uipc_shm.c revision 224914 
1/*-
2 * Copyright (c) 2006 Robert N. M. Watson
3 * All rights reserved.
4 *
5 * Redistribution and use in source and binary forms, with or without
6 * modification, are permitted provided that the following conditions
7 * are met:
8 * 1. Redistributions of source code must retain the above copyright
9 *    notice, this list of conditions and the following disclaimer.
10 * 2. Redistributions in binary form must reproduce the above copyright
11 *    notice, this list of conditions and the following disclaimer in the
12 *    documentation and/or other materials provided with the distribution.
13 *
14 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
15 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
16 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
17 * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
18 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
19 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
20 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
21 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
22 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
23 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
24 * SUCH DAMAGE.
25 */
26
27/*
28 * Support for shared swap-backed anonymous memory objects via
29 * shm_open(2) and shm_unlink(2).  While most of the implementation is
30 * here, vm_mmap.c contains mapping logic changes.
31 *
32 * TODO:
33 *
34 * (2) Need to export data to a userland tool via a sysctl.  Should ipcs(1)
35 *     and ipcrm(1) be expanded or should new tools to manage both POSIX
36 *     kernel semaphores and POSIX shared memory be written?
37 *
38 * (3) Add support for this file type to fstat(1).
39 *
40 * (4) Resource limits?  Does this need its own resource limits or are the
41 *     existing limits in mmap(2) sufficient?
42 *
43 * (5) Partial page truncation.  vnode_pager_setsize() will zero any parts
44 *     of a partially mapped page as a result of ftruncate(2)/truncate(2).
45 *     We can do the same (with the same pmap evil), but do we need to
46 *     worry about the bits on disk if the page is swapped out or will the
47 *     swapper zero the parts of a page that are invalid if the page is
48 *     swapped back in for us?
49 *
50 * (6) Add MAC support in mac_biba(4) and mac_mls(4).
51 *
52 * (7) Add a MAC check_create() hook for creating new named objects.
53 */
54
55#include <sys/cdefs.h>
56__FBSDID("$FreeBSD: head/sys/kern/uipc_shm.c 224914 2011-08-16 20:07:47Z kib $");
57
58#include "opt_capsicum.h"
59
60#include <sys/param.h>
61#include <sys/capability.h>
62#include <sys/fcntl.h>
63#include <sys/file.h>
64#include <sys/filedesc.h>
65#include <sys/fnv_hash.h>
66#include <sys/kernel.h>
67#include <sys/lock.h>
68#include <sys/malloc.h>
69#include <sys/mman.h>
70#include <sys/mutex.h>
71#include <sys/priv.h>
72#include <sys/proc.h>
73#include <sys/refcount.h>
74#include <sys/resourcevar.h>
75#include <sys/stat.h>
76#include <sys/sysctl.h>
77#include <sys/sysproto.h>
78#include <sys/systm.h>
79#include <sys/sx.h>
80#include <sys/time.h>
81#include <sys/vnode.h>
82
83#include <security/mac/mac_framework.h>
84
85#include <vm/vm.h>
86#include <vm/vm_param.h>
87#include <vm/pmap.h>
88#include <vm/vm_map.h>
89#include <vm/vm_object.h>
90#include <vm/vm_page.h>
91#include <vm/vm_pager.h>
92#include <vm/swap_pager.h>
93
94struct shm_mapping {
95	char		*sm_path;
96	Fnv32_t		sm_fnv;
97	struct shmfd	*sm_shmfd;
98	LIST_ENTRY(shm_mapping) sm_link;
99};
100
101static MALLOC_DEFINE(M_SHMFD, "shmfd", "shared memory file descriptor");
102static LIST_HEAD(, shm_mapping) *shm_dictionary;
103static struct sx shm_dict_lock;
104static struct mtx shm_timestamp_lock;
105static u_long shm_hash;
106
107#define	SHM_HASH(fnv)	(&shm_dictionary[(fnv) & shm_hash])
108
109static int	shm_access(struct shmfd *shmfd, struct ucred *ucred, int flags);
110static struct shmfd *shm_alloc(struct ucred *ucred, mode_t mode);
111static void	shm_dict_init(void *arg);
112static void	shm_drop(struct shmfd *shmfd);
113static struct shmfd *shm_hold(struct shmfd *shmfd);
114static void	shm_insert(char *path, Fnv32_t fnv, struct shmfd *shmfd);
115static struct shmfd *shm_lookup(char *path, Fnv32_t fnv);
116static int	shm_remove(char *path, Fnv32_t fnv, struct ucred *ucred);
117static int	shm_dotruncate(struct shmfd *shmfd, off_t length);
118
119static fo_rdwr_t	shm_read;
120static fo_rdwr_t	shm_write;
121static fo_truncate_t	shm_truncate;
122static fo_ioctl_t	shm_ioctl;
123static fo_poll_t	shm_poll;
124static fo_kqfilter_t	shm_kqfilter;
125static fo_stat_t	shm_stat;
126static fo_close_t	shm_close;
127static fo_chmod_t	shm_chmod;
128static fo_chown_t	shm_chown;
129
130/* File descriptor operations. */
131static struct fileops shm_ops = {
132	.fo_read = shm_read,
133	.fo_write = shm_write,
134	.fo_truncate = shm_truncate,
135	.fo_ioctl = shm_ioctl,
136	.fo_poll = shm_poll,
137	.fo_kqfilter = shm_kqfilter,
138	.fo_stat = shm_stat,
139	.fo_close = shm_close,
140	.fo_chmod = shm_chmod,
141	.fo_chown = shm_chown,
142	.fo_flags = DFLAG_PASSABLE
143};
144
145FEATURE(posix_shm, "POSIX shared memory");
146
147static int
148shm_read(struct file *fp, struct uio *uio, struct ucred *active_cred,
149    int flags, struct thread *td)
150{
151
152	return (EOPNOTSUPP);
153}
154
155static int
156shm_write(struct file *fp, struct uio *uio, struct ucred *active_cred,
157    int flags, struct thread *td)
158{
159
160	return (EOPNOTSUPP);
161}
162
163static int
164shm_truncate(struct file *fp, off_t length, struct ucred *active_cred,
165    struct thread *td)
166{
167	struct shmfd *shmfd;
168#ifdef MAC
169	int error;
170#endif
171
172	shmfd = fp->f_data;
173#ifdef MAC
174	error = mac_posixshm_check_truncate(active_cred, fp->f_cred, shmfd);
175	if (error)
176		return (error);
177#endif
178	return (shm_dotruncate(shmfd, length));
179}
180
181static int
182shm_ioctl(struct file *fp, u_long com, void *data,
183    struct ucred *active_cred, struct thread *td)
184{
185
186	return (EOPNOTSUPP);
187}
188
189static int
190shm_poll(struct file *fp, int events, struct ucred *active_cred,
191    struct thread *td)
192{
193
194	return (EOPNOTSUPP);
195}
196
197static int
198shm_kqfilter(struct file *fp, struct knote *kn)
199{
200
201	return (EOPNOTSUPP);
202}
203
204static int
205shm_stat(struct file *fp, struct stat *sb, struct ucred *active_cred,
206    struct thread *td)
207{
208	struct shmfd *shmfd;
209#ifdef MAC
210	int error;
211#endif
212
213	shmfd = fp->f_data;
214
215#ifdef MAC
216	error = mac_posixshm_check_stat(active_cred, fp->f_cred, shmfd);
217	if (error)
218		return (error);
219#endif
220
221	/*
222	 * Attempt to return sanish values for fstat() on a memory file
223	 * descriptor.
224	 */
225	bzero(sb, sizeof(*sb));
226	sb->st_blksize = PAGE_SIZE;
227	sb->st_size = shmfd->shm_size;
228	sb->st_blocks = (sb->st_size + sb->st_blksize - 1) / sb->st_blksize;
229	mtx_lock(&shm_timestamp_lock);
230	sb->st_atim = shmfd->shm_atime;
231	sb->st_ctim = shmfd->shm_ctime;
232	sb->st_mtim = shmfd->shm_mtime;
233	sb->st_birthtim = shmfd->shm_birthtime;
234	sb->st_mode = S_IFREG | shmfd->shm_mode;		/* XXX */
235	sb->st_uid = shmfd->shm_uid;
236	sb->st_gid = shmfd->shm_gid;
237	mtx_unlock(&shm_timestamp_lock);
238
239	return (0);
240}
241
242static int
243shm_close(struct file *fp, struct thread *td)
244{
245	struct shmfd *shmfd;
246
247	shmfd = fp->f_data;
248	fp->f_data = NULL;
249	shm_drop(shmfd);
250
251	return (0);
252}
253
254static int
255shm_dotruncate(struct shmfd *shmfd, off_t length)
256{
257	vm_object_t object;
258	vm_page_t m;
259	vm_pindex_t nobjsize;
260	vm_ooffset_t delta;
261
262	object = shmfd->shm_object;
263	VM_OBJECT_LOCK(object);
264	if (length == shmfd->shm_size) {
265		VM_OBJECT_UNLOCK(object);
266		return (0);
267	}
268	nobjsize = OFF_TO_IDX(length + PAGE_MASK);
269
270	/* Are we shrinking?  If so, trim the end. */
271	if (length < shmfd->shm_size) {
272		delta = ptoa(object->size - nobjsize);
273
274		/* Toss in memory pages. */
275		if (nobjsize < object->size)
276			vm_object_page_remove(object, nobjsize, object->size,
277			    0);
278
279		/* Toss pages from swap. */
280		if (object->type == OBJT_SWAP)
281			swap_pager_freespace(object, nobjsize, delta);
282
283		/* Free the swap accounted for shm */
284		swap_release_by_cred(delta, object->cred);
285		object->charge -= delta;
286
287		/*
288		 * If the last page is partially mapped, then zero out
289		 * the garbage at the end of the page.  See comments
290		 * in vnode_pager_setsize() for more details.
291		 *
292		 * XXXJHB: This handles in memory pages, but what about
293		 * a page swapped out to disk?
294		 */
295		if ((length & PAGE_MASK) &&
296		    (m = vm_page_lookup(object, OFF_TO_IDX(length))) != NULL &&
297		    m->valid != 0) {
298			int base = (int)length & PAGE_MASK;
299			int size = PAGE_SIZE - base;
300
301			pmap_zero_page_area(m, base, size);
302
303			/*
304			 * Update the valid bits to reflect the blocks that
305			 * have been zeroed.  Some of these valid bits may
306			 * have already been set.
307			 */
308			vm_page_set_valid(m, base, size);
309
310			/*
311			 * Round "base" to the next block boundary so that the
312			 * dirty bit for a partially zeroed block is not
313			 * cleared.
314			 */
315			base = roundup2(base, DEV_BSIZE);
316
317			vm_page_clear_dirty(m, base, PAGE_SIZE - base);
318		} else if ((length & PAGE_MASK) &&
319		    __predict_false(object->cache != NULL)) {
320			vm_page_cache_free(object, OFF_TO_IDX(length),
321			    nobjsize);
322		}
323	} else {
324
325		/* Attempt to reserve the swap */
326		delta = ptoa(nobjsize - object->size);
327		if (!swap_reserve_by_cred(delta, object->cred)) {
328			VM_OBJECT_UNLOCK(object);
329			return (ENOMEM);
330		}
331		object->charge += delta;
332	}
333	shmfd->shm_size = length;
334	mtx_lock(&shm_timestamp_lock);
335	vfs_timestamp(&shmfd->shm_ctime);
336	shmfd->shm_mtime = shmfd->shm_ctime;
337	mtx_unlock(&shm_timestamp_lock);
338	object->size = nobjsize;
339	VM_OBJECT_UNLOCK(object);
340	return (0);
341}
342
343/*
344 * shmfd object management including creation and reference counting
345 * routines.
346 */
347static struct shmfd *
348shm_alloc(struct ucred *ucred, mode_t mode)
349{
350	struct shmfd *shmfd;
351
352	shmfd = malloc(sizeof(*shmfd), M_SHMFD, M_WAITOK | M_ZERO);
353	shmfd->shm_size = 0;
354	shmfd->shm_uid = ucred->cr_uid;
355	shmfd->shm_gid = ucred->cr_gid;
356	shmfd->shm_mode = mode;
357	shmfd->shm_object = vm_pager_allocate(OBJT_DEFAULT, NULL,
358	    shmfd->shm_size, VM_PROT_DEFAULT, 0, ucred);
359	KASSERT(shmfd->shm_object != NULL, ("shm_create: vm_pager_allocate"));
360	VM_OBJECT_LOCK(shmfd->shm_object);
361	vm_object_clear_flag(shmfd->shm_object, OBJ_ONEMAPPING);
362	vm_object_set_flag(shmfd->shm_object, OBJ_NOSPLIT);
363	VM_OBJECT_UNLOCK(shmfd->shm_object);
364	vfs_timestamp(&shmfd->shm_birthtime);
365	shmfd->shm_atime = shmfd->shm_mtime = shmfd->shm_ctime =
366	    shmfd->shm_birthtime;
367	refcount_init(&shmfd->shm_refs, 1);
368#ifdef MAC
369	mac_posixshm_init(shmfd);
370	mac_posixshm_create(ucred, shmfd);
371#endif
372
373	return (shmfd);
374}
375
376static struct shmfd *
377shm_hold(struct shmfd *shmfd)
378{
379
380	refcount_acquire(&shmfd->shm_refs);
381	return (shmfd);
382}
383
384static void
385shm_drop(struct shmfd *shmfd)
386{
387
388	if (refcount_release(&shmfd->shm_refs)) {
389#ifdef MAC
390		mac_posixshm_destroy(shmfd);
391#endif
392		vm_object_deallocate(shmfd->shm_object);
393		free(shmfd, M_SHMFD);
394	}
395}
396
397/*
398 * Determine if the credentials have sufficient permissions for a
399 * specified combination of FREAD and FWRITE.
400 */
401static int
402shm_access(struct shmfd *shmfd, struct ucred *ucred, int flags)
403{
404	accmode_t accmode;
405	int error;
406
407	accmode = 0;
408	if (flags & FREAD)
409		accmode |= VREAD;
410	if (flags & FWRITE)
411		accmode |= VWRITE;
412	mtx_lock(&shm_timestamp_lock);
413	error = vaccess(VREG, shmfd->shm_mode, shmfd->shm_uid, shmfd->shm_gid,
414	    accmode, ucred, NULL);
415	mtx_unlock(&shm_timestamp_lock);
416	return (error);
417}
418
419/*
420 * Dictionary management.  We maintain an in-kernel dictionary to map
421 * paths to shmfd objects.  We use the FNV hash on the path to store
422 * the mappings in a hash table.
423 */
424static void
425shm_dict_init(void *arg)
426{
427
428	mtx_init(&shm_timestamp_lock, "shm timestamps", NULL, MTX_DEF);
429	sx_init(&shm_dict_lock, "shm dictionary");
430	shm_dictionary = hashinit(1024, M_SHMFD, &shm_hash);
431}
432SYSINIT(shm_dict_init, SI_SUB_SYSV_SHM, SI_ORDER_ANY, shm_dict_init, NULL);
433
434static struct shmfd *
435shm_lookup(char *path, Fnv32_t fnv)
436{
437	struct shm_mapping *map;
438
439	LIST_FOREACH(map, SHM_HASH(fnv), sm_link) {
440		if (map->sm_fnv != fnv)
441			continue;
442		if (strcmp(map->sm_path, path) == 0)
443			return (map->sm_shmfd);
444	}
445
446	return (NULL);
447}
448
449static void
450shm_insert(char *path, Fnv32_t fnv, struct shmfd *shmfd)
451{
452	struct shm_mapping *map;
453
454	map = malloc(sizeof(struct shm_mapping), M_SHMFD, M_WAITOK);
455	map->sm_path = path;
456	map->sm_fnv = fnv;
457	map->sm_shmfd = shm_hold(shmfd);
458	LIST_INSERT_HEAD(SHM_HASH(fnv), map, sm_link);
459}
460
461static int
462shm_remove(char *path, Fnv32_t fnv, struct ucred *ucred)
463{
464	struct shm_mapping *map;
465	int error;
466
467	LIST_FOREACH(map, SHM_HASH(fnv), sm_link) {
468		if (map->sm_fnv != fnv)
469			continue;
470		if (strcmp(map->sm_path, path) == 0) {
471#ifdef MAC
472			error = mac_posixshm_check_unlink(ucred, map->sm_shmfd);
473			if (error)
474				return (error);
475#endif
476			error = shm_access(map->sm_shmfd, ucred,
477			    FREAD | FWRITE);
478			if (error)
479				return (error);
480			LIST_REMOVE(map, sm_link);
481			shm_drop(map->sm_shmfd);
482			free(map->sm_path, M_SHMFD);
483			free(map, M_SHMFD);
484			return (0);
485		}
486	}
487
488	return (ENOENT);
489}
490
491/* System calls. */
492int
493shm_open(struct thread *td, struct shm_open_args *uap)
494{
495	struct filedesc *fdp;
496	struct shmfd *shmfd;
497	struct file *fp;
498	char *path;
499	Fnv32_t fnv;
500	mode_t cmode;
501	int fd, error;
502
503#ifdef CAPABILITY_MODE
504	/*
505	 * shm_open(2) is only allowed for anonymous objects.
506	 */
507	if (IN_CAPABILITY_MODE(td) && (uap->path != SHM_ANON))
508		return (ECAPMODE);
509#endif
510
511	if ((uap->flags & O_ACCMODE) != O_RDONLY &&
512	    (uap->flags & O_ACCMODE) != O_RDWR)
513		return (EINVAL);
514
515	if ((uap->flags & ~(O_ACCMODE | O_CREAT | O_EXCL | O_TRUNC)) != 0)
516		return (EINVAL);
517
518	fdp = td->td_proc->p_fd;
519	cmode = (uap->mode & ~fdp->fd_cmask) & ACCESSPERMS;
520
521	error = falloc(td, &fp, &fd, 0);
522	if (error)
523		return (error);
524
525	/* A SHM_ANON path pointer creates an anonymous object. */
526	if (uap->path == SHM_ANON) {
527		/* A read-only anonymous object is pointless. */
528		if ((uap->flags & O_ACCMODE) == O_RDONLY) {
529			fdclose(fdp, fp, fd, td);
530			fdrop(fp, td);
531			return (EINVAL);
532		}
533		shmfd = shm_alloc(td->td_ucred, cmode);
534	} else {
535		path = malloc(MAXPATHLEN, M_SHMFD, M_WAITOK);
536		error = copyinstr(uap->path, path, MAXPATHLEN, NULL);
537
538		/* Require paths to start with a '/' character. */
539		if (error == 0 && path[0] != '/')
540			error = EINVAL;
541		if (error) {
542			fdclose(fdp, fp, fd, td);
543			fdrop(fp, td);
544			free(path, M_SHMFD);
545			return (error);
546		}
547
548		fnv = fnv_32_str(path, FNV1_32_INIT);
549		sx_xlock(&shm_dict_lock);
550		shmfd = shm_lookup(path, fnv);
551		if (shmfd == NULL) {
552			/* Object does not yet exist, create it if requested. */
553			if (uap->flags & O_CREAT) {
554				shmfd = shm_alloc(td->td_ucred, cmode);
555				shm_insert(path, fnv, shmfd);
556			} else {
557				free(path, M_SHMFD);
558				error = ENOENT;
559			}
560		} else {
561			/*
562			 * Object already exists, obtain a new
563			 * reference if requested and permitted.
564			 */
565			free(path, M_SHMFD);
566			if ((uap->flags & (O_CREAT | O_EXCL)) ==
567			    (O_CREAT | O_EXCL))
568				error = EEXIST;
569			else {
570#ifdef MAC
571				error = mac_posixshm_check_open(td->td_ucred,
572				    shmfd);
573				if (error == 0)
574#endif
575				error = shm_access(shmfd, td->td_ucred,
576				    FFLAGS(uap->flags & O_ACCMODE));
577			}
578
579			/*
580			 * Truncate the file back to zero length if
581			 * O_TRUNC was specified and the object was
582			 * opened with read/write.
583			 */
584			if (error == 0 &&
585			    (uap->flags & (O_ACCMODE | O_TRUNC)) ==
586			    (O_RDWR | O_TRUNC)) {
587#ifdef MAC
588				error = mac_posixshm_check_truncate(
589					td->td_ucred, fp->f_cred, shmfd);
590				if (error == 0)
591#endif
592					shm_dotruncate(shmfd, 0);
593			}
594			if (error == 0)
595				shm_hold(shmfd);
596		}
597		sx_xunlock(&shm_dict_lock);
598
599		if (error) {
600			fdclose(fdp, fp, fd, td);
601			fdrop(fp, td);
602			return (error);
603		}
604	}
605
606	finit(fp, FFLAGS(uap->flags & O_ACCMODE), DTYPE_SHM, shmfd, &shm_ops);
607
608	FILEDESC_XLOCK(fdp);
609	if (fdp->fd_ofiles[fd] == fp)
610		fdp->fd_ofileflags[fd] |= UF_EXCLOSE;
611	FILEDESC_XUNLOCK(fdp);
612	td->td_retval[0] = fd;
613	fdrop(fp, td);
614
615	return (0);
616}
617
618int
619shm_unlink(struct thread *td, struct shm_unlink_args *uap)
620{
621	char *path;
622	Fnv32_t fnv;
623	int error;
624
625	path = malloc(MAXPATHLEN, M_TEMP, M_WAITOK);
626	error = copyinstr(uap->path, path, MAXPATHLEN, NULL);
627	if (error) {
628		free(path, M_TEMP);
629		return (error);
630	}
631
632	fnv = fnv_32_str(path, FNV1_32_INIT);
633	sx_xlock(&shm_dict_lock);
634	error = shm_remove(path, fnv, td->td_ucred);
635	sx_xunlock(&shm_dict_lock);
636	free(path, M_TEMP);
637
638	return (error);
639}
640
641/*
642 * mmap() helper to validate mmap() requests against shm object state
643 * and give mmap() the vm_object to use for the mapping.
644 */
645int
646shm_mmap(struct shmfd *shmfd, vm_size_t objsize, vm_ooffset_t foff,
647    vm_object_t *obj)
648{
649
650	/*
651	 * XXXRW: This validation is probably insufficient, and subject to
652	 * sign errors.  It should be fixed.
653	 */
654	if (foff >= shmfd->shm_size ||
655	    foff + objsize > round_page(shmfd->shm_size))
656		return (EINVAL);
657
658	mtx_lock(&shm_timestamp_lock);
659	vfs_timestamp(&shmfd->shm_atime);
660	mtx_unlock(&shm_timestamp_lock);
661	vm_object_reference(shmfd->shm_object);
662	*obj = shmfd->shm_object;
663	return (0);
664}
665
666static int
667shm_chmod(struct file *fp, mode_t mode, struct ucred *active_cred,
668    struct thread *td)
669{
670	struct shmfd *shmfd;
671	int error;
672
673	error = 0;
674	shmfd = fp->f_data;
675	mtx_lock(&shm_timestamp_lock);
676	/*
677	 * SUSv4 says that x bits of permission need not be affected.
678	 * Be consistent with our shm_open there.
679	 */
680#ifdef MAC
681	error = mac_posixshm_check_setmode(active_cred, shmfd, mode);
682	if (error != 0)
683		goto out;
684#endif
685	error = vaccess(VREG, shmfd->shm_mode, shmfd->shm_uid,
686	    shmfd->shm_gid, VADMIN, active_cred, NULL);
687	if (error != 0)
688		goto out;
689	shmfd->shm_mode = mode & ACCESSPERMS;
690out:
691	mtx_unlock(&shm_timestamp_lock);
692	return (error);
693}
694
695static int
696shm_chown(struct file *fp, uid_t uid, gid_t gid, struct ucred *active_cred,
697    struct thread *td)
698{
699	struct shmfd *shmfd;
700	int error;
701
702	shmfd = fp->f_data;
703	mtx_lock(&shm_timestamp_lock);
704#ifdef MAC
705	error = mac_posixshm_check_setowner(active_cred, shmfd, uid, gid);
706	if (error != 0)
707		goto out;
708#endif
709	if (uid == (uid_t)-1)
710		uid = shmfd->shm_uid;
711	if (gid == (gid_t)-1)
712                 gid = shmfd->shm_gid;
713	if (((uid != shmfd->shm_uid && uid != active_cred->cr_uid) ||
714	    (gid != shmfd->shm_gid && !groupmember(gid, active_cred))) &&
715	    (error = priv_check_cred(active_cred, PRIV_VFS_CHOWN, 0)))
716		goto out;
717	shmfd->shm_uid = uid;
718	shmfd->shm_gid = gid;
719out:
720	mtx_unlock(&shm_timestamp_lock);
721	return (error);
722}
723