1KCOV: code coverage for fuzzing 2=============================== 3 4KCOV collects and exposes kernel code coverage information in a form suitable 5for coverage-guided fuzzing. Coverage data of a running kernel is exported via 6the ``kcov`` debugfs file. Coverage collection is enabled on a task basis, and 7thus KCOV can capture precise coverage of a single system call. 8 9Note that KCOV does not aim to collect as much coverage as possible. It aims 10to collect more or less stable coverage that is a function of syscall inputs. 11To achieve this goal, it does not collect coverage in soft/hard interrupts 12(unless remove coverage collection is enabled, see below) and from some 13inherently non-deterministic parts of the kernel (e.g. scheduler, locking). 14 15Besides collecting code coverage, KCOV can also collect comparison operands. 16See the "Comparison operands collection" section for details. 17 18Besides collecting coverage data from syscall handlers, KCOV can also collect 19coverage for annotated parts of the kernel executing in background kernel 20tasks or soft interrupts. See the "Remote coverage collection" section for 21details. 22 23Prerequisites 24------------- 25 26KCOV relies on compiler instrumentation and requires GCC 6.1.0 or later 27or any Clang version supported by the kernel. 28 29Collecting comparison operands is supported with GCC 8+ or with Clang. 30 31To enable KCOV, configure the kernel with:: 32 33 CONFIG_KCOV=y 34 35To enable comparison operands collection, set:: 36 37 CONFIG_KCOV_ENABLE_COMPARISONS=y 38 39Coverage data only becomes accessible once debugfs has been mounted:: 40 41 mount -t debugfs none /sys/kernel/debug 42 43Coverage collection 44------------------- 45 46The following program demonstrates how to use KCOV to collect coverage for a 47single syscall from within a test program: 48 49.. code-block:: c 50 51 #include <stdio.h> 52 #include <stddef.h> 53 #include <stdint.h> 54 #include <stdlib.h> 55 #include <sys/types.h> 56 #include <sys/stat.h> 57 #include <sys/ioctl.h> 58 #include <sys/mman.h> 59 #include <unistd.h> 60 #include <fcntl.h> 61 #include <linux/types.h> 62 63 #define KCOV_INIT_TRACE _IOR('c', 1, unsigned long) 64 #define KCOV_ENABLE _IO('c', 100) 65 #define KCOV_DISABLE _IO('c', 101) 66 #define COVER_SIZE (64<<10) 67 68 #define KCOV_TRACE_PC 0 69 #define KCOV_TRACE_CMP 1 70 71 int main(int argc, char **argv) 72 { 73 int fd; 74 unsigned long *cover, n, i; 75 76 /* A single fd descriptor allows coverage collection on a single 77 * thread. 78 */ 79 fd = open("/sys/kernel/debug/kcov", O_RDWR); 80 if (fd == -1) 81 perror("open"), exit(1); 82 /* Setup trace mode and trace size. */ 83 if (ioctl(fd, KCOV_INIT_TRACE, COVER_SIZE)) 84 perror("ioctl"), exit(1); 85 /* Mmap buffer shared between kernel- and user-space. */ 86 cover = (unsigned long*)mmap(NULL, COVER_SIZE * sizeof(unsigned long), 87 PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0); 88 if ((void*)cover == MAP_FAILED) 89 perror("mmap"), exit(1); 90 /* Enable coverage collection on the current thread. */ 91 if (ioctl(fd, KCOV_ENABLE, KCOV_TRACE_PC)) 92 perror("ioctl"), exit(1); 93 /* Reset coverage from the tail of the ioctl() call. */ 94 __atomic_store_n(&cover[0], 0, __ATOMIC_RELAXED); 95 /* Call the target syscall call. */ 96 read(-1, NULL, 0); 97 /* Read number of PCs collected. */ 98 n = __atomic_load_n(&cover[0], __ATOMIC_RELAXED); 99 for (i = 0; i < n; i++) 100 printf("0x%lx\n", cover[i + 1]); 101 /* Disable coverage collection for the current thread. After this call 102 * coverage can be enabled for a different thread. 103 */ 104 if (ioctl(fd, KCOV_DISABLE, 0)) 105 perror("ioctl"), exit(1); 106 /* Free resources. */ 107 if (munmap(cover, COVER_SIZE * sizeof(unsigned long))) 108 perror("munmap"), exit(1); 109 if (close(fd)) 110 perror("close"), exit(1); 111 return 0; 112 } 113 114After piping through ``addr2line`` the output of the program looks as follows:: 115 116 SyS_read 117 fs/read_write.c:562 118 __fdget_pos 119 fs/file.c:774 120 __fget_light 121 fs/file.c:746 122 __fget_light 123 fs/file.c:750 124 __fget_light 125 fs/file.c:760 126 __fdget_pos 127 fs/file.c:784 128 SyS_read 129 fs/read_write.c:562 130 131If a program needs to collect coverage from several threads (independently), 132it needs to open ``/sys/kernel/debug/kcov`` in each thread separately. 133 134The interface is fine-grained to allow efficient forking of test processes. 135That is, a parent process opens ``/sys/kernel/debug/kcov``, enables trace mode, 136mmaps coverage buffer, and then forks child processes in a loop. The child 137processes only need to enable coverage (it gets disabled automatically when 138a thread exits). 139 140Comparison operands collection 141------------------------------ 142 143Comparison operands collection is similar to coverage collection: 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 comparisons. 154 * 155 * Bit 0 shows whether one of the arguments is a compile-time constant. 156 * Bits 1 & 2 contain log2 of the argument size, up to 8 bytes. 157 */ 158 159 #define KCOV_CMP_CONST (1 << 0) 160 #define KCOV_CMP_SIZE(n) ((n) << 1) 161 #define KCOV_CMP_MASK KCOV_CMP_SIZE(3) 162 163 int main(int argc, char **argv) 164 { 165 int fd; 166 uint64_t *cover, type, arg1, arg2, is_const, size; 167 unsigned long n, i; 168 169 fd = open("/sys/kernel/debug/kcov", O_RDWR); 170 if (fd == -1) 171 perror("open"), exit(1); 172 if (ioctl(fd, KCOV_INIT_TRACE, COVER_SIZE)) 173 perror("ioctl"), exit(1); 174 /* 175 * Note that the buffer pointer is of type uint64_t*, because all 176 * the comparison operands are promoted to uint64_t. 177 */ 178 cover = (uint64_t *)mmap(NULL, COVER_SIZE * sizeof(unsigned long), 179 PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0); 180 if ((void*)cover == MAP_FAILED) 181 perror("mmap"), exit(1); 182 /* Note KCOV_TRACE_CMP instead of KCOV_TRACE_PC. */ 183 if (ioctl(fd, KCOV_ENABLE, KCOV_TRACE_CMP)) 184 perror("ioctl"), exit(1); 185 __atomic_store_n(&cover[0], 0, __ATOMIC_RELAXED); 186 read(-1, NULL, 0); 187 /* Read number of comparisons collected. */ 188 n = __atomic_load_n(&cover[0], __ATOMIC_RELAXED); 189 for (i = 0; i < n; i++) { 190 uint64_t ip; 191 192 type = cover[i * KCOV_WORDS_PER_CMP + 1]; 193 /* arg1 and arg2 - operands of the comparison. */ 194 arg1 = cover[i * KCOV_WORDS_PER_CMP + 2]; 195 arg2 = cover[i * KCOV_WORDS_PER_CMP + 3]; 196 /* ip - caller address. */ 197 ip = cover[i * KCOV_WORDS_PER_CMP + 4]; 198 /* size of the operands. */ 199 size = 1 << ((type & KCOV_CMP_MASK) >> 1); 200 /* is_const - true if either operand is a compile-time constant.*/ 201 is_const = type & KCOV_CMP_CONST; 202 printf("ip: 0x%lx type: 0x%lx, arg1: 0x%lx, arg2: 0x%lx, " 203 "size: %lu, %s\n", 204 ip, type, arg1, arg2, size, 205 is_const ? "const" : "non-const"); 206 } 207 if (ioctl(fd, KCOV_DISABLE, 0)) 208 perror("ioctl"), exit(1); 209 /* Free resources. */ 210 if (munmap(cover, COVER_SIZE * sizeof(unsigned long))) 211 perror("munmap"), exit(1); 212 if (close(fd)) 213 perror("close"), exit(1); 214 return 0; 215 } 216 217Note that the KCOV modes (collection of code coverage or comparison operands) 218are mutually exclusive. 219 220Remote coverage collection 221-------------------------- 222 223Besides collecting coverage data from handlers of syscalls issued from a 224userspace process, KCOV can also collect coverage for parts of the kernel 225executing in other contexts - so-called "remote" coverage. 226 227Using KCOV to collect remote coverage requires: 228 2291. Modifying kernel code to annotate the code section from where coverage 230 should be collected with ``kcov_remote_start`` and ``kcov_remote_stop``. 231 2322. Using ``KCOV_REMOTE_ENABLE`` instead of ``KCOV_ENABLE`` in the userspace 233 process that collects coverage. 234 235Both ``kcov_remote_start`` and ``kcov_remote_stop`` annotations and the 236``KCOV_REMOTE_ENABLE`` ioctl accept handles that identify particular coverage 237collection sections. The way a handle is used depends on the context where the 238matching code section executes. 239 240KCOV supports collecting remote coverage from the following contexts: 241 2421. Global kernel background tasks. These are the tasks that are spawned during 243 kernel boot in a limited number of instances (e.g. one USB ``hub_event`` 244 worker is spawned per one USB HCD). 245 2462. Local kernel background tasks. These are spawned when a userspace process 247 interacts with some kernel interface and are usually killed when the process 248 exits (e.g. vhost workers). 249 2503. Soft interrupts. 251 252For #1 and #3, a unique global handle must be chosen and passed to the 253corresponding ``kcov_remote_start`` call. Then a userspace process must pass 254this handle to ``KCOV_REMOTE_ENABLE`` in the ``handles`` array field of the 255``kcov_remote_arg`` struct. This will attach the used KCOV device to the code 256section referenced by this handle. Multiple global handles identifying 257different code sections can be passed at once. 258 259For #2, the userspace process instead must pass a non-zero handle through the 260``common_handle`` field of the ``kcov_remote_arg`` struct. This common handle 261gets saved to the ``kcov_handle`` field in the current ``task_struct`` and 262needs to be passed to the newly spawned local tasks via custom kernel code 263modifications. Those tasks should in turn use the passed handle in their 264``kcov_remote_start`` and ``kcov_remote_stop`` annotations. 265 266KCOV follows a predefined format for both global and common handles. Each 267handle is a ``u64`` integer. Currently, only the one top and the lower 4 bytes 268are used. Bytes 4-7 are reserved and must be zero. 269 270For global handles, the top byte of the handle denotes the id of a subsystem 271this handle belongs to. For example, KCOV uses ``1`` as the USB subsystem id. 272The lower 4 bytes of a global handle denote the id of a task instance within 273that subsystem. For example, each ``hub_event`` worker uses the USB bus number 274as the task instance id. 275 276For common handles, a reserved value ``0`` is used as a subsystem id, as such 277handles don't belong to a particular subsystem. The lower 4 bytes of a common 278handle identify a collective instance of all local tasks spawned by the 279userspace process that passed a common handle to ``KCOV_REMOTE_ENABLE``. 280 281In practice, any value can be used for common handle instance id if coverage 282is only collected from a single userspace process on the system. However, if 283common handles are used by multiple processes, unique instance ids must be 284used for each process. One option is to use the process id as the common 285handle instance id. 286 287The following program demonstrates using KCOV to collect coverage from both 288local tasks spawned by the process and the global task that handles USB bus #1: 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 _IOR('c', 1, unsigned long) 303 #define KCOV_DISABLE _IO('c', 101) 304 #define KCOV_REMOTE_ENABLE _IOW('c', 102, struct kcov_remote_arg) 305 306 #define COVER_SIZE (64 << 10) 307 308 #define KCOV_TRACE_PC 0 309 310 #define KCOV_SUBSYSTEM_COMMON (0x00ull << 56) 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(__u64 subsys, __u64 inst) 317 { 318 if (subsys & ~KCOV_SUBSYSTEM_MASK || inst & ~KCOV_INSTANCE_MASK) 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_RDWR); 333 if (fd == -1) 334 perror("open"), exit(1); 335 if (ioctl(fd, KCOV_INIT_TRACE, COVER_SIZE)) 336 perror("ioctl"), exit(1); 337 cover = (unsigned long*)mmap(NULL, COVER_SIZE * sizeof(unsigned long), 338 PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0); 339 if ((void*)cover == MAP_FAILED) 340 perror("mmap"), exit(1); 341 342 /* Enable coverage collection via common handle and from USB bus #1. */ 343 arg = calloc(1, sizeof(*arg) + sizeof(uint64_t)); 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_handle(KCOV_SUBSYSTEM_COMMON, 350 KCOV_COMMON_ID); 351 arg->handles[0] = kcov_remote_handle(KCOV_SUBSYSTEM_USB, 352 KCOV_USB_BUS_NUM); 353 if (ioctl(fd, KCOV_REMOTE_ENABLE, arg)) 354 perror("ioctl"), free(arg), exit(1); 355 free(arg); 356 357 /* 358 * Here the user needs to trigger execution of a kernel code section 359 * that is either annotated with the common handle, or to trigger some 360 * activity on USB bus #1. 361 */ 362 sleep(2); 363 364 n = __atomic_load_n(&cover[0], __ATOMIC_RELAXED); 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(unsigned long))) 370 perror("munmap"), exit(1); 371 if (close(fd)) 372 perror("close"), exit(1); 373 return 0; 374 } 375