Linux/Documentation/dev-tools/kcov.rst

Version: ~ [ linux-6.12-rc7 ] ~ [ linux-6.11.7 ] ~ [ linux-6.10.14 ] ~ [ linux-6.9.12 ] ~ [ linux-6.8.12 ] ~ [ linux-6.7.12 ] ~ [ linux-6.6.60 ] ~ [ linux-6.5.13 ] ~ [ linux-6.4.16 ] ~ [ linux-6.3.13 ] ~ [ linux-6.2.16 ] ~ [ linux-6.1.116 ] ~ [ linux-6.0.19 ] ~ [ linux-5.19.17 ] ~ [ linux-5.18.19 ] ~ [ linux-5.17.15 ] ~ [ linux-5.16.20 ] ~ [ linux-5.15.171 ] ~ [ linux-5.14.21 ] ~ [ linux-5.13.19 ] ~ [ linux-5.12.19 ] ~ [ linux-5.11.22 ] ~ [ linux-5.10.229 ] ~ [ linux-5.9.16 ] ~ [ linux-5.8.18 ] ~ [ linux-5.7.19 ] ~ [ linux-5.6.19 ] ~ [ linux-5.5.19 ] ~ [ linux-5.4.285 ] ~ [ linux-5.3.18 ] ~ [ linux-5.2.21 ] ~ [ linux-5.1.21 ] ~ [ linux-5.0.21 ] ~ [ linux-4.20.17 ] ~ [ linux-4.19.323 ] ~ [ linux-4.18.20 ] ~ [ linux-4.17.19 ] ~ [ linux-4.16.18 ] ~ [ linux-4.15.18 ] ~ [ linux-4.14.336 ] ~ [ linux-4.13.16 ] ~ [ linux-4.12.14 ] ~ [ linux-4.11.12 ] ~ [ linux-4.10.17 ] ~ [ linux-4.9.337 ] ~ [ linux-4.4.302 ] ~ [ linux-3.10.108 ] ~ [ linux-2.6.32.71 ] ~ [ linux-2.6.0 ] ~ [ linux-2.4.37.11 ] ~ [ unix-v6-master ] ~ [ ccs-tools-1.8.12 ] ~ [ policy-sample ] ~
Architecture: ~ [ i386 ] ~ [ alpha ] ~ [ m68k ] ~ [ mips ] ~ [ ppc ] ~ [ sparc ] ~ [ sparc64 ] ~

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1 KCOV: code coverage for fuzzing !! 1 kcov: code coverage for fuzzing 2 =============================== 2 =============================== 3 3 4 KCOV collects and exposes kernel code coverage !! 4 kcov exposes kernel code coverage information in a form suitable for coverage- 5 for coverage-guided fuzzing. Coverage data of !! 5 guided fuzzing (randomized testing). Coverage data of a running kernel is 6 the ``kcov`` debugfs file. Coverage collection !! 6 exported via the "kcov" debugfs file. Coverage collection is enabled on a task 7 thus KCOV can capture precise coverage of a si !! 7 basis, and thus it can capture precise coverage of a single system call. 8 !! 8 9 Note that KCOV does not aim to collect as much !! 9 Note that kcov does not aim to collect as much coverage as possible. It aims 10 to collect more or less stable coverage that i !! 10 to collect more or less stable coverage that is function of syscall inputs. 11 To achieve this goal, it does not collect cove !! 11 To achieve this goal it does not collect coverage in soft/hard interrupts 12 (unless remove coverage collection is enabled, !! 12 and instrumentation of some inherently non-deterministic parts of kernel is 13 inherently non-deterministic parts of the kern !! 13 disabled (e.g. scheduler, locking). 14 << 15 Besides collecting code coverage, KCOV can als << 16 See the "Comparison operands collection" secti << 17 << 18 Besides collecting coverage data from syscall << 19 coverage for annotated parts of the kernel exe << 20 tasks or soft interrupts. See the "Remote cove << 21 details. << 22 << 23 Prerequisites << 24 ------------- << 25 14 26 KCOV relies on compiler instrumentation and re !! 15 Usage 27 or any Clang version supported by the kernel. !! 16 ----- 28 17 29 Collecting comparison operands is supported wi !! 18 Configure the kernel with:: 30 << 31 To enable KCOV, configure the kernel with:: << 32 19 33 CONFIG_KCOV=y 20 CONFIG_KCOV=y 34 21 35 To enable comparison operands collection, set: !! 22 CONFIG_KCOV requires gcc built on revision 231296 or later. 36 !! 23 Profiling data will only become accessible once debugfs has been mounted:: 37 CONFIG_KCOV_ENABLE_COMPARISONS=y << 38 << 39 Coverage data only becomes accessible once deb << 40 24 41 mount -t debugfs none /sys/kernel/debu 25 mount -t debugfs none /sys/kernel/debug 42 26 43 Coverage collection !! 27 The following program demonstrates kcov usage from within a test program: 44 ------------------- << 45 << 46 The following program demonstrates how to use << 47 single syscall from within a test program: << 48 28 49 .. code-block:: c 29 .. code-block:: c 50 30 51 #include <stdio.h> 31 #include <stdio.h> 52 #include <stddef.h> 32 #include <stddef.h> 53 #include <stdint.h> 33 #include <stdint.h> 54 #include <stdlib.h> 34 #include <stdlib.h> 55 #include <sys/types.h> 35 #include <sys/types.h> 56 #include <sys/stat.h> 36 #include <sys/stat.h> 57 #include <sys/ioctl.h> 37 #include <sys/ioctl.h> 58 #include <sys/mman.h> 38 #include <sys/mman.h> 59 #include <unistd.h> 39 #include <unistd.h> 60 #include <fcntl.h> 40 #include <fcntl.h> 61 #include <linux/types.h> << 62 41 63 #define KCOV_INIT_TRACE 42 #define KCOV_INIT_TRACE _IOR('c', 1, unsigned long) 64 #define KCOV_ENABLE _IO('c 43 #define KCOV_ENABLE _IO('c', 100) 65 #define KCOV_DISABLE 44 #define KCOV_DISABLE _IO('c', 101) 66 #define COVER_SIZE (64<<1 45 #define COVER_SIZE (64<<10) 67 46 68 #define KCOV_TRACE_PC 0 << 69 #define KCOV_TRACE_CMP 1 << 70 << 71 int main(int argc, char **argv) 47 int main(int argc, char **argv) 72 { 48 { 73 int fd; 49 int fd; 74 unsigned long *cover, n, i; 50 unsigned long *cover, n, i; 75 51 76 /* A single fd descriptor allows cover 52 /* A single fd descriptor allows coverage collection on a single 77 * thread. 53 * thread. 78 */ 54 */ 79 fd = open("/sys/kernel/debug/kcov", O_ 55 fd = open("/sys/kernel/debug/kcov", O_RDWR); 80 if (fd == -1) 56 if (fd == -1) 81 perror("open"), exit(1); 57 perror("open"), exit(1); 82 /* Setup trace mode and trace size. */ 58 /* Setup trace mode and trace size. */ 83 if (ioctl(fd, KCOV_INIT_TRACE, COVER_S 59 if (ioctl(fd, KCOV_INIT_TRACE, COVER_SIZE)) 84 perror("ioctl"), exit(1); 60 perror("ioctl"), exit(1); 85 /* Mmap buffer shared between kernel- 61 /* Mmap buffer shared between kernel- and user-space. */ 86 cover = (unsigned long*)mmap(NULL, COV 62 cover = (unsigned long*)mmap(NULL, COVER_SIZE * sizeof(unsigned long), 87 PROT_READ 63 PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0); 88 if ((void*)cover == MAP_FAILED) 64 if ((void*)cover == MAP_FAILED) 89 perror("mmap"), exit(1); 65 perror("mmap"), exit(1); 90 /* Enable coverage collection on the c 66 /* Enable coverage collection on the current thread. */ 91 if (ioctl(fd, KCOV_ENABLE, KCOV_TRACE_ !! 67 if (ioctl(fd, KCOV_ENABLE, 0)) 92 perror("ioctl"), exit(1); 68 perror("ioctl"), exit(1); 93 /* Reset coverage from the tail of the 69 /* Reset coverage from the tail of the ioctl() call. */ 94 __atomic_store_n(&cover[0], 0, __ATOMI 70 __atomic_store_n(&cover[0], 0, __ATOMIC_RELAXED); 95 /* Call the target syscall call. */ !! 71 /* That's the target syscal call. */ 96 read(-1, NULL, 0); 72 read(-1, NULL, 0); 97 /* Read number of PCs collected. */ 73 /* Read number of PCs collected. */ 98 n = __atomic_load_n(&cover[0], __ATOMI 74 n = __atomic_load_n(&cover[0], __ATOMIC_RELAXED); 99 for (i = 0; i < n; i++) 75 for (i = 0; i < n; i++) 100 printf("0x%lx\n", cover[i + 1] 76 printf("0x%lx\n", cover[i + 1]); 101 /* Disable coverage collection for the 77 /* Disable coverage collection for the current thread. After this call 102 * coverage can be enabled for a diffe 78 * coverage can be enabled for a different thread. 103 */ 79 */ 104 if (ioctl(fd, KCOV_DISABLE, 0)) 80 if (ioctl(fd, KCOV_DISABLE, 0)) 105 perror("ioctl"), exit(1); 81 perror("ioctl"), exit(1); 106 /* Free resources. */ 82 /* Free resources. */ 107 if (munmap(cover, COVER_SIZE * sizeof( 83 if (munmap(cover, COVER_SIZE * sizeof(unsigned long))) 108 perror("munmap"), exit(1); 84 perror("munmap"), exit(1); 109 if (close(fd)) 85 if (close(fd)) 110 perror("close"), exit(1); 86 perror("close"), exit(1); 111 return 0; 87 return 0; 112 } 88 } 113 89 114 After piping through ``addr2line`` the output !! 90 After piping through addr2line output of the program looks as follows:: 115 91 116 SyS_read 92 SyS_read 117 fs/read_write.c:562 93 fs/read_write.c:562 118 __fdget_pos 94 __fdget_pos 119 fs/file.c:774 95 fs/file.c:774 120 __fget_light 96 __fget_light 121 fs/file.c:746 97 fs/file.c:746 122 __fget_light 98 __fget_light 123 fs/file.c:750 99 fs/file.c:750 124 __fget_light 100 __fget_light 125 fs/file.c:760 101 fs/file.c:760 126 __fdget_pos 102 __fdget_pos 127 fs/file.c:784 103 fs/file.c:784 128 SyS_read 104 SyS_read 129 fs/read_write.c:562 105 fs/read_write.c:562 130 106 131 If a program needs to collect coverage from se 107 If a program needs to collect coverage from several threads (independently), 132 it needs to open ``/sys/kernel/debug/kcov`` in !! 108 it needs to open /sys/kernel/debug/kcov in each thread separately. 133 109 134 The interface is fine-grained to allow efficie 110 The interface is fine-grained to allow efficient forking of test processes. 135 That is, a parent process opens ``/sys/kernel/ !! 111 That is, a parent process opens /sys/kernel/debug/kcov, enables trace mode, 136 mmaps coverage buffer, and then forks child pr !! 112 mmaps coverage buffer and then forks child processes in a loop. Child processes 137 processes only need to enable coverage (it get !! 113 only need to enable coverage (disable happens automatically on thread end). 138 a thread exits). << 139 << 140 Comparison operands collection << 141 ------------------------------ << 142 << 143 Comparison operands collection is similar to c << 144 << 145 .. code-block:: c << 146 << 147 /* Same includes and defines as above. */ << 148 << 149 /* Number of 64-bit words per record. */ << 150 #define KCOV_WORDS_PER_CMP 4 << 151 << 152 /* << 153 * The format for the types of collected c << 154 * << 155 * Bit 0 shows whether one of the argument << 156 * Bits 1 & 2 contain log2 of the argument << 157 */ << 158 << 159 #define KCOV_CMP_CONST (1 << 0) << 160 #define KCOV_CMP_SIZE(n) ((n) << 1) << 161 #define KCOV_CMP_MASK KCOV_CMP_S << 162 << 163 int main(int argc, char **argv) << 164 { << 165 int fd; << 166 uint64_t *cover, type, arg1, arg2, is_ << 167 unsigned long n, i; << 168 << 169 fd = open("/sys/kernel/debug/kcov", O_ << 170 if (fd == -1) << 171 perror("open"), exit(1); << 172 if (ioctl(fd, KCOV_INIT_TRACE, COVER_S << 173 perror("ioctl"), exit(1); << 174 /* << 175 * Note that the buffer pointer is of t << 176 * the comparison operands are promoted << 177 */ << 178 cover = (uint64_t *)mmap(NULL, COVER_S << 179 PROT_READ << 180 if ((void*)cover == MAP_FAILED) << 181 perror("mmap"), exit(1); << 182 /* Note KCOV_TRACE_CMP instead of KCOV << 183 if (ioctl(fd, KCOV_ENABLE, KCOV_TRACE_ << 184 perror("ioctl"), exit(1); << 185 __atomic_store_n(&cover[0], 0, __ATOMI << 186 read(-1, NULL, 0); << 187 /* Read number of comparisons collecte << 188 n = __atomic_load_n(&cover[0], __ATOMI << 189 for (i = 0; i < n; i++) { << 190 uint64_t ip; << 191 << 192 type = cover[i * KCOV_WORDS_PE << 193 /* arg1 and arg2 - operands of << 194 arg1 = cover[i * KCOV_WORDS_PE << 195 arg2 = cover[i * KCOV_WORDS_PE << 196 /* ip - caller address. */ << 197 ip = cover[i * KCOV_WORDS_PER_ << 198 /* size of the operands. */ << 199 size = 1 << ((type & KCOV_CMP_ << 200 /* is_const - true if either o << 201 is_const = type & KCOV_CMP_CON << 202 printf("ip: 0x%lx type: 0x%lx, << 203 "size: %lu, %s\n", << 204 ip, type, arg1, arg2, << 205 is_const ? "const" : "non-cons << 206 } << 207 if (ioctl(fd, KCOV_DISABLE, 0)) << 208 perror("ioctl"), exit(1); << 209 /* Free resources. */ << 210 if (munmap(cover, COVER_SIZE * sizeof( << 211 perror("munmap"), exit(1); << 212 if (close(fd)) << 213 perror("close"), exit(1); << 214 return 0; << 215 } << 216 << 217 Note that the KCOV modes (collection of code c << 218 are mutually exclusive. << 219 << 220 Remote coverage collection << 221 -------------------------- << 222 << 223 Besides collecting coverage data from handlers << 224 userspace process, KCOV can also collect cover << 225 executing in other contexts - so-called "remot << 226 << 227 Using KCOV to collect remote coverage requires << 228 << 229 1. Modifying kernel code to annotate the code << 230 should be collected with ``kcov_remote_star << 231 << 232 2. Using ``KCOV_REMOTE_ENABLE`` instead of ``K << 233 process that collects coverage. << 234 << 235 Both ``kcov_remote_start`` and ``kcov_remote_s << 236 ``KCOV_REMOTE_ENABLE`` ioctl accept handles th << 237 collection sections. The way a handle is used << 238 matching code section executes. << 239 << 240 KCOV supports collecting remote coverage from << 241 << 242 1. Global kernel background tasks. These are t << 243 kernel boot in a limited number of instance << 244 worker is spawned per one USB HCD). << 245 << 246 2. Local kernel background tasks. These are sp << 247 interacts with some kernel interface and ar << 248 exits (e.g. vhost workers). << 249 << 250 3. Soft interrupts. << 251 << 252 For #1 and #3, a unique global handle must be << 253 corresponding ``kcov_remote_start`` call. Then << 254 this handle to ``KCOV_REMOTE_ENABLE`` in the ` << 255 ``kcov_remote_arg`` struct. This will attach t << 256 section referenced by this handle. Multiple gl << 257 different code sections can be passed at once. << 258 << 259 For #2, the userspace process instead must pas << 260 ``common_handle`` field of the ``kcov_remote_a << 261 gets saved to the ``kcov_handle`` field in the << 262 needs to be passed to the newly spawned local << 263 modifications. Those tasks should in turn use << 264 ``kcov_remote_start`` and ``kcov_remote_stop`` << 265 << 266 KCOV follows a predefined format for both glob << 267 handle is a ``u64`` integer. Currently, only t << 268 are used. Bytes 4-7 are reserved and must be z << 269 << 270 For global handles, the top byte of the handle << 271 this handle belongs to. For example, KCOV uses << 272 The lower 4 bytes of a global handle denote th << 273 that subsystem. For example, each ``hub_event` << 274 as the task instance id. << 275 << 276 For common handles, a reserved value ``0`` is << 277 handles don't belong to a particular subsystem << 278 handle identify a collective instance of all l << 279 userspace process that passed a common handle << 280 << 281 In practice, any value can be used for common << 282 is only collected from a single userspace proc << 283 common handles are used by multiple processes, << 284 used for each process. One option is to use th << 285 handle instance id. << 286 << 287 The following program demonstrates using KCOV << 288 local tasks spawned by the process and the glo << 289 << 290 .. code-block:: c << 291 << 292 /* Same includes and defines as above. */ << 293 << 294 struct kcov_remote_arg { << 295 __u32 trace_mode; << 296 __u32 area_size; << 297 __u32 num_handles; << 298 __aligned_u64 common_handle; << 299 __aligned_u64 handles[0]; << 300 }; << 301 << 302 #define KCOV_INIT_TRACE << 303 #define KCOV_DISABLE << 304 #define KCOV_REMOTE_ENABLE _IOW(' << 305 << 306 #define COVER_SIZE (64 << 10) << 307 << 308 #define KCOV_TRACE_PC 0 << 309 << 310 #define KCOV_SUBSYSTEM_COMMON (0x00u << 311 #define KCOV_SUBSYSTEM_USB (0x01ull << 56 << 312 << 313 #define KCOV_SUBSYSTEM_MASK (0xffull << 56 << 314 #define KCOV_INSTANCE_MASK (0xffffffffull << 315 << 316 static inline __u64 kcov_remote_handle(__u << 317 { << 318 if (subsys & ~KCOV_SUBSYSTEM_MASK || i << 319 return 0; << 320 return subsys | inst; << 321 } << 322 << 323 #define KCOV_COMMON_ID 0x42 << 324 #define KCOV_USB_BUS_NUM 1 << 325 << 326 int main(int argc, char **argv) << 327 { << 328 int fd; << 329 unsigned long *cover, n, i; << 330 struct kcov_remote_arg *arg; << 331 << 332 fd = open("/sys/kernel/debug/kcov", O_ << 333 if (fd == -1) << 334 perror("open"), exit(1); << 335 if (ioctl(fd, KCOV_INIT_TRACE, COVER_S << 336 perror("ioctl"), exit(1); << 337 cover = (unsigned long*)mmap(NULL, COV << 338 PROT_READ << 339 if ((void*)cover == MAP_FAILED) << 340 perror("mmap"), exit(1); << 341 << 342 /* Enable coverage collection via comm << 343 arg = calloc(1, sizeof(*arg) + sizeof( << 344 if (!arg) << 345 perror("calloc"), exit(1); << 346 arg->trace_mode = KCOV_TRACE_PC; << 347 arg->area_size = COVER_SIZE; << 348 arg->num_handles = 1; << 349 arg->common_handle = kcov_remote_handl << 350 << 351 arg->handles[0] = kcov_remote_handle(K << 352 << 353 if (ioctl(fd, KCOV_REMOTE_ENABLE, arg) << 354 perror("ioctl"), free(arg), ex << 355 free(arg); << 356 << 357 /* << 358 * Here the user needs to trigger exec << 359 * that is either annotated with the c << 360 * activity on USB bus #1. << 361 */ << 362 sleep(2); << 363 << 364 n = __atomic_load_n(&cover[0], __ATOMI << 365 for (i = 0; i < n; i++) << 366 printf("0x%lx\n", cover[i + 1] << 367 if (ioctl(fd, KCOV_DISABLE, 0)) << 368 perror("ioctl"), exit(1); << 369 if (munmap(cover, COVER_SIZE * sizeof( << 370 perror("munmap"), exit(1); << 371 if (close(fd)) << 372 perror("close"), exit(1); << 373 return 0; << 374 } <<

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