1 // SPDX-License-Identifier: GPL-2.0-only
2 /*
3 * Userfaultfd tests util functions
4 *
5 * Copyright (C) 2015-2023 Red Hat, Inc.
6 */
7
8 #include "uffd-common.h"
9
10 #define BASE_PMD_ADDR ((void *)(1UL << 30))
11
12 volatile bool test_uffdio_copy_eexist = true;
13 unsigned long nr_cpus, nr_pages, nr_pages_per_cpu, page_size;
14 char *area_src, *area_src_alias, *area_dst, *area_dst_alias, *area_remap;
15 int uffd = -1, uffd_flags, finished, *pipefd, test_type;
16 bool map_shared;
17 bool test_uffdio_wp = true;
18 unsigned long long *count_verify;
19 uffd_test_ops_t *uffd_test_ops;
20 uffd_test_case_ops_t *uffd_test_case_ops;
21 pthread_barrier_t ready_for_fork;
22
23 static int uffd_mem_fd_create(off_t mem_size, bool hugetlb)
24 {
25 unsigned int memfd_flags = 0;
26 int mem_fd;
27
28 if (hugetlb)
29 memfd_flags = MFD_HUGETLB;
30 mem_fd = memfd_create("uffd-test", memfd_flags);
31 if (mem_fd < 0)
32 err("memfd_create");
33 if (ftruncate(mem_fd, mem_size))
34 err("ftruncate");
35 if (fallocate(mem_fd,
36 FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE, 0,
37 mem_size))
38 err("fallocate");
39
40 return mem_fd;
41 }
42
43 static void anon_release_pages(char *rel_area)
44 {
45 if (madvise(rel_area, nr_pages * page_size, MADV_DONTNEED))
46 err("madvise(MADV_DONTNEED) failed");
47 }
48
49 static int anon_allocate_area(void **alloc_area, bool is_src)
50 {
51 *alloc_area = mmap(NULL, nr_pages * page_size, PROT_READ | PROT_WRITE,
52 MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
53 if (*alloc_area == MAP_FAILED) {
54 *alloc_area = NULL;
55 return -errno;
56 }
57 return 0;
58 }
59
60 static void noop_alias_mapping(__u64 *start, size_t len, unsigned long offset)
61 {
62 }
63
64 static void hugetlb_release_pages(char *rel_area)
65 {
66 if (!map_shared) {
67 if (madvise(rel_area, nr_pages * page_size, MADV_DONTNEED))
68 err("madvise(MADV_DONTNEED) failed");
69 } else {
70 if (madvise(rel_area, nr_pages * page_size, MADV_REMOVE))
71 err("madvise(MADV_REMOVE) failed");
72 }
73 }
74
75 static int hugetlb_allocate_area(void **alloc_area, bool is_src)
76 {
77 off_t size = nr_pages * page_size;
78 off_t offset = is_src ? 0 : size;
79 void *area_alias = NULL;
80 char **alloc_area_alias;
81 int mem_fd = uffd_mem_fd_create(size * 2, true);
82
83 *alloc_area = mmap(NULL, size, PROT_READ | PROT_WRITE,
84 (map_shared ? MAP_SHARED : MAP_PRIVATE) |
85 (is_src ? 0 : MAP_NORESERVE),
86 mem_fd, offset);
87 if (*alloc_area == MAP_FAILED) {
88 *alloc_area = NULL;
89 return -errno;
90 }
91
92 if (map_shared) {
93 area_alias = mmap(NULL, size, PROT_READ | PROT_WRITE,
94 MAP_SHARED, mem_fd, offset);
95 if (area_alias == MAP_FAILED)
96 return -errno;
97 }
98
99 if (is_src) {
100 alloc_area_alias = &area_src_alias;
101 } else {
102 alloc_area_alias = &area_dst_alias;
103 }
104 if (area_alias)
105 *alloc_area_alias = area_alias;
106
107 close(mem_fd);
108 return 0;
109 }
110
111 static void hugetlb_alias_mapping(__u64 *start, size_t len, unsigned long offset)
112 {
113 if (!map_shared)
114 return;
115
116 *start = (unsigned long) area_dst_alias + offset;
117 }
118
119 static void shmem_release_pages(char *rel_area)
120 {
121 if (madvise(rel_area, nr_pages * page_size, MADV_REMOVE))
122 err("madvise(MADV_REMOVE) failed");
123 }
124
125 static int shmem_allocate_area(void **alloc_area, bool is_src)
126 {
127 void *area_alias = NULL;
128 size_t bytes = nr_pages * page_size, hpage_size = read_pmd_pagesize();
129 unsigned long offset = is_src ? 0 : bytes;
130 char *p = NULL, *p_alias = NULL;
131 int mem_fd = uffd_mem_fd_create(bytes * 2, false);
132
133 /* TODO: clean this up. Use a static addr is ugly */
134 p = BASE_PMD_ADDR;
135 if (!is_src)
136 /* src map + alias + interleaved hpages */
137 p += 2 * (bytes + hpage_size);
138 p_alias = p;
139 p_alias += bytes;
140 p_alias += hpage_size; /* Prevent src/dst VMA merge */
141
142 *alloc_area = mmap(p, bytes, PROT_READ | PROT_WRITE, MAP_SHARED,
143 mem_fd, offset);
144 if (*alloc_area == MAP_FAILED) {
145 *alloc_area = NULL;
146 return -errno;
147 }
148 if (*alloc_area != p)
149 err("mmap of memfd failed at %p", p);
150
151 area_alias = mmap(p_alias, bytes, PROT_READ | PROT_WRITE, MAP_SHARED,
152 mem_fd, offset);
153 if (area_alias == MAP_FAILED) {
154 munmap(*alloc_area, bytes);
155 *alloc_area = NULL;
156 return -errno;
157 }
158 if (area_alias != p_alias)
159 err("mmap of anonymous memory failed at %p", p_alias);
160
161 if (is_src)
162 area_src_alias = area_alias;
163 else
164 area_dst_alias = area_alias;
165
166 close(mem_fd);
167 return 0;
168 }
169
170 static void shmem_alias_mapping(__u64 *start, size_t len, unsigned long offset)
171 {
172 *start = (unsigned long)area_dst_alias + offset;
173 }
174
175 static void shmem_check_pmd_mapping(void *p, int expect_nr_hpages)
176 {
177 if (!check_huge_shmem(area_dst_alias, expect_nr_hpages,
178 read_pmd_pagesize()))
179 err("Did not find expected %d number of hugepages",
180 expect_nr_hpages);
181 }
182
183 struct uffd_test_ops anon_uffd_test_ops = {
184 .allocate_area = anon_allocate_area,
185 .release_pages = anon_release_pages,
186 .alias_mapping = noop_alias_mapping,
187 .check_pmd_mapping = NULL,
188 };
189
190 struct uffd_test_ops shmem_uffd_test_ops = {
191 .allocate_area = shmem_allocate_area,
192 .release_pages = shmem_release_pages,
193 .alias_mapping = shmem_alias_mapping,
194 .check_pmd_mapping = shmem_check_pmd_mapping,
195 };
196
197 struct uffd_test_ops hugetlb_uffd_test_ops = {
198 .allocate_area = hugetlb_allocate_area,
199 .release_pages = hugetlb_release_pages,
200 .alias_mapping = hugetlb_alias_mapping,
201 .check_pmd_mapping = NULL,
202 };
203
204 void uffd_stats_report(struct uffd_args *args, int n_cpus)
205 {
206 int i;
207 unsigned long long miss_total = 0, wp_total = 0, minor_total = 0;
208
209 for (i = 0; i < n_cpus; i++) {
210 miss_total += args[i].missing_faults;
211 wp_total += args[i].wp_faults;
212 minor_total += args[i].minor_faults;
213 }
214
215 printf("userfaults: ");
216 if (miss_total) {
217 printf("%llu missing (", miss_total);
218 for (i = 0; i < n_cpus; i++)
219 printf("%lu+", args[i].missing_faults);
220 printf("\b) ");
221 }
222 if (wp_total) {
223 printf("%llu wp (", wp_total);
224 for (i = 0; i < n_cpus; i++)
225 printf("%lu+", args[i].wp_faults);
226 printf("\b) ");
227 }
228 if (minor_total) {
229 printf("%llu minor (", minor_total);
230 for (i = 0; i < n_cpus; i++)
231 printf("%lu+", args[i].minor_faults);
232 printf("\b)");
233 }
234 printf("\n");
235 }
236
237 int userfaultfd_open(uint64_t *features)
238 {
239 struct uffdio_api uffdio_api;
240
241 uffd = uffd_open(UFFD_FLAGS);
242 if (uffd < 0)
243 return -1;
244 uffd_flags = fcntl(uffd, F_GETFD, NULL);
245
246 uffdio_api.api = UFFD_API;
247 uffdio_api.features = *features;
248 if (ioctl(uffd, UFFDIO_API, &uffdio_api))
249 /* Probably lack of CAP_PTRACE? */
250 return -1;
251 if (uffdio_api.api != UFFD_API)
252 err("UFFDIO_API error: %" PRIu64, (uint64_t)uffdio_api.api);
253
254 *features = uffdio_api.features;
255 return 0;
256 }
257
258 static inline void munmap_area(void **area)
259 {
260 if (*area)
261 if (munmap(*area, nr_pages * page_size))
262 err("munmap");
263
264 *area = NULL;
265 }
266
267 void uffd_test_ctx_clear(void)
268 {
269 size_t i;
270
271 if (pipefd) {
272 for (i = 0; i < nr_cpus * 2; ++i) {
273 if (close(pipefd[i]))
274 err("close pipefd");
275 }
276 free(pipefd);
277 pipefd = NULL;
278 }
279
280 if (count_verify) {
281 free(count_verify);
282 count_verify = NULL;
283 }
284
285 if (uffd != -1) {
286 if (close(uffd))
287 err("close uffd");
288 uffd = -1;
289 }
290
291 munmap_area((void **)&area_src);
292 munmap_area((void **)&area_src_alias);
293 munmap_area((void **)&area_dst);
294 munmap_area((void **)&area_dst_alias);
295 munmap_area((void **)&area_remap);
296 }
297
298 int uffd_test_ctx_init(uint64_t features, const char **errmsg)
299 {
300 unsigned long nr, cpu;
301 int ret;
302
303 if (uffd_test_case_ops && uffd_test_case_ops->pre_alloc) {
304 ret = uffd_test_case_ops->pre_alloc(errmsg);
305 if (ret)
306 return ret;
307 }
308
309 ret = uffd_test_ops->allocate_area((void **)&area_src, true);
310 ret |= uffd_test_ops->allocate_area((void **)&area_dst, false);
311 if (ret) {
312 if (errmsg)
313 *errmsg = "memory allocation failed";
314 return ret;
315 }
316
317 if (uffd_test_case_ops && uffd_test_case_ops->post_alloc) {
318 ret = uffd_test_case_ops->post_alloc(errmsg);
319 if (ret)
320 return ret;
321 }
322
323 ret = userfaultfd_open(&features);
324 if (ret) {
325 if (errmsg)
326 *errmsg = "possible lack of priviledge";
327 return ret;
328 }
329
330 count_verify = malloc(nr_pages * sizeof(unsigned long long));
331 if (!count_verify)
332 err("count_verify");
333
334 for (nr = 0; nr < nr_pages; nr++) {
335 *area_mutex(area_src, nr) =
336 (pthread_mutex_t)PTHREAD_MUTEX_INITIALIZER;
337 count_verify[nr] = *area_count(area_src, nr) = 1;
338 /*
339 * In the transition between 255 to 256, powerpc will
340 * read out of order in my_bcmp and see both bytes as
341 * zero, so leave a placeholder below always non-zero
342 * after the count, to avoid my_bcmp to trigger false
343 * positives.
344 */
345 *(area_count(area_src, nr) + 1) = 1;
346 }
347
348 /*
349 * After initialization of area_src, we must explicitly release pages
350 * for area_dst to make sure it's fully empty. Otherwise we could have
351 * some area_dst pages be errornously initialized with zero pages,
352 * hence we could hit memory corruption later in the test.
353 *
354 * One example is when THP is globally enabled, above allocate_area()
355 * calls could have the two areas merged into a single VMA (as they
356 * will have the same VMA flags so they're mergeable). When we
357 * initialize the area_src above, it's possible that some part of
358 * area_dst could have been faulted in via one huge THP that will be
359 * shared between area_src and area_dst. It could cause some of the
360 * area_dst won't be trapped by missing userfaults.
361 *
362 * This release_pages() will guarantee even if that happened, we'll
363 * proactively split the thp and drop any accidentally initialized
364 * pages within area_dst.
365 */
366 uffd_test_ops->release_pages(area_dst);
367
368 pipefd = malloc(sizeof(int) * nr_cpus * 2);
369 if (!pipefd)
370 err("pipefd");
371 for (cpu = 0; cpu < nr_cpus; cpu++)
372 if (pipe2(&pipefd[cpu * 2], O_CLOEXEC | O_NONBLOCK))
373 err("pipe");
374
375 return 0;
376 }
377
378 void wp_range(int ufd, __u64 start, __u64 len, bool wp)
379 {
380 struct uffdio_writeprotect prms;
381
382 /* Write protection page faults */
383 prms.range.start = start;
384 prms.range.len = len;
385 /* Undo write-protect, do wakeup after that */
386 prms.mode = wp ? UFFDIO_WRITEPROTECT_MODE_WP : 0;
387
388 if (ioctl(ufd, UFFDIO_WRITEPROTECT, &prms))
389 err("clear WP failed: address=0x%"PRIx64, (uint64_t)start);
390 }
391
392 static void continue_range(int ufd, __u64 start, __u64 len, bool wp)
393 {
394 struct uffdio_continue req;
395 int ret;
396
397 req.range.start = start;
398 req.range.len = len;
399 req.mode = 0;
400 if (wp)
401 req.mode |= UFFDIO_CONTINUE_MODE_WP;
402
403 if (ioctl(ufd, UFFDIO_CONTINUE, &req))
404 err("UFFDIO_CONTINUE failed for address 0x%" PRIx64,
405 (uint64_t)start);
406
407 /*
408 * Error handling within the kernel for continue is subtly different
409 * from copy or zeropage, so it may be a source of bugs. Trigger an
410 * error (-EEXIST) on purpose, to verify doing so doesn't cause a BUG.
411 */
412 req.mapped = 0;
413 ret = ioctl(ufd, UFFDIO_CONTINUE, &req);
414 if (ret >= 0 || req.mapped != -EEXIST)
415 err("failed to exercise UFFDIO_CONTINUE error handling, ret=%d, mapped=%" PRId64,
416 ret, (int64_t) req.mapped);
417 }
418
419 int uffd_read_msg(int ufd, struct uffd_msg *msg)
420 {
421 int ret = read(uffd, msg, sizeof(*msg));
422
423 if (ret != sizeof(*msg)) {
424 if (ret < 0) {
425 if (errno == EAGAIN || errno == EINTR)
426 return 1;
427 err("blocking read error");
428 } else {
429 err("short read");
430 }
431 }
432
433 return 0;
434 }
435
436 void uffd_handle_page_fault(struct uffd_msg *msg, struct uffd_args *args)
437 {
438 unsigned long offset;
439
440 if (msg->event != UFFD_EVENT_PAGEFAULT)
441 err("unexpected msg event %u", msg->event);
442
443 if (msg->arg.pagefault.flags & UFFD_PAGEFAULT_FLAG_WP) {
444 /* Write protect page faults */
445 wp_range(uffd, msg->arg.pagefault.address, page_size, false);
446 args->wp_faults++;
447 } else if (msg->arg.pagefault.flags & UFFD_PAGEFAULT_FLAG_MINOR) {
448 uint8_t *area;
449 int b;
450
451 /*
452 * Minor page faults
453 *
454 * To prove we can modify the original range for testing
455 * purposes, we're going to bit flip this range before
456 * continuing.
457 *
458 * Note that this requires all minor page fault tests operate on
459 * area_dst (non-UFFD-registered) and area_dst_alias
460 * (UFFD-registered).
461 */
462
463 area = (uint8_t *)(area_dst +
464 ((char *)msg->arg.pagefault.address -
465 area_dst_alias));
466 for (b = 0; b < page_size; ++b)
467 area[b] = ~area[b];
468 continue_range(uffd, msg->arg.pagefault.address, page_size,
469 args->apply_wp);
470 args->minor_faults++;
471 } else {
472 /*
473 * Missing page faults.
474 *
475 * Here we force a write check for each of the missing mode
476 * faults. It's guaranteed because the only threads that
477 * will trigger uffd faults are the locking threads, and
478 * their first instruction to touch the missing page will
479 * always be pthread_mutex_lock().
480 *
481 * Note that here we relied on an NPTL glibc impl detail to
482 * always read the lock type at the entry of the lock op
483 * (pthread_mutex_t.__data.__type, offset 0x10) before
484 * doing any locking operations to guarantee that. It's
485 * actually not good to rely on this impl detail because
486 * logically a pthread-compatible lib can implement the
487 * locks without types and we can fail when linking with
488 * them. However since we used to find bugs with this
489 * strict check we still keep it around. Hopefully this
490 * could be a good hint when it fails again. If one day
491 * it'll break on some other impl of glibc we'll revisit.
492 */
493 if (msg->arg.pagefault.flags & UFFD_PAGEFAULT_FLAG_WRITE)
494 err("unexpected write fault");
495
496 offset = (char *)(unsigned long)msg->arg.pagefault.address - area_dst;
497 offset &= ~(page_size-1);
498
499 if (copy_page(uffd, offset, args->apply_wp))
500 args->missing_faults++;
501 }
502 }
503
504 void *uffd_poll_thread(void *arg)
505 {
506 struct uffd_args *args = (struct uffd_args *)arg;
507 unsigned long cpu = args->cpu;
508 struct pollfd pollfd[2];
509 struct uffd_msg msg;
510 struct uffdio_register uffd_reg;
511 int ret;
512 char tmp_chr;
513
514 if (!args->handle_fault)
515 args->handle_fault = uffd_handle_page_fault;
516
517 pollfd[0].fd = uffd;
518 pollfd[0].events = POLLIN;
519 pollfd[1].fd = pipefd[cpu*2];
520 pollfd[1].events = POLLIN;
521
522 /* Ready for parent thread to fork */
523 pthread_barrier_wait(&ready_for_fork);
524
525 for (;;) {
526 ret = poll(pollfd, 2, -1);
527 if (ret <= 0) {
528 if (errno == EINTR || errno == EAGAIN)
529 continue;
530 err("poll error: %d", ret);
531 }
532 if (pollfd[1].revents) {
533 if (!(pollfd[1].revents & POLLIN))
534 err("pollfd[1].revents %d", pollfd[1].revents);
535 if (read(pollfd[1].fd, &tmp_chr, 1) != 1)
536 err("read pipefd error");
537 break;
538 }
539 if (!(pollfd[0].revents & POLLIN))
540 err("pollfd[0].revents %d", pollfd[0].revents);
541 if (uffd_read_msg(uffd, &msg))
542 continue;
543 switch (msg.event) {
544 default:
545 err("unexpected msg event %u\n", msg.event);
546 break;
547 case UFFD_EVENT_PAGEFAULT:
548 args->handle_fault(&msg, args);
549 break;
550 case UFFD_EVENT_FORK:
551 close(uffd);
552 uffd = msg.arg.fork.ufd;
553 pollfd[0].fd = uffd;
554 break;
555 case UFFD_EVENT_REMOVE:
556 uffd_reg.range.start = msg.arg.remove.start;
557 uffd_reg.range.len = msg.arg.remove.end -
558 msg.arg.remove.start;
559 if (ioctl(uffd, UFFDIO_UNREGISTER, &uffd_reg.range))
560 err("remove failure");
561 break;
562 case UFFD_EVENT_REMAP:
563 area_remap = area_dst; /* save for later unmap */
564 area_dst = (char *)(unsigned long)msg.arg.remap.to;
565 break;
566 }
567 }
568
569 return NULL;
570 }
571
572 static void retry_copy_page(int ufd, struct uffdio_copy *uffdio_copy,
573 unsigned long offset)
574 {
575 uffd_test_ops->alias_mapping(&uffdio_copy->dst,
576 uffdio_copy->len,
577 offset);
578 if (ioctl(ufd, UFFDIO_COPY, uffdio_copy)) {
579 /* real retval in ufdio_copy.copy */
580 if (uffdio_copy->copy != -EEXIST)
581 err("UFFDIO_COPY retry error: %"PRId64,
582 (int64_t)uffdio_copy->copy);
583 } else {
584 err("UFFDIO_COPY retry unexpected: %"PRId64,
585 (int64_t)uffdio_copy->copy);
586 }
587 }
588
589 static void wake_range(int ufd, unsigned long addr, unsigned long len)
590 {
591 struct uffdio_range uffdio_wake;
592
593 uffdio_wake.start = addr;
594 uffdio_wake.len = len;
595
596 if (ioctl(ufd, UFFDIO_WAKE, &uffdio_wake))
597 fprintf(stderr, "error waking %lu\n",
598 addr), exit(1);
599 }
600
601 int __copy_page(int ufd, unsigned long offset, bool retry, bool wp)
602 {
603 struct uffdio_copy uffdio_copy;
604
605 if (offset >= nr_pages * page_size)
606 err("unexpected offset %lu\n", offset);
607 uffdio_copy.dst = (unsigned long) area_dst + offset;
608 uffdio_copy.src = (unsigned long) area_src + offset;
609 uffdio_copy.len = page_size;
610 if (wp)
611 uffdio_copy.mode = UFFDIO_COPY_MODE_WP;
612 else
613 uffdio_copy.mode = 0;
614 uffdio_copy.copy = 0;
615 if (ioctl(ufd, UFFDIO_COPY, &uffdio_copy)) {
616 /* real retval in ufdio_copy.copy */
617 if (uffdio_copy.copy != -EEXIST)
618 err("UFFDIO_COPY error: %"PRId64,
619 (int64_t)uffdio_copy.copy);
620 wake_range(ufd, uffdio_copy.dst, page_size);
621 } else if (uffdio_copy.copy != page_size) {
622 err("UFFDIO_COPY error: %"PRId64, (int64_t)uffdio_copy.copy);
623 } else {
624 if (test_uffdio_copy_eexist && retry) {
625 test_uffdio_copy_eexist = false;
626 retry_copy_page(ufd, &uffdio_copy, offset);
627 }
628 return 1;
629 }
630 return 0;
631 }
632
633 int copy_page(int ufd, unsigned long offset, bool wp)
634 {
635 return __copy_page(ufd, offset, false, wp);
636 }
637
638 int move_page(int ufd, unsigned long offset, unsigned long len)
639 {
640 struct uffdio_move uffdio_move;
641
642 if (offset + len > nr_pages * page_size)
643 err("unexpected offset %lu and length %lu\n", offset, len);
644 uffdio_move.dst = (unsigned long) area_dst + offset;
645 uffdio_move.src = (unsigned long) area_src + offset;
646 uffdio_move.len = len;
647 uffdio_move.mode = UFFDIO_MOVE_MODE_ALLOW_SRC_HOLES;
648 uffdio_move.move = 0;
649 if (ioctl(ufd, UFFDIO_MOVE, &uffdio_move)) {
650 /* real retval in uffdio_move.move */
651 if (uffdio_move.move != -EEXIST)
652 err("UFFDIO_MOVE error: %"PRId64,
653 (int64_t)uffdio_move.move);
654 wake_range(ufd, uffdio_move.dst, len);
655 } else if (uffdio_move.move != len) {
656 err("UFFDIO_MOVE error: %"PRId64, (int64_t)uffdio_move.move);
657 } else
658 return 1;
659 return 0;
660 }
661
662 int uffd_open_dev(unsigned int flags)
663 {
664 int fd, uffd;
665
666 fd = open("/dev/userfaultfd", O_RDWR | O_CLOEXEC);
667 if (fd < 0)
668 return fd;
669 uffd = ioctl(fd, USERFAULTFD_IOC_NEW, flags);
670 close(fd);
671
672 return uffd;
673 }
674
675 int uffd_open_sys(unsigned int flags)
676 {
677 return syscall(__NR_userfaultfd, flags);
678 }
679
680 int uffd_open(unsigned int flags)
681 {
682 int uffd = uffd_open_sys(flags);
683
684 if (uffd < 0)
685 uffd = uffd_open_dev(flags);
686
687 return uffd;
688 }
689
690 int uffd_get_features(uint64_t *features)
691 {
692 struct uffdio_api uffdio_api = { .api = UFFD_API, .features = 0 };
693 /*
694 * This should by default work in most kernels; the feature list
695 * will be the same no matter what we pass in here.
696 */
697 int fd = uffd_open(UFFD_USER_MODE_ONLY);
698
699 if (fd < 0)
700 /* Maybe the kernel is older than user-only mode? */
701 fd = uffd_open(0);
702
703 if (fd < 0)
704 return fd;
705
706 if (ioctl(fd, UFFDIO_API, &uffdio_api)) {
707 close(fd);
708 return -errno;
709 }
710
711 *features = uffdio_api.features;
712 close(fd);
713
714 return 0;
715 }
716
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