]>
Commit | Line | Data |
---|---|---|
1 | .\" Copyright (c) 2016, 2019 by Michael Kerrisk <mtk.manpages@gmail.com> | |
2 | .\" | |
3 | .\" %%%LICENSE_START(VERBATIM) | |
4 | .\" Permission is granted to make and distribute verbatim copies of this | |
5 | .\" manual provided the copyright notice and this permission notice are | |
6 | .\" preserved on all copies. | |
7 | .\" | |
8 | .\" Permission is granted to copy and distribute modified versions of this | |
9 | .\" manual under the conditions for verbatim copying, provided that the | |
10 | .\" entire resulting derived work is distributed under the terms of a | |
11 | .\" permission notice identical to this one. | |
12 | .\" | |
13 | .\" Since the Linux kernel and libraries are constantly changing, this | |
14 | .\" manual page may be incorrect or out-of-date. The author(s) assume no | |
15 | .\" responsibility for errors or omissions, or for damages resulting from | |
16 | .\" the use of the information contained herein. The author(s) may not | |
17 | .\" have taken the same level of care in the production of this manual, | |
18 | .\" which is licensed free of charge, as they might when working | |
19 | .\" professionally. | |
20 | .\" | |
21 | .\" Formatted or processed versions of this manual, if unaccompanied by | |
22 | .\" the source, must acknowledge the copyright and authors of this work. | |
23 | .\" %%%LICENSE_END | |
24 | .\" | |
25 | .\" | |
26 | .TH MOUNT_NAMESPACES 7 2019-08-02 "Linux" "Linux Programmer's Manual" | |
27 | .SH NAME | |
28 | mount_namespaces \- overview of Linux mount namespaces | |
29 | .SH DESCRIPTION | |
30 | For an overview of namespaces, see | |
31 | .BR namespaces (7). | |
32 | .PP | |
33 | Mount namespaces provide isolation of the list of mount points seen | |
34 | by the processes in each namespace instance. | |
35 | Thus, the processes in each of the mount namespace instances | |
36 | will see distinct single-directory hierarchies. | |
37 | .PP | |
38 | The views provided by the | |
39 | .IR /proc/[pid]/mounts , | |
40 | .IR /proc/[pid]/mountinfo , | |
41 | and | |
42 | .IR /proc/[pid]/mountstats | |
43 | files (all described in | |
44 | .BR proc (5)) | |
45 | correspond to the mount namespace in which the process with the PID | |
46 | .IR [pid] | |
47 | resides. | |
48 | (All of the processes that reside in the same mount namespace | |
49 | will see the same view in these files.) | |
50 | .PP | |
51 | A new mount namespace is created using either | |
52 | .BR clone (2) | |
53 | or | |
54 | .BR unshare (2) | |
55 | with the | |
56 | .BR CLONE_NEWNS | |
57 | flag. | |
58 | When a new mount namespace is created, | |
59 | its mount point list is initialized as follows: | |
60 | .IP * 3 | |
61 | If the namespace is created using | |
62 | .BR clone (2), | |
63 | the mount point list of the child's namespace is a copy | |
64 | of the mount point list in the parent's namespace. | |
65 | .IP * | |
66 | If the namespace is created using | |
67 | .BR unshare (2), | |
68 | the mount point list of the new namespace is a copy of | |
69 | the mount point list in the caller's previous mount namespace. | |
70 | .PP | |
71 | Subsequent modifications to the mount point list | |
72 | .RB ( mount (2) | |
73 | and | |
74 | .BR umount (2)) | |
75 | in either mount namespace will not (by default) affect the | |
76 | mount point list seen in the other namespace | |
77 | (but see the following discussion of shared subtrees). | |
78 | .\" | |
79 | .\" ============================================================ | |
80 | .\" | |
81 | .SS Restrictions on mount namespaces | |
82 | Note the following points with respect to mount namespaces: | |
83 | .IP * 3 | |
84 | Each mount namespace has an owner user namespace. | |
85 | As explained above, when a new mount namespace is created, | |
86 | its mount point list is initialized as a copy of the mount point list | |
87 | of another mount namespace. | |
88 | If the new namespace and the namespace from which the mount point list | |
89 | was copied are owned by different user namespaces, | |
90 | then the new mount namespace is considered | |
91 | .IR "less privileged" . | |
92 | .IP * | |
93 | When creating a less privileged mount namespace, | |
94 | shared mounts are reduced to slave mounts. | |
95 | (Shared and slave mounts are discussed below.) | |
96 | This ensures that mappings performed in less | |
97 | privileged mount namespaces will not propagate to more privileged | |
98 | mount namespaces. | |
99 | .IP * | |
100 | .\" FIXME . | |
101 | .\" What does "come as a single unit from more privileged mount" mean? | |
102 | Mounts that come as a single unit from more privileged mount are | |
103 | locked together and may not be separated in a less privileged mount | |
104 | namespace. | |
105 | (The | |
106 | .BR unshare (2) | |
107 | .B CLONE_NEWNS | |
108 | operation brings across all of the mounts from the original | |
109 | mount namespace as a single unit, | |
110 | and recursive mounts that propagate between | |
111 | mount namespaces propagate as a single unit.) | |
112 | .IP * | |
113 | The | |
114 | .BR mount (2) | |
115 | flags | |
116 | .BR MS_RDONLY , | |
117 | .BR MS_NOSUID , | |
118 | .BR MS_NOEXEC , | |
119 | and the "atime" flags | |
120 | .RB ( MS_NOATIME , | |
121 | .BR MS_NODIRATIME , | |
122 | .BR MS_RELATIME ) | |
123 | settings become locked | |
124 | .\" commit 9566d6742852c527bf5af38af5cbb878dad75705 | |
125 | .\" Author: Eric W. Biederman <ebiederm@xmission.com> | |
126 | .\" Date: Mon Jul 28 17:26:07 2014 -0700 | |
127 | .\" | |
128 | .\" mnt: Correct permission checks in do_remount | |
129 | .\" | |
130 | when propagated from a more privileged to | |
131 | a less privileged mount namespace, | |
132 | and may not be changed in the less privileged mount namespace. | |
133 | .IP * | |
134 | .\" (As of 3.18-rc1 (in Al Viro's 2014-08-30 vfs.git#for-next tree)) | |
135 | A file or directory that is a mount point in one namespace that is not | |
136 | a mount point in another namespace, may be renamed, unlinked, or removed | |
137 | .RB ( rmdir (2)) | |
138 | in the mount namespace in which it is not a mount point | |
139 | (subject to the usual permission checks). | |
140 | Consequently, the mount point is removed in the mount namespace | |
141 | where it was a mount point. | |
142 | .IP | |
143 | Previously (before Linux 3.18), | |
144 | .\" mtk: The change was in Linux 3.18, I think, with this commit: | |
145 | .\" commit 8ed936b5671bfb33d89bc60bdcc7cf0470ba52fe | |
146 | .\" Author: Eric W. Biederman <ebiederman@twitter.com> | |
147 | .\" Date: Tue Oct 1 18:33:48 2013 -0700 | |
148 | .\" | |
149 | .\" vfs: Lazily remove mounts on unlinked files and directories. | |
150 | attempting to unlink, rename, or remove a file or directory | |
151 | that was a mount point in another mount namespace would result in the error | |
152 | .BR EBUSY . | |
153 | That behavior had technical problems of enforcement (e.g., for NFS) | |
154 | and permitted denial-of-service attacks against more privileged users. | |
155 | (i.e., preventing individual files from being updated | |
156 | by bind mounting on top of them). | |
157 | .\" | |
158 | .SH SHARED SUBTREES | |
159 | After the implementation of mount namespaces was completed, | |
160 | experience showed that the isolation that they provided was, | |
161 | in some cases, too great. | |
162 | For example, in order to make a newly loaded optical disk | |
163 | available in all mount namespaces, | |
164 | a mount operation was required in each namespace. | |
165 | For this use case, and others, | |
166 | the shared subtree feature was introduced in Linux 2.6.15. | |
167 | This feature allows for automatic, controlled propagation of mount and unmount | |
168 | .I events | |
169 | between namespaces | |
170 | (or, more precisely, between the members of a | |
171 | .IR "peer group" | |
172 | that are propagating events to one another). | |
173 | .PP | |
174 | Each mount point is marked (via | |
175 | .BR mount (2)) | |
176 | as having one of the following | |
177 | .IR "propagation types" : | |
178 | .TP | |
179 | .BR MS_SHARED | |
180 | This mount point shares events with members of a peer group. | |
181 | Mount and unmount events immediately under this mount point will propagate | |
182 | to the other mount points that are members of the peer group. | |
183 | .I Propagation | |
184 | here means that the same mount or unmount will automatically occur | |
185 | under all of the other mount points in the peer group. | |
186 | Conversely, mount and unmount events that take place under | |
187 | peer mount points will propagate to this mount point. | |
188 | .TP | |
189 | .BR MS_PRIVATE | |
190 | This mount point is private; it does not have a peer group. | |
191 | Mount and unmount events do not propagate into or out of this mount point. | |
192 | .TP | |
193 | .BR MS_SLAVE | |
194 | Mount and unmount events propagate into this mount point from | |
195 | a (master) shared peer group. | |
196 | Mount and unmount events under this mount point do not propagate to any peer. | |
197 | .IP | |
198 | Note that a mount point can be the slave of another peer group | |
199 | while at the same time sharing mount and unmount events | |
200 | with a peer group of which it is a member. | |
201 | (More precisely, one peer group can be the slave of another peer group.) | |
202 | .TP | |
203 | .BR MS_UNBINDABLE | |
204 | This is like a private mount, | |
205 | and in addition this mount can't be bind mounted. | |
206 | Attempts to bind mount this mount | |
207 | .RB ( mount (2) | |
208 | with the | |
209 | .BR MS_BIND | |
210 | flag) will fail. | |
211 | .IP | |
212 | When a recursive bind mount | |
213 | .RB ( mount (2) | |
214 | with the | |
215 | .BR MS_BIND | |
216 | and | |
217 | .BR MS_REC | |
218 | flags) is performed on a directory subtree, | |
219 | any bind mounts within the subtree are automatically pruned | |
220 | (i.e., not replicated) | |
221 | when replicating that subtree to produce the target subtree. | |
222 | .PP | |
223 | For a discussion of the propagation type assigned to a new mount, | |
224 | see NOTES. | |
225 | .PP | |
226 | The propagation type is a per-mount-point setting; | |
227 | some mount points may be marked as shared | |
228 | (with each shared mount point being a member of a distinct peer group), | |
229 | while others are private | |
230 | (or slaved or unbindable). | |
231 | .PP | |
232 | Note that a mount's propagation type determines whether | |
233 | mounts and unmounts of mount points | |
234 | .I "immediately under" | |
235 | the mount point are propagated. | |
236 | Thus, the propagation type does not affect propagation of events for | |
237 | grandchildren and further removed descendant mount points. | |
238 | What happens if the mount point itself is unmounted is determined by | |
239 | the propagation type that is in effect for the | |
240 | .I parent | |
241 | of the mount point. | |
242 | .PP | |
243 | Members are added to a | |
244 | .IR "peer group" | |
245 | when a mount point is marked as shared and either: | |
246 | .IP * 3 | |
247 | the mount point is replicated during the creation of a new mount namespace; or | |
248 | .IP * | |
249 | a new bind mount is created from the mount point. | |
250 | .PP | |
251 | In both of these cases, the new mount point joins the peer group | |
252 | of which the existing mount point is a member. | |
253 | .PP | |
254 | A new peer group is also created when a child mount point is created under | |
255 | an existing mount point that is marked as shared. | |
256 | In this case, the new child mount point is also marked as shared and | |
257 | the resulting peer group consists of all the mount points | |
258 | that are replicated under the peers of parent mount. | |
259 | .PP | |
260 | A mount ceases to be a member of a peer group when either | |
261 | the mount is explicitly unmounted, | |
262 | or when the mount is implicitly unmounted because a mount namespace is removed | |
263 | (because it has no more member processes). | |
264 | .PP | |
265 | The propagation type of the mount points in a mount namespace | |
266 | can be discovered via the "optional fields" exposed in | |
267 | .IR /proc/[pid]/mountinfo . | |
268 | (See | |
269 | .BR proc (5) | |
270 | for details of this file.) | |
271 | The following tags can appear in the optional fields | |
272 | for a record in that file: | |
273 | .TP | |
274 | .I shared:X | |
275 | This mount point is shared in peer group | |
276 | .IR X . | |
277 | Each peer group has a unique ID that is automatically | |
278 | generated by the kernel, | |
279 | and all mount points in the same peer group will show the same ID. | |
280 | (These IDs are assigned starting from the value 1, | |
281 | and may be recycled when a peer group ceases to have any members.) | |
282 | .TP | |
283 | .I master:X | |
284 | This mount is a slave to shared peer group | |
285 | .IR X . | |
286 | .TP | |
287 | .IR propagate_from:X " (since Linux 2.6.26)" | |
288 | .\" commit 97e7e0f71d6d948c25f11f0a33878d9356d9579e | |
289 | This mount is a slave and receives propagation from shared peer group | |
290 | .IR X . | |
291 | This tag will always appear in conjunction with a | |
292 | .IR master:X | |
293 | tag. | |
294 | Here, | |
295 | .IR X | |
296 | is the closest dominant peer group under the process's root directory. | |
297 | If | |
298 | .IR X | |
299 | is the immediate master of the mount, | |
300 | or if there is no dominant peer group under the same root, | |
301 | then only the | |
302 | .IR master:X | |
303 | field is present and not the | |
304 | .IR propagate_from:X | |
305 | field. | |
306 | For further details, see below. | |
307 | .TP | |
308 | .IR unbindable | |
309 | This is an unbindable mount. | |
310 | .PP | |
311 | If none of the above tags is present, then this is a private mount. | |
312 | .SS MS_SHARED and MS_PRIVATE example | |
313 | Suppose that on a terminal in the initial mount namespace, | |
314 | we mark one mount point as shared and another as private, | |
315 | and then view the mounts in | |
316 | .IR /proc/self/mountinfo : | |
317 | .PP | |
318 | .in +4n | |
319 | .EX | |
320 | sh1# \fBmount \-\-make\-shared /mntS\fP | |
321 | sh1# \fBmount \-\-make\-private /mntP\fP | |
322 | sh1# \fBcat /proc/self/mountinfo | grep \(aq/mnt\(aq | sed \(aqs/ \- .*//\(aq\fP | |
323 | 77 61 8:17 / /mntS rw,relatime shared:1 | |
324 | 83 61 8:15 / /mntP rw,relatime | |
325 | .EE | |
326 | .in | |
327 | .PP | |
328 | From the | |
329 | .IR /proc/self/mountinfo | |
330 | output, we see that | |
331 | .IR /mntS | |
332 | is a shared mount in peer group 1, and that | |
333 | .IR /mntP | |
334 | has no optional tags, indicating that it is a private mount. | |
335 | The first two fields in each record in this file are the unique | |
336 | ID for this mount, and the mount ID of the parent mount. | |
337 | We can further inspect this file to see that the parent mount point of | |
338 | .IR /mntS | |
339 | and | |
340 | .IR /mntP | |
341 | is the root directory, | |
342 | .IR / , | |
343 | which is mounted as private: | |
344 | .PP | |
345 | .in +4n | |
346 | .EX | |
347 | sh1# \fBcat /proc/self/mountinfo | awk \(aq$1 == 61\(aq | sed \(aqs/ \- .*//\(aq\fP | |
348 | 61 0 8:2 / / rw,relatime | |
349 | .EE | |
350 | .in | |
351 | .PP | |
352 | On a second terminal, | |
353 | we create a new mount namespace where we run a second shell | |
354 | and inspect the mounts: | |
355 | .PP | |
356 | .in +4n | |
357 | .EX | |
358 | $ \fBPS1=\(aqsh2# \(aq sudo unshare \-m \-\-propagation unchanged sh\fP | |
359 | sh2# \fBcat /proc/self/mountinfo | grep \(aq/mnt\(aq | sed \(aqs/ \- .*//\(aq\fP | |
360 | 222 145 8:17 / /mntS rw,relatime shared:1 | |
361 | 225 145 8:15 / /mntP rw,relatime | |
362 | .EE | |
363 | .in | |
364 | .PP | |
365 | The new mount namespace received a copy of the initial mount namespace's | |
366 | mount points. | |
367 | These new mount points maintain the same propagation types, | |
368 | but have unique mount IDs. | |
369 | (The | |
370 | .IR \-\-propagation\ unchanged | |
371 | option prevents | |
372 | .BR unshare (1) | |
373 | from marking all mounts as private when creating a new mount namespace, | |
374 | .\" Since util-linux 2.27 | |
375 | which it does by default.) | |
376 | .PP | |
377 | In the second terminal, we then create submounts under each of | |
378 | .IR /mntS | |
379 | and | |
380 | .IR /mntP | |
381 | and inspect the set-up: | |
382 | .PP | |
383 | .in +4n | |
384 | .EX | |
385 | sh2# \fBmkdir /mntS/a\fP | |
386 | sh2# \fBmount /dev/sdb6 /mntS/a\fP | |
387 | sh2# \fBmkdir /mntP/b\fP | |
388 | sh2# \fBmount /dev/sdb7 /mntP/b\fP | |
389 | sh2# \fBcat /proc/self/mountinfo | grep \(aq/mnt\(aq | sed \(aqs/ \- .*//\(aq\fP | |
390 | 222 145 8:17 / /mntS rw,relatime shared:1 | |
391 | 225 145 8:15 / /mntP rw,relatime | |
392 | 178 222 8:22 / /mntS/a rw,relatime shared:2 | |
393 | 230 225 8:23 / /mntP/b rw,relatime | |
394 | .EE | |
395 | .in | |
396 | .PP | |
397 | From the above, it can be seen that | |
398 | .IR /mntS/a | |
399 | was created as shared (inheriting this setting from its parent mount) and | |
400 | .IR /mntP/b | |
401 | was created as a private mount. | |
402 | .PP | |
403 | Returning to the first terminal and inspecting the set-up, | |
404 | we see that the new mount created under the shared mount point | |
405 | .IR /mntS | |
406 | propagated to its peer mount (in the initial mount namespace), | |
407 | but the new mount created under the private mount point | |
408 | .IR /mntP | |
409 | did not propagate: | |
410 | .PP | |
411 | .in +4n | |
412 | .EX | |
413 | sh1# \fBcat /proc/self/mountinfo | grep \(aq/mnt\(aq | sed \(aqs/ \- .*//\(aq\fP | |
414 | 77 61 8:17 / /mntS rw,relatime shared:1 | |
415 | 83 61 8:15 / /mntP rw,relatime | |
416 | 179 77 8:22 / /mntS/a rw,relatime shared:2 | |
417 | .EE | |
418 | .in | |
419 | .\" | |
420 | .SS MS_SLAVE example | |
421 | Making a mount point a slave allows it to receive propagated | |
422 | mount and unmount events from a master shared peer group, | |
423 | while preventing it from propagating events to that master. | |
424 | This is useful if we want to (say) receive a mount event when | |
425 | an optical disk is mounted in the master shared peer group | |
426 | (in another mount namespace), | |
427 | but want to prevent mount and unmount events under the slave mount | |
428 | from having side effects in other namespaces. | |
429 | .PP | |
430 | We can demonstrate the effect of slaving by first marking | |
431 | two mount points as shared in the initial mount namespace: | |
432 | .PP | |
433 | .in +4n | |
434 | .EX | |
435 | sh1# \fBmount \-\-make\-shared /mntX\fP | |
436 | sh1# \fBmount \-\-make\-shared /mntY\fP | |
437 | sh1# \fBcat /proc/self/mountinfo | grep \(aq/mnt\(aq | sed \(aqs/ \- .*//\(aq\fP | |
438 | 132 83 8:23 / /mntX rw,relatime shared:1 | |
439 | 133 83 8:22 / /mntY rw,relatime shared:2 | |
440 | .EE | |
441 | .in | |
442 | .PP | |
443 | On a second terminal, | |
444 | we create a new mount namespace and inspect the mount points: | |
445 | .PP | |
446 | .in +4n | |
447 | .EX | |
448 | sh2# \fBunshare \-m \-\-propagation unchanged sh\fP | |
449 | sh2# \fBcat /proc/self/mountinfo | grep \(aq/mnt\(aq | sed \(aqs/ \- .*//\(aq\fP | |
450 | 168 167 8:23 / /mntX rw,relatime shared:1 | |
451 | 169 167 8:22 / /mntY rw,relatime shared:2 | |
452 | .EE | |
453 | .in | |
454 | .PP | |
455 | In the new mount namespace, we then mark one of the mount points as a slave: | |
456 | .PP | |
457 | .in +4n | |
458 | .EX | |
459 | sh2# \fBmount \-\-make\-slave /mntY\fP | |
460 | sh2# \fBcat /proc/self/mountinfo | grep \(aq/mnt\(aq | sed \(aqs/ \- .*//\(aq\fP | |
461 | 168 167 8:23 / /mntX rw,relatime shared:1 | |
462 | 169 167 8:22 / /mntY rw,relatime master:2 | |
463 | .EE | |
464 | .in | |
465 | .PP | |
466 | From the above output, we see that | |
467 | .IR /mntY | |
468 | is now a slave mount that is receiving propagation events from | |
469 | the shared peer group with the ID 2. | |
470 | .PP | |
471 | Continuing in the new namespace, we create submounts under each of | |
472 | .IR /mntX | |
473 | and | |
474 | .IR /mntY : | |
475 | .PP | |
476 | .in +4n | |
477 | .EX | |
478 | sh2# \fBmkdir /mntX/a\fP | |
479 | sh2# \fBmount /dev/sda3 /mntX/a\fP | |
480 | sh2# \fBmkdir /mntY/b\fP | |
481 | sh2# \fBmount /dev/sda5 /mntY/b\fP | |
482 | .EE | |
483 | .in | |
484 | .PP | |
485 | When we inspect the state of the mount points in the new mount namespace, | |
486 | we see that | |
487 | .IR /mntX/a | |
488 | was created as a new shared mount | |
489 | (inheriting the "shared" setting from its parent mount) and | |
490 | .IR /mntY/b | |
491 | was created as a private mount: | |
492 | .PP | |
493 | .in +4n | |
494 | .EX | |
495 | sh2# \fBcat /proc/self/mountinfo | grep \(aq/mnt\(aq | sed \(aqs/ \- .*//\(aq\fP | |
496 | 168 167 8:23 / /mntX rw,relatime shared:1 | |
497 | 169 167 8:22 / /mntY rw,relatime master:2 | |
498 | 173 168 8:3 / /mntX/a rw,relatime shared:3 | |
499 | 175 169 8:5 / /mntY/b rw,relatime | |
500 | .EE | |
501 | .in | |
502 | .PP | |
503 | Returning to the first terminal (in the initial mount namespace), | |
504 | we see that the mount | |
505 | .IR /mntX/a | |
506 | propagated to the peer (the shared | |
507 | .IR /mntX ), | |
508 | but the mount | |
509 | .IR /mntY/b | |
510 | was not propagated: | |
511 | .PP | |
512 | .in +4n | |
513 | .EX | |
514 | sh1# \fBcat /proc/self/mountinfo | grep \(aq/mnt\(aq | sed \(aqs/ \- .*//\(aq\fP | |
515 | 132 83 8:23 / /mntX rw,relatime shared:1 | |
516 | 133 83 8:22 / /mntY rw,relatime shared:2 | |
517 | 174 132 8:3 / /mntX/a rw,relatime shared:3 | |
518 | .EE | |
519 | .in | |
520 | .PP | |
521 | Now we create a new mount point under | |
522 | .IR /mntY | |
523 | in the first shell: | |
524 | .PP | |
525 | .in +4n | |
526 | .EX | |
527 | sh1# \fBmkdir /mntY/c\fP | |
528 | sh1# \fBmount /dev/sda1 /mntY/c\fP | |
529 | sh1# \fBcat /proc/self/mountinfo | grep '/mnt' | sed 's/ \- .*//'\fP | |
530 | 132 83 8:23 / /mntX rw,relatime shared:1 | |
531 | 133 83 8:22 / /mntY rw,relatime shared:2 | |
532 | 174 132 8:3 / /mntX/a rw,relatime shared:3 | |
533 | 178 133 8:1 / /mntY/c rw,relatime shared:4 | |
534 | .EE | |
535 | .in | |
536 | .PP | |
537 | When we examine the mount points in the second mount namespace, | |
538 | we see that in this case the new mount has been propagated | |
539 | to the slave mount point, | |
540 | and that the new mount is itself a slave mount (to peer group 4): | |
541 | .PP | |
542 | .in +4n | |
543 | .EX | |
544 | sh2# \fBcat /proc/self/mountinfo | grep \(aq/mnt\(aq | sed \(aqs/ \- .*//\(aq\fP | |
545 | 168 167 8:23 / /mntX rw,relatime shared:1 | |
546 | 169 167 8:22 / /mntY rw,relatime master:2 | |
547 | 173 168 8:3 / /mntX/a rw,relatime shared:3 | |
548 | 175 169 8:5 / /mntY/b rw,relatime | |
549 | 179 169 8:1 / /mntY/c rw,relatime master:4 | |
550 | .EE | |
551 | .in | |
552 | .\" | |
553 | .SS MS_UNBINDABLE example | |
554 | One of the primary purposes of unbindable mounts is to avoid | |
555 | the "mount point explosion" problem when repeatedly performing bind mounts | |
556 | of a higher-level subtree at a lower-level mount point. | |
557 | The problem is illustrated by the following shell session. | |
558 | .PP | |
559 | Suppose we have a system with the following mount points: | |
560 | .PP | |
561 | .in +4n | |
562 | .EX | |
563 | # \fBmount | awk \(aq{print $1, $2, $3}\(aq\fP | |
564 | /dev/sda1 on / | |
565 | /dev/sdb6 on /mntX | |
566 | /dev/sdb7 on /mntY | |
567 | .EE | |
568 | .in | |
569 | .PP | |
570 | Suppose furthermore that we wish to recursively bind mount | |
571 | the root directory under several users' home directories. | |
572 | We do this for the first user, and inspect the mount points: | |
573 | .PP | |
574 | .in +4n | |
575 | .EX | |
576 | # \fBmount \-\-rbind / /home/cecilia/\fP | |
577 | # \fBmount | awk \(aq{print $1, $2, $3}\(aq\fP | |
578 | /dev/sda1 on / | |
579 | /dev/sdb6 on /mntX | |
580 | /dev/sdb7 on /mntY | |
581 | /dev/sda1 on /home/cecilia | |
582 | /dev/sdb6 on /home/cecilia/mntX | |
583 | /dev/sdb7 on /home/cecilia/mntY | |
584 | .EE | |
585 | .in | |
586 | .PP | |
587 | When we repeat this operation for the second user, | |
588 | we start to see the explosion problem: | |
589 | .PP | |
590 | .in +4n | |
591 | .EX | |
592 | # \fBmount \-\-rbind / /home/henry\fP | |
593 | # \fBmount | awk \(aq{print $1, $2, $3}\(aq\fP | |
594 | /dev/sda1 on / | |
595 | /dev/sdb6 on /mntX | |
596 | /dev/sdb7 on /mntY | |
597 | /dev/sda1 on /home/cecilia | |
598 | /dev/sdb6 on /home/cecilia/mntX | |
599 | /dev/sdb7 on /home/cecilia/mntY | |
600 | /dev/sda1 on /home/henry | |
601 | /dev/sdb6 on /home/henry/mntX | |
602 | /dev/sdb7 on /home/henry/mntY | |
603 | /dev/sda1 on /home/henry/home/cecilia | |
604 | /dev/sdb6 on /home/henry/home/cecilia/mntX | |
605 | /dev/sdb7 on /home/henry/home/cecilia/mntY | |
606 | .EE | |
607 | .in | |
608 | .PP | |
609 | Under | |
610 | .IR /home/henry , | |
611 | we have not only recursively added the | |
612 | .IR /mntX | |
613 | and | |
614 | .IR /mntY | |
615 | mounts, but also the recursive mounts of those directories under | |
616 | .IR /home/cecilia | |
617 | that were created in the previous step. | |
618 | Upon repeating the step for a third user, | |
619 | it becomes obvious that the explosion is exponential in nature: | |
620 | .PP | |
621 | .in +4n | |
622 | .EX | |
623 | # \fBmount \-\-rbind / /home/otto\fP | |
624 | # \fBmount | awk \(aq{print $1, $2, $3}\(aq\fP | |
625 | /dev/sda1 on / | |
626 | /dev/sdb6 on /mntX | |
627 | /dev/sdb7 on /mntY | |
628 | /dev/sda1 on /home/cecilia | |
629 | /dev/sdb6 on /home/cecilia/mntX | |
630 | /dev/sdb7 on /home/cecilia/mntY | |
631 | /dev/sda1 on /home/henry | |
632 | /dev/sdb6 on /home/henry/mntX | |
633 | /dev/sdb7 on /home/henry/mntY | |
634 | /dev/sda1 on /home/henry/home/cecilia | |
635 | /dev/sdb6 on /home/henry/home/cecilia/mntX | |
636 | /dev/sdb7 on /home/henry/home/cecilia/mntY | |
637 | /dev/sda1 on /home/otto | |
638 | /dev/sdb6 on /home/otto/mntX | |
639 | /dev/sdb7 on /home/otto/mntY | |
640 | /dev/sda1 on /home/otto/home/cecilia | |
641 | /dev/sdb6 on /home/otto/home/cecilia/mntX | |
642 | /dev/sdb7 on /home/otto/home/cecilia/mntY | |
643 | /dev/sda1 on /home/otto/home/henry | |
644 | /dev/sdb6 on /home/otto/home/henry/mntX | |
645 | /dev/sdb7 on /home/otto/home/henry/mntY | |
646 | /dev/sda1 on /home/otto/home/henry/home/cecilia | |
647 | /dev/sdb6 on /home/otto/home/henry/home/cecilia/mntX | |
648 | /dev/sdb7 on /home/otto/home/henry/home/cecilia/mntY | |
649 | .EE | |
650 | .in | |
651 | .PP | |
652 | The mount explosion problem in the above scenario can be avoided | |
653 | by making each of the new mounts unbindable. | |
654 | The effect of doing this is that recursive mounts of the root | |
655 | directory will not replicate the unbindable mounts. | |
656 | We make such a mount for the first user: | |
657 | .PP | |
658 | .in +4n | |
659 | .EX | |
660 | # \fBmount \-\-rbind \-\-make\-unbindable / /home/cecilia\fP | |
661 | .EE | |
662 | .in | |
663 | .PP | |
664 | Before going further, we show that unbindable mounts are indeed unbindable: | |
665 | .PP | |
666 | .in +4n | |
667 | .EX | |
668 | # \fBmkdir /mntZ\fP | |
669 | # \fBmount \-\-bind /home/cecilia /mntZ\fP | |
670 | mount: wrong fs type, bad option, bad superblock on /home/cecilia, | |
671 | missing codepage or helper program, or other error | |
672 | ||
673 | In some cases useful info is found in syslog \- try | |
674 | dmesg | tail or so. | |
675 | .EE | |
676 | .in | |
677 | .PP | |
678 | Now we create unbindable recursive bind mounts for the other two users: | |
679 | .PP | |
680 | .in +4n | |
681 | .EX | |
682 | # \fBmount \-\-rbind \-\-make\-unbindable / /home/henry\fP | |
683 | # \fBmount \-\-rbind \-\-make\-unbindable / /home/otto\fP | |
684 | .EE | |
685 | .in | |
686 | .PP | |
687 | Upon examining the list of mount points, | |
688 | we see there has been no explosion of mount points, | |
689 | because the unbindable mounts were not replicated | |
690 | under each user's directory: | |
691 | .PP | |
692 | .in +4n | |
693 | .EX | |
694 | # \fBmount | awk \(aq{print $1, $2, $3}\(aq\fP | |
695 | /dev/sda1 on / | |
696 | /dev/sdb6 on /mntX | |
697 | /dev/sdb7 on /mntY | |
698 | /dev/sda1 on /home/cecilia | |
699 | /dev/sdb6 on /home/cecilia/mntX | |
700 | /dev/sdb7 on /home/cecilia/mntY | |
701 | /dev/sda1 on /home/henry | |
702 | /dev/sdb6 on /home/henry/mntX | |
703 | /dev/sdb7 on /home/henry/mntY | |
704 | /dev/sda1 on /home/otto | |
705 | /dev/sdb6 on /home/otto/mntX | |
706 | /dev/sdb7 on /home/otto/mntY | |
707 | .EE | |
708 | .in | |
709 | .\" | |
710 | .SS Propagation type transitions | |
711 | The following table shows the effect that applying a new propagation type | |
712 | (i.e., | |
713 | .IR "mount \-\-make\-xxxx") | |
714 | has on the existing propagation type of a mount point. | |
715 | The rows correspond to existing propagation types, | |
716 | and the columns are the new propagation settings. | |
717 | For reasons of space, "private" is abbreviated as "priv" and | |
718 | "unbindable" as "unbind". | |
719 | .TS | |
720 | lb2 lb2 lb2 lb2 lb1 | |
721 | lb l l l l l. | |
722 | make-shared make-slave make-priv make-unbind | |
723 | shared shared slave/priv [1] priv unbind | |
724 | slave slave+shared slave [2] priv unbind | |
725 | slave+shared slave+shared slave priv unbind | |
726 | private shared priv [2] priv unbind | |
727 | unbindable shared unbind [2] priv unbind | |
728 | .TE | |
729 | .sp 1 | |
730 | Note the following details to the table: | |
731 | .IP [1] 4 | |
732 | If a shared mount is the only mount in its peer group, | |
733 | making it a slave automatically makes it private. | |
734 | .IP [2] | |
735 | Slaving a nonshared mount has no effect on the mount. | |
736 | .\" | |
737 | .SS Bind (MS_BIND) semantics | |
738 | Suppose that the following command is performed: | |
739 | .PP | |
740 | .in +4n | |
741 | .EX | |
742 | mount \-\-bind A/a B/b | |
743 | .EE | |
744 | .in | |
745 | .PP | |
746 | Here, | |
747 | .I A | |
748 | is the source mount point, | |
749 | .I B | |
750 | is the destination mount point, | |
751 | .I a | |
752 | is a subdirectory path under the mount point | |
753 | .IR A , | |
754 | and | |
755 | .I b | |
756 | is a subdirectory path under the mount point | |
757 | .IR B . | |
758 | The propagation type of the resulting mount, | |
759 | .IR B/b , | |
760 | depends on the propagation types of the mount points | |
761 | .IR A | |
762 | and | |
763 | .IR B , | |
764 | and is summarized in the following table. | |
765 | .PP | |
766 | .TS | |
767 | lb2 lb1 lb2 lb2 lb2 lb0 | |
768 | lb2 lb1 lb2 lb2 lb2 lb0 | |
769 | lb lb l l l l l. | |
770 | source(A) | |
771 | shared private slave unbind | |
772 | _ | |
773 | dest(B) shared | shared shared slave+shared invalid | |
774 | nonshared | shared private slave invalid | |
775 | .TE | |
776 | .sp 1 | |
777 | Note that a recursive bind of a subtree follows the same semantics | |
778 | as for a bind operation on each mount in the subtree. | |
779 | (Unbindable mounts are automatically pruned at the target mount point.) | |
780 | .PP | |
781 | For further details, see | |
782 | .I Documentation/filesystems/sharedsubtree.txt | |
783 | in the kernel source tree. | |
784 | .\" | |
785 | .SS Move (MS_MOVE) semantics | |
786 | Suppose that the following command is performed: | |
787 | .PP | |
788 | .in +4n | |
789 | .EX | |
790 | mount \-\-move A B/b | |
791 | .EE | |
792 | .in | |
793 | .PP | |
794 | Here, | |
795 | .I A | |
796 | is the source mount point, | |
797 | .I B | |
798 | is the destination mount point, and | |
799 | .I b | |
800 | is a subdirectory path under the mount point | |
801 | .IR B . | |
802 | The propagation type of the resulting mount, | |
803 | .IR B/b , | |
804 | depends on the propagation types of the mount points | |
805 | .IR A | |
806 | and | |
807 | .IR B , | |
808 | and is summarized in the following table. | |
809 | .PP | |
810 | .TS | |
811 | lb2 lb1 lb2 lb2 lb2 lb0 | |
812 | lb2 lb1 lb2 lb2 lb2 lb0 | |
813 | lb lb l l l l l. | |
814 | source(A) | |
815 | shared private slave unbind | |
816 | _ | |
817 | dest(B) shared | shared shared slave+shared invalid | |
818 | nonshared | shared private slave unbindable | |
819 | .TE | |
820 | .sp 1 | |
821 | Note: moving a mount that resides under a shared mount is invalid. | |
822 | .PP | |
823 | For further details, see | |
824 | .I Documentation/filesystems/sharedsubtree.txt | |
825 | in the kernel source tree. | |
826 | .\" | |
827 | .SS Mount semantics | |
828 | Suppose that we use the following command to create a mount point: | |
829 | .PP | |
830 | .in +4n | |
831 | .EX | |
832 | mount device B/b | |
833 | .EE | |
834 | .in | |
835 | .PP | |
836 | Here, | |
837 | .I B | |
838 | is the destination mount point, and | |
839 | .I b | |
840 | is a subdirectory path under the mount point | |
841 | .IR B . | |
842 | The propagation type of the resulting mount, | |
843 | .IR B/b , | |
844 | follows the same rules as for a bind mount, | |
845 | where the propagation type of the source mount | |
846 | is considered always to be private. | |
847 | .\" | |
848 | .SS Unmount semantics | |
849 | Suppose that we use the following command to tear down a mount point: | |
850 | .PP | |
851 | .in +4n | |
852 | .EX | |
853 | unmount A | |
854 | .EE | |
855 | .in | |
856 | .PP | |
857 | Here, | |
858 | .I A | |
859 | is a mount point on | |
860 | .IR B/b , | |
861 | where | |
862 | .I B | |
863 | is the parent mount and | |
864 | .I b | |
865 | is a subdirectory path under the mount point | |
866 | .IR B . | |
867 | If | |
868 | .B B | |
869 | is shared, then all most-recently-mounted mounts at | |
870 | .I b | |
871 | on mounts that receive propagation from mount | |
872 | .I B | |
873 | and do not have submounts under them are unmounted. | |
874 | .\" | |
875 | .SS The /proc/[pid]/mountinfo "propagate_from" tag | |
876 | The | |
877 | .I propagate_from:X | |
878 | tag is shown in the optional fields of a | |
879 | .IR /proc/[pid]/mountinfo | |
880 | record in cases where a process can't see a slave's immediate master | |
881 | (i.e., the pathname of the master is not reachable from | |
882 | the filesystem root directory) | |
883 | and so cannot determine the | |
884 | chain of propagation between the mounts it can see. | |
885 | .PP | |
886 | In the following example, we first create a two-link master-slave chain | |
887 | between the mounts | |
888 | .IR /mnt , | |
889 | .IR /tmp/etc , | |
890 | and | |
891 | .IR /mnt/tmp/etc . | |
892 | Then the | |
893 | .BR chroot (1) | |
894 | command is used to make the | |
895 | .IR /tmp/etc | |
896 | mount point unreachable from the root directory, | |
897 | creating a situation where the master of | |
898 | .IR /mnt/tmp/etc | |
899 | is not reachable from the (new) root directory of the process. | |
900 | .PP | |
901 | First, we bind mount the root directory onto | |
902 | .IR /mnt | |
903 | and then bind mount | |
904 | .IR /proc | |
905 | at | |
906 | .IR /mnt/proc | |
907 | so that after the later | |
908 | .BR chroot (1) | |
909 | the | |
910 | .BR proc (5) | |
911 | filesystem remains visible at the correct location | |
912 | in the chroot-ed environment. | |
913 | .PP | |
914 | .in +4n | |
915 | .EX | |
916 | # \fBmkdir \-p /mnt/proc\fP | |
917 | # \fBmount \-\-bind / /mnt\fP | |
918 | # \fBmount \-\-bind /proc /mnt/proc\fP | |
919 | .EE | |
920 | .in | |
921 | .PP | |
922 | Next, we ensure that the | |
923 | .IR /mnt | |
924 | mount is a shared mount in a new peer group (with no peers): | |
925 | .PP | |
926 | .in +4n | |
927 | .EX | |
928 | # \fBmount \-\-make\-private /mnt\fP # Isolate from any previous peer group | |
929 | # \fBmount \-\-make\-shared /mnt\fP | |
930 | # \fBcat /proc/self/mountinfo | grep \(aq/mnt\(aq | sed \(aqs/ \- .*//\(aq\fP | |
931 | 239 61 8:2 / /mnt ... shared:102 | |
932 | 248 239 0:4 / /mnt/proc ... shared:5 | |
933 | .EE | |
934 | .in | |
935 | .PP | |
936 | Next, we bind mount | |
937 | .IR /mnt/etc | |
938 | onto | |
939 | .IR /tmp/etc : | |
940 | .PP | |
941 | .in +4n | |
942 | .EX | |
943 | # \fBmkdir \-p /tmp/etc\fP | |
944 | # \fBmount \-\-bind /mnt/etc /tmp/etc\fP | |
945 | # \fBcat /proc/self/mountinfo | egrep \(aq/mnt|/tmp/\(aq | sed \(aqs/ \- .*//\(aq\fP | |
946 | 239 61 8:2 / /mnt ... shared:102 | |
947 | 248 239 0:4 / /mnt/proc ... shared:5 | |
948 | 267 40 8:2 /etc /tmp/etc ... shared:102 | |
949 | .EE | |
950 | .in | |
951 | .PP | |
952 | Initially, these two mount points are in the same peer group, | |
953 | but we then make the | |
954 | .IR /tmp/etc | |
955 | a slave of | |
956 | .IR /mnt/etc , | |
957 | and then make | |
958 | .IR /tmp/etc | |
959 | shared as well, | |
960 | so that it can propagate events to the next slave in the chain: | |
961 | .PP | |
962 | .in +4n | |
963 | .EX | |
964 | # \fBmount \-\-make\-slave /tmp/etc\fP | |
965 | # \fBmount \-\-make\-shared /tmp/etc\fP | |
966 | # \fBcat /proc/self/mountinfo | egrep \(aq/mnt|/tmp/\(aq | sed \(aqs/ \- .*//\(aq\fP | |
967 | 239 61 8:2 / /mnt ... shared:102 | |
968 | 248 239 0:4 / /mnt/proc ... shared:5 | |
969 | 267 40 8:2 /etc /tmp/etc ... shared:105 master:102 | |
970 | .EE | |
971 | .in | |
972 | .PP | |
973 | Then we bind mount | |
974 | .IR /tmp/etc | |
975 | onto | |
976 | .IR /mnt/tmp/etc . | |
977 | Again, the two mount points are initially in the same peer group, | |
978 | but we then make | |
979 | .IR /mnt/tmp/etc | |
980 | a slave of | |
981 | .IR /tmp/etc : | |
982 | .PP | |
983 | .in +4n | |
984 | .EX | |
985 | # \fBmkdir \-p /mnt/tmp/etc\fP | |
986 | # \fBmount \-\-bind /tmp/etc /mnt/tmp/etc\fP | |
987 | # \fBmount \-\-make\-slave /mnt/tmp/etc\fP | |
988 | # \fBcat /proc/self/mountinfo | egrep \(aq/mnt|/tmp/\(aq | sed \(aqs/ \- .*//\(aq\fP | |
989 | 239 61 8:2 / /mnt ... shared:102 | |
990 | 248 239 0:4 / /mnt/proc ... shared:5 | |
991 | 267 40 8:2 /etc /tmp/etc ... shared:105 master:102 | |
992 | 273 239 8:2 /etc /mnt/tmp/etc ... master:105 | |
993 | .EE | |
994 | .in | |
995 | .PP | |
996 | From the above, we see that | |
997 | .IR /mnt | |
998 | is the master of the slave | |
999 | .IR /tmp/etc , | |
1000 | which in turn is the master of the slave | |
1001 | .IR /mnt/tmp/etc . | |
1002 | .PP | |
1003 | We then | |
1004 | .BR chroot (1) | |
1005 | to the | |
1006 | .IR /mnt | |
1007 | directory, which renders the mount with ID 267 unreachable | |
1008 | from the (new) root directory: | |
1009 | .PP | |
1010 | .in +4n | |
1011 | .EX | |
1012 | # \fBchroot /mnt\fP | |
1013 | .EE | |
1014 | .in | |
1015 | .PP | |
1016 | When we examine the state of the mounts inside the chroot-ed environment, | |
1017 | we see the following: | |
1018 | .PP | |
1019 | .in +4n | |
1020 | .EX | |
1021 | # \fBcat /proc/self/mountinfo | sed \(aqs/ \- .*//\(aq\fP | |
1022 | 239 61 8:2 / / ... shared:102 | |
1023 | 248 239 0:4 / /proc ... shared:5 | |
1024 | 273 239 8:2 /etc /tmp/etc ... master:105 propagate_from:102 | |
1025 | .EE | |
1026 | .in | |
1027 | .PP | |
1028 | Above, we see that the mount with ID 273 | |
1029 | is a slave whose master is the peer group 105. | |
1030 | The mount point for that master is unreachable, and so a | |
1031 | .IR propagate_from | |
1032 | tag is displayed, indicating that the closest dominant peer group | |
1033 | (i.e., the nearest reachable mount in the slave chain) | |
1034 | is the peer group with the ID 102 (corresponding to the | |
1035 | .IR /mnt | |
1036 | mount point before the | |
1037 | .BR chroot (1) | |
1038 | was performed. | |
1039 | .\" | |
1040 | .SH VERSIONS | |
1041 | Mount namespaces first appeared in Linux 2.4.19. | |
1042 | .SH CONFORMING TO | |
1043 | Namespaces are a Linux-specific feature. | |
1044 | .\" | |
1045 | .SH NOTES | |
1046 | The propagation type assigned to a new mount point depends | |
1047 | on the propagation type of the parent mount. | |
1048 | If the mount point has a parent (i.e., it is a non-root mount | |
1049 | point) and the propagation type of the parent is | |
1050 | .BR MS_SHARED , | |
1051 | then the propagation type of the new mount is also | |
1052 | .BR MS_SHARED . | |
1053 | Otherwise, the propagation type of the new mount is | |
1054 | .BR MS_PRIVATE . | |
1055 | .PP | |
1056 | Notwithstanding the fact that the default propagation type | |
1057 | for new mount points is in many cases | |
1058 | .BR MS_PRIVATE , | |
1059 | .BR MS_SHARED | |
1060 | is typically more useful. | |
1061 | For this reason, | |
1062 | .BR systemd (1) | |
1063 | automatically remounts all mount points as | |
1064 | .BR MS_SHARED | |
1065 | on system startup. | |
1066 | Thus, on most modern systems, the default propagation type is in practice | |
1067 | .BR MS_SHARED . | |
1068 | .PP | |
1069 | Since, when one uses | |
1070 | .BR unshare (1) | |
1071 | to create a mount namespace, | |
1072 | the goal is commonly to provide full isolation of the mount points | |
1073 | in the new namespace, | |
1074 | .BR unshare (1) | |
1075 | (since | |
1076 | .IR util-linux | |
1077 | version 2.27) in turn reverses the step performed by | |
1078 | .BR systemd (1), | |
1079 | by making all mount points private in the new namespace. | |
1080 | That is, | |
1081 | .BR unshare (1) | |
1082 | performs the equivalent of the following in the new mount namespace: | |
1083 | .PP | |
1084 | .in +4n | |
1085 | .EX | |
1086 | mount \-\-make\-rprivate / | |
1087 | .EE | |
1088 | .in | |
1089 | .PP | |
1090 | To prevent this, one can use the | |
1091 | .IR "\-\-propagation\ unchanged" | |
1092 | option to | |
1093 | .BR unshare (1). | |
1094 | .PP | |
1095 | An application that creates a new mount namespace directly using | |
1096 | .BR clone (2) | |
1097 | or | |
1098 | .BR unshare (2) | |
1099 | may desire to prevent propagation of mount events to other mount namespaces | |
1100 | (as is done by | |
1101 | .BR unshare (1)). | |
1102 | This can be done by changing the propagation type of | |
1103 | mount points in the new namespace to either | |
1104 | .BR MS_SLAVE | |
1105 | or | |
1106 | .BR MS_PRIVATE . | |
1107 | using a call such as the following: | |
1108 | .IP | |
1109 | .in +4n | |
1110 | .EX | |
1111 | mount(NULL, "/", MS_SLAVE | MS_REC, NULL); | |
1112 | .EE | |
1113 | .in | |
1114 | .PP | |
1115 | For a discussion of propagation types when moving mounts | |
1116 | .RB ( MS_MOVE ) | |
1117 | and creating bind mounts | |
1118 | .RB ( MS_BIND ), | |
1119 | see | |
1120 | .IR Documentation/filesystems/sharedsubtree.txt . | |
1121 | .SH EXAMPLE | |
1122 | See | |
1123 | .BR pivot_root (2). | |
1124 | .SH SEE ALSO | |
1125 | .BR unshare (1), | |
1126 | .BR clone (2), | |
1127 | .BR mount (2), | |
1128 | .BR pivot_root (2), | |
1129 | .BR setns (2), | |
1130 | .BR umount (2), | |
1131 | .BR unshare (2), | |
1132 | .BR proc (5), | |
1133 | .BR namespaces (7), | |
1134 | .BR user_namespaces (7), | |
1135 | .BR findmnt (8), | |
1136 | .BR pivot_root (8) | |
1137 | .PP | |
1138 | .IR Documentation/filesystems/sharedsubtree.txt | |
1139 | in the kernel source tree. |