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Linux/tools/testing/selftests/memfd/fuse_test.c

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  1 /*
  2  * memfd GUP test-case
  3  * This tests memfd interactions with get_user_pages(). We require the
  4  * fuse_mnt.c program to provide a fake direct-IO FUSE mount-point for us. This
  5  * file-system delays _all_ reads by 1s and forces direct-IO. This means, any
  6  * read() on files in that file-system will pin the receive-buffer pages for at
  7  * least 1s via get_user_pages().
  8  *
  9  * We use this trick to race ADD_SEALS against a write on a memfd object. The
 10  * ADD_SEALS must fail if the memfd pages are still pinned. Note that we use
 11  * the read() syscall with our memory-mapped memfd object as receive buffer to
 12  * force the kernel to write into our memfd object.
 13  */
 14 
 15 #define _GNU_SOURCE
 16 #define __EXPORTED_HEADERS__
 17 
 18 #include <errno.h>
 19 #include <inttypes.h>
 20 #include <limits.h>
 21 #include <linux/falloc.h>
 22 #include <linux/fcntl.h>
 23 #include <linux/memfd.h>
 24 #include <sched.h>
 25 #include <stdio.h>
 26 #include <stdlib.h>
 27 #include <signal.h>
 28 #include <string.h>
 29 #include <sys/mman.h>
 30 #include <sys/stat.h>
 31 #include <sys/syscall.h>
 32 #include <sys/wait.h>
 33 #include <unistd.h>
 34 
 35 #define MFD_DEF_SIZE 8192
 36 #define STACK_SIZE 65535
 37 
 38 static int sys_memfd_create(const char *name,
 39                             unsigned int flags)
 40 {
 41         return syscall(__NR_memfd_create, name, flags);
 42 }
 43 
 44 static int mfd_assert_new(const char *name, loff_t sz, unsigned int flags)
 45 {
 46         int r, fd;
 47 
 48         fd = sys_memfd_create(name, flags);
 49         if (fd < 0) {
 50                 printf("memfd_create(\"%s\", %u) failed: %m\n",
 51                        name, flags);
 52                 abort();
 53         }
 54 
 55         r = ftruncate(fd, sz);
 56         if (r < 0) {
 57                 printf("ftruncate(%llu) failed: %m\n", (unsigned long long)sz);
 58                 abort();
 59         }
 60 
 61         return fd;
 62 }
 63 
 64 static __u64 mfd_assert_get_seals(int fd)
 65 {
 66         long r;
 67 
 68         r = fcntl(fd, F_GET_SEALS);
 69         if (r < 0) {
 70                 printf("GET_SEALS(%d) failed: %m\n", fd);
 71                 abort();
 72         }
 73 
 74         return r;
 75 }
 76 
 77 static void mfd_assert_has_seals(int fd, __u64 seals)
 78 {
 79         __u64 s;
 80 
 81         s = mfd_assert_get_seals(fd);
 82         if (s != seals) {
 83                 printf("%llu != %llu = GET_SEALS(%d)\n",
 84                        (unsigned long long)seals, (unsigned long long)s, fd);
 85                 abort();
 86         }
 87 }
 88 
 89 static void mfd_assert_add_seals(int fd, __u64 seals)
 90 {
 91         long r;
 92         __u64 s;
 93 
 94         s = mfd_assert_get_seals(fd);
 95         r = fcntl(fd, F_ADD_SEALS, seals);
 96         if (r < 0) {
 97                 printf("ADD_SEALS(%d, %llu -> %llu) failed: %m\n",
 98                        fd, (unsigned long long)s, (unsigned long long)seals);
 99                 abort();
100         }
101 }
102 
103 static int mfd_busy_add_seals(int fd, __u64 seals)
104 {
105         long r;
106         __u64 s;
107 
108         r = fcntl(fd, F_GET_SEALS);
109         if (r < 0)
110                 s = 0;
111         else
112                 s = r;
113 
114         r = fcntl(fd, F_ADD_SEALS, seals);
115         if (r < 0 && errno != EBUSY) {
116                 printf("ADD_SEALS(%d, %llu -> %llu) didn't fail as expected with EBUSY: %m\n",
117                        fd, (unsigned long long)s, (unsigned long long)seals);
118                 abort();
119         }
120 
121         return r;
122 }
123 
124 static void *mfd_assert_mmap_shared(int fd)
125 {
126         void *p;
127 
128         p = mmap(NULL,
129                  MFD_DEF_SIZE,
130                  PROT_READ | PROT_WRITE,
131                  MAP_SHARED,
132                  fd,
133                  0);
134         if (p == MAP_FAILED) {
135                 printf("mmap() failed: %m\n");
136                 abort();
137         }
138 
139         return p;
140 }
141 
142 static void *mfd_assert_mmap_private(int fd)
143 {
144         void *p;
145 
146         p = mmap(NULL,
147                  MFD_DEF_SIZE,
148                  PROT_READ | PROT_WRITE,
149                  MAP_PRIVATE,
150                  fd,
151                  0);
152         if (p == MAP_FAILED) {
153                 printf("mmap() failed: %m\n");
154                 abort();
155         }
156 
157         return p;
158 }
159 
160 static int global_mfd = -1;
161 static void *global_p = NULL;
162 
163 static int sealing_thread_fn(void *arg)
164 {
165         int sig, r;
166 
167         /*
168          * This thread first waits 200ms so any pending operation in the parent
169          * is correctly started. After that, it tries to seal @global_mfd as
170          * SEAL_WRITE. This _must_ fail as the parent thread has a read() into
171          * that memory mapped object still ongoing.
172          * We then wait one more second and try sealing again. This time it
173          * must succeed as there shouldn't be anyone else pinning the pages.
174          */
175 
176         /* wait 200ms for FUSE-request to be active */
177         usleep(200000);
178 
179         /* unmount mapping before sealing to avoid i_mmap_writable failures */
180         munmap(global_p, MFD_DEF_SIZE);
181 
182         /* Try sealing the global file; expect EBUSY or success. Current
183          * kernels will never succeed, but in the future, kernels might
184          * implement page-replacements or other fancy ways to avoid racing
185          * writes. */
186         r = mfd_busy_add_seals(global_mfd, F_SEAL_WRITE);
187         if (r >= 0) {
188                 printf("HURRAY! This kernel fixed GUP races!\n");
189         } else {
190                 /* wait 1s more so the FUSE-request is done */
191                 sleep(1);
192 
193                 /* try sealing the global file again */
194                 mfd_assert_add_seals(global_mfd, F_SEAL_WRITE);
195         }
196 
197         return 0;
198 }
199 
200 static pid_t spawn_sealing_thread(void)
201 {
202         uint8_t *stack;
203         pid_t pid;
204 
205         stack = malloc(STACK_SIZE);
206         if (!stack) {
207                 printf("malloc(STACK_SIZE) failed: %m\n");
208                 abort();
209         }
210 
211         pid = clone(sealing_thread_fn,
212                     stack + STACK_SIZE,
213                     SIGCHLD | CLONE_FILES | CLONE_FS | CLONE_VM,
214                     NULL);
215         if (pid < 0) {
216                 printf("clone() failed: %m\n");
217                 abort();
218         }
219 
220         return pid;
221 }
222 
223 static void join_sealing_thread(pid_t pid)
224 {
225         waitpid(pid, NULL, 0);
226 }
227 
228 int main(int argc, char **argv)
229 {
230         static const char zero[MFD_DEF_SIZE];
231         int fd, mfd, r;
232         void *p;
233         int was_sealed;
234         pid_t pid;
235 
236         if (argc < 2) {
237                 printf("error: please pass path to file in fuse_mnt mount-point\n");
238                 abort();
239         }
240 
241         /* open FUSE memfd file for GUP testing */
242         printf("opening: %s\n", argv[1]);
243         fd = open(argv[1], O_RDONLY | O_CLOEXEC);
244         if (fd < 0) {
245                 printf("cannot open(\"%s\"): %m\n", argv[1]);
246                 abort();
247         }
248 
249         /* create new memfd-object */
250         mfd = mfd_assert_new("kern_memfd_fuse",
251                              MFD_DEF_SIZE,
252                              MFD_CLOEXEC | MFD_ALLOW_SEALING);
253 
254         /* mmap memfd-object for writing */
255         p = mfd_assert_mmap_shared(mfd);
256 
257         /* pass mfd+mapping to a separate sealing-thread which tries to seal
258          * the memfd objects with SEAL_WRITE while we write into it */
259         global_mfd = mfd;
260         global_p = p;
261         pid = spawn_sealing_thread();
262 
263         /* Use read() on the FUSE file to read into our memory-mapped memfd
264          * object. This races the other thread which tries to seal the
265          * memfd-object.
266          * If @fd is on the memfd-fake-FUSE-FS, the read() is delayed by 1s.
267          * This guarantees that the receive-buffer is pinned for 1s until the
268          * data is written into it. The racing ADD_SEALS should thus fail as
269          * the pages are still pinned. */
270         r = read(fd, p, MFD_DEF_SIZE);
271         if (r < 0) {
272                 printf("read() failed: %m\n");
273                 abort();
274         } else if (!r) {
275                 printf("unexpected EOF on read()\n");
276                 abort();
277         }
278 
279         was_sealed = mfd_assert_get_seals(mfd) & F_SEAL_WRITE;
280 
281         /* Wait for sealing-thread to finish and verify that it
282          * successfully sealed the file after the second try. */
283         join_sealing_thread(pid);
284         mfd_assert_has_seals(mfd, F_SEAL_WRITE);
285 
286         /* *IF* the memfd-object was sealed at the time our read() returned,
287          * then the kernel did a page-replacement or canceled the read() (or
288          * whatever magic it did..). In that case, the memfd object is still
289          * all zero.
290          * In case the memfd-object was *not* sealed, the read() was successfull
291          * and the memfd object must *not* be all zero.
292          * Note that in real scenarios, there might be a mixture of both, but
293          * in this test-cases, we have explicit 200ms delays which should be
294          * enough to avoid any in-flight writes. */
295 
296         p = mfd_assert_mmap_private(mfd);
297         if (was_sealed && memcmp(p, zero, MFD_DEF_SIZE)) {
298                 printf("memfd sealed during read() but data not discarded\n");
299                 abort();
300         } else if (!was_sealed && !memcmp(p, zero, MFD_DEF_SIZE)) {
301                 printf("memfd sealed after read() but data discarded\n");
302                 abort();
303         }
304 
305         close(mfd);
306         close(fd);
307 
308         printf("fuse: DONE\n");
309 
310         return 0;
311 }
312 

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