1 // SPDX-License-Identifier: GPL-2.0
4 * This tests memfd interactions with get_user_pages(). We require the
5 * fuse_mnt.c program to provide a fake direct-IO FUSE mount-point for us. This
6 * file-system delays _all_ reads by 1s and forces direct-IO. This means, any
7 * read() on files in that file-system will pin the receive-buffer pages for at
8 * least 1s via get_user_pages().
10 * We use this trick to race ADD_SEALS against a write on a memfd object. The
11 * ADD_SEALS must fail if the memfd pages are still pinned. Note that we use
12 * the read() syscall with our memory-mapped memfd object as receive buffer to
13 * force the kernel to write into our memfd object.
17 #define __EXPORTED_HEADERS__
22 #include <linux/falloc.h>
23 #include <linux/fcntl.h>
24 #include <linux/memfd.h>
32 #include <sys/syscall.h>
38 #define MFD_DEF_SIZE 8192
39 #define STACK_SIZE 65536
41 static size_t mfd_def_size = MFD_DEF_SIZE;
43 static int mfd_assert_new(const char *name, loff_t sz, unsigned int flags)
47 fd = sys_memfd_create(name, flags);
49 printf("memfd_create(\"%s\", %u) failed: %m\n",
54 r = ftruncate(fd, sz);
56 printf("ftruncate(%llu) failed: %m\n", (unsigned long long)sz);
63 static __u64 mfd_assert_get_seals(int fd)
67 r = fcntl(fd, F_GET_SEALS);
69 printf("GET_SEALS(%d) failed: %m\n", fd);
76 static void mfd_assert_has_seals(int fd, __u64 seals)
80 s = mfd_assert_get_seals(fd);
82 printf("%llu != %llu = GET_SEALS(%d)\n",
83 (unsigned long long)seals, (unsigned long long)s, fd);
88 static void mfd_assert_add_seals(int fd, __u64 seals)
93 s = mfd_assert_get_seals(fd);
94 r = fcntl(fd, F_ADD_SEALS, seals);
96 printf("ADD_SEALS(%d, %llu -> %llu) failed: %m\n",
97 fd, (unsigned long long)s, (unsigned long long)seals);
102 static int mfd_busy_add_seals(int fd, __u64 seals)
107 r = fcntl(fd, F_GET_SEALS);
113 r = fcntl(fd, F_ADD_SEALS, seals);
114 if (r < 0 && errno != EBUSY) {
115 printf("ADD_SEALS(%d, %llu -> %llu) didn't fail as expected with EBUSY: %m\n",
116 fd, (unsigned long long)s, (unsigned long long)seals);
123 static void *mfd_assert_mmap_shared(int fd)
129 PROT_READ | PROT_WRITE,
133 if (p == MAP_FAILED) {
134 printf("mmap() failed: %m\n");
141 static void *mfd_assert_mmap_private(int fd)
147 PROT_READ | PROT_WRITE,
151 if (p == MAP_FAILED) {
152 printf("mmap() failed: %m\n");
159 static int global_mfd = -1;
160 static void *global_p = NULL;
162 static int sealing_thread_fn(void *arg)
167 * This thread first waits 200ms so any pending operation in the parent
168 * is correctly started. After that, it tries to seal @global_mfd as
169 * SEAL_WRITE. This _must_ fail as the parent thread has a read() into
170 * that memory mapped object still ongoing.
171 * We then wait one more second and try sealing again. This time it
172 * must succeed as there shouldn't be anyone else pinning the pages.
175 /* wait 200ms for FUSE-request to be active */
178 /* unmount mapping before sealing to avoid i_mmap_writable failures */
179 munmap(global_p, mfd_def_size);
181 /* Try sealing the global file; expect EBUSY or success. Current
182 * kernels will never succeed, but in the future, kernels might
183 * implement page-replacements or other fancy ways to avoid racing
185 r = mfd_busy_add_seals(global_mfd, F_SEAL_WRITE);
187 printf("HURRAY! This kernel fixed GUP races!\n");
189 /* wait 1s more so the FUSE-request is done */
192 /* try sealing the global file again */
193 mfd_assert_add_seals(global_mfd, F_SEAL_WRITE);
199 static pid_t spawn_sealing_thread(void)
204 stack = malloc(STACK_SIZE);
206 printf("malloc(STACK_SIZE) failed: %m\n");
210 pid = clone(sealing_thread_fn,
212 SIGCHLD | CLONE_FILES | CLONE_FS | CLONE_VM,
215 printf("clone() failed: %m\n");
222 static void join_sealing_thread(pid_t pid)
224 waitpid(pid, NULL, 0);
227 int main(int argc, char **argv)
236 printf("error: please pass path to file in fuse_mnt mount-point\n");
241 if (!strcmp(argv[2], "hugetlbfs")) {
242 unsigned long hpage_size = default_huge_page_size();
245 printf("Unable to determine huge page size\n");
250 mfd_def_size = hpage_size * 2;
252 printf("Unknown option: %s\n", argv[2]);
257 zero = calloc(sizeof(*zero), mfd_def_size);
259 /* open FUSE memfd file for GUP testing */
260 printf("opening: %s\n", argv[1]);
261 fd = open(argv[1], O_RDONLY | O_CLOEXEC);
263 printf("cannot open(\"%s\"): %m\n", argv[1]);
267 /* create new memfd-object */
268 mfd = mfd_assert_new("kern_memfd_fuse",
270 MFD_CLOEXEC | MFD_ALLOW_SEALING);
272 /* mmap memfd-object for writing */
273 p = mfd_assert_mmap_shared(mfd);
275 /* pass mfd+mapping to a separate sealing-thread which tries to seal
276 * the memfd objects with SEAL_WRITE while we write into it */
279 pid = spawn_sealing_thread();
281 /* Use read() on the FUSE file to read into our memory-mapped memfd
282 * object. This races the other thread which tries to seal the
284 * If @fd is on the memfd-fake-FUSE-FS, the read() is delayed by 1s.
285 * This guarantees that the receive-buffer is pinned for 1s until the
286 * data is written into it. The racing ADD_SEALS should thus fail as
287 * the pages are still pinned. */
288 r = read(fd, p, mfd_def_size);
290 printf("read() failed: %m\n");
293 printf("unexpected EOF on read()\n");
297 was_sealed = mfd_assert_get_seals(mfd) & F_SEAL_WRITE;
299 /* Wait for sealing-thread to finish and verify that it
300 * successfully sealed the file after the second try. */
301 join_sealing_thread(pid);
302 mfd_assert_has_seals(mfd, F_SEAL_WRITE);
304 /* *IF* the memfd-object was sealed at the time our read() returned,
305 * then the kernel did a page-replacement or canceled the read() (or
306 * whatever magic it did..). In that case, the memfd object is still
308 * In case the memfd-object was *not* sealed, the read() was successfull
309 * and the memfd object must *not* be all zero.
310 * Note that in real scenarios, there might be a mixture of both, but
311 * in this test-cases, we have explicit 200ms delays which should be
312 * enough to avoid any in-flight writes. */
314 p = mfd_assert_mmap_private(mfd);
315 if (was_sealed && memcmp(p, zero, mfd_def_size)) {
316 printf("memfd sealed during read() but data not discarded\n");
318 } else if (!was_sealed && !memcmp(p, zero, mfd_def_size)) {
319 printf("memfd sealed after read() but data discarded\n");
326 printf("fuse: DONE\n");