == \-1) { | |
1030 | perror("fanotify_mark"); | |
1031 | exit(EXIT_FAILURE); | |
1032 | } | |
1033 | ||
1034 | printf("Listening for events.\e\n"); | |
1035 | ||
1036 | /* Read events from the event queue into a buffer */ | |
1037 | ||
1038 | len = read(fd, (void *) &events_buf, sizeof(events_buf)); | |
1039 | if (len == \-1 && errno != EAGAIN) { | |
1040 | perror("read"); | |
1041 | exit(EXIT_FAILURE); | |
1042 | } | |
1043 | ||
1044 | /* Process all events within the buffer */ | |
1045 | ||
1046 | for (metadata = (struct fanotify_event_metadata *) events_buf; | |
1047 | FAN_EVENT_OK(metadata, len); | |
1048 | metadata = FAN_EVENT_NEXT(metadata, len)) { | |
1049 | fid = (struct fanotify_event_info_fid *) (metadata + 1); | |
1050 | file_handle = (struct file_handle *) fid->handle; | |
1051 | ||
1052 | /* Ensure that the event info is of the correct type */ | |
1053 | ||
1054 | if (fid->hdr.info_type != FAN_EVENT_INFO_TYPE_FID) { | |
1055 | fprintf(stderr, "Received unexpected event info type.\e\n"); | |
1056 | exit(EXIT_FAILURE); | |
1057 | } | |
1058 | ||
1059 | if (metadata->mask == FAN_CREATE) | |
1060 | printf("FAN_CREATE (file created):"); | |
1061 | ||
1062 | if (metadata->mask == FAN_CREATE | FAN_ONDIR) | |
1063 | printf("FAN_CREATE | FAN_ONDIR (subdirectory created):"); | |
1064 | ||
1065 | /* metadata->fd is set to FAN_NOFD when FAN_REPORT_FID is enabled. | |
1066 | To obtain a file descriptor for the file object corresponding to | |
1067 | an event you can use the struct file_handle that\(aqs provided | |
1068 | within the fanotify_event_info_fid in conjunction with the | |
1069 | open_by_handle_at(2) system call. A check for ESTALE is done | |
1070 | to accommodate for the situation where the file handle for the | |
1071 | object was deleted prior to this system call. */ | |
1072 | ||
1073 | event_fd = open_by_handle_at(AT_FDCWD, file_handle, O_RDONLY); | |
1074 | if (ret == \-1) { | |
1075 | if (errno == ESTALE) { | |
1076 | printf("File handle is no longer valid. " | |
1077 | "File has been deleted\e\n"); | |
1078 | continue; | |
1079 | } else { | |
1080 | perror("open_by_handle_at"); | |
1081 | exit(EXIT_FAILURE); | |
1082 | } | |
1083 | } | |
1084 | ||
1085 | snprintf(procfd_path, sizeof(procfd_path), "/proc/self/fd/%d", | |
1086 | event_fd); | |
1087 | ||
1088 | /* Retrieve and print the path of the modified dentry */ | |
1089 | ||
1090 | path_len = readlink(procfd_path, path, sizeof(path) \- 1); | |
1091 | if (path_len == \-1) { | |
1092 | perror("readlink"); | |
1093 | exit(EXIT_FAILURE); | |
1094 | } | |
1095 | ||
1096 | path[path_len] = \(aq\e\0\(aq; | |
1097 | printf("\etDirectory \(aq%s\(aq has been modified.\e\n", path); | |
1098 | ||
1099 | /* Close associated file descriptor for this event */ | |
1100 | ||
1101 | close(event_fd); | |
1102 | } | |
1103 | ||
1104 | printf("All events processed successfully. Program exiting.\e\n"); | |
1105 | exit(EXIT_SUCCESS); | |
1106 | } | |
1107 | .EE | |
1108 | .SH SEE ALSO | |
1109 | .ad l | |
1110 | .BR fanotify_init (2), | |
1111 | .BR fanotify_mark (2), | |
1112 | .BR inotify (7) |