1kcov: code coverage for fuzzing 2=============================== 3 4kcov exposes kernel code coverage information in a form suitable for coverage- 5guided fuzzing (randomized testing). Coverage data of a running kernel is 6exported via the "kcov" debugfs file. Coverage collection is enabled on a task 7basis, and thus it 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 function of syscall inputs. 11To achieve this goal it does not collect coverage in soft/hard interrupts 12and instrumentation of some inherently non-deterministic parts of kernel is 13disabled (e.g. scheduler, locking). 14 15kcov is also able to collect comparison operands from the instrumented code 16(this feature currently requires that the kernel is compiled with clang). 17 18Prerequisites 19------------- 20 21Configure the kernel with:: 22 23 CONFIG_KCOV=y 24 25CONFIG_KCOV requires gcc 6.1.0 or later. 26 27If the comparison operands need to be collected, set:: 28 29 CONFIG_KCOV_ENABLE_COMPARISONS=y 30 31Profiling data will only become accessible once debugfs has been mounted:: 32 33 mount -t debugfs none /sys/kernel/debug 34 35Coverage collection 36------------------- 37 38The following program demonstrates coverage collection from within a test 39program using kcov: 40 41.. code-block:: c 42 43 #include <stdio.h> 44 #include <stddef.h> 45 #include <stdint.h> 46 #include <stdlib.h> 47 #include <sys/types.h> 48 #include <sys/stat.h> 49 #include <sys/ioctl.h> 50 #include <sys/mman.h> 51 #include <unistd.h> 52 #include <fcntl.h> 53 54 #define KCOV_INIT_TRACE _IOR('c', 1, unsigned long) 55 #define KCOV_ENABLE _IO('c', 100) 56 #define KCOV_DISABLE _IO('c', 101) 57 #define COVER_SIZE (64<<10) 58 59 #define KCOV_TRACE_PC 0 60 #define KCOV_TRACE_CMP 1 61 62 int main(int argc, char **argv) 63 { 64 int fd; 65 unsigned long *cover, n, i; 66 67 /* A single fd descriptor allows coverage collection on a single 68 * thread. 69 */ 70 fd = open("/sys/kernel/debug/kcov", O_RDWR); 71 if (fd == -1) 72 perror("open"), exit(1); 73 /* Setup trace mode and trace size. */ 74 if (ioctl(fd, KCOV_INIT_TRACE, COVER_SIZE)) 75 perror("ioctl"), exit(1); 76 /* Mmap buffer shared between kernel- and user-space. */ 77 cover = (unsigned long*)mmap(NULL, COVER_SIZE * sizeof(unsigned long), 78 PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0); 79 if ((void*)cover == MAP_FAILED) 80 perror("mmap"), exit(1); 81 /* Enable coverage collection on the current thread. */ 82 if (ioctl(fd, KCOV_ENABLE, KCOV_TRACE_PC)) 83 perror("ioctl"), exit(1); 84 /* Reset coverage from the tail of the ioctl() call. */ 85 __atomic_store_n(&cover[0], 0, __ATOMIC_RELAXED); 86 /* That's the target syscal call. */ 87 read(-1, NULL, 0); 88 /* Read number of PCs collected. */ 89 n = __atomic_load_n(&cover[0], __ATOMIC_RELAXED); 90 for (i = 0; i < n; i++) 91 printf("0x%lx\n", cover[i + 1]); 92 /* Disable coverage collection for the current thread. After this call 93 * coverage can be enabled for a different thread. 94 */ 95 if (ioctl(fd, KCOV_DISABLE, 0)) 96 perror("ioctl"), exit(1); 97 /* Free resources. */ 98 if (munmap(cover, COVER_SIZE * sizeof(unsigned long))) 99 perror("munmap"), exit(1); 100 if (close(fd)) 101 perror("close"), exit(1); 102 return 0; 103 } 104 105After piping through addr2line output of the program looks as follows:: 106 107 SyS_read 108 fs/read_write.c:562 109 __fdget_pos 110 fs/file.c:774 111 __fget_light 112 fs/file.c:746 113 __fget_light 114 fs/file.c:750 115 __fget_light 116 fs/file.c:760 117 __fdget_pos 118 fs/file.c:784 119 SyS_read 120 fs/read_write.c:562 121 122If a program needs to collect coverage from several threads (independently), 123it needs to open /sys/kernel/debug/kcov in each thread separately. 124 125The interface is fine-grained to allow efficient forking of test processes. 126That is, a parent process opens /sys/kernel/debug/kcov, enables trace mode, 127mmaps coverage buffer and then forks child processes in a loop. Child processes 128only need to enable coverage (disable happens automatically on thread end). 129 130Comparison operands collection 131------------------------------ 132 133Comparison operands collection is similar to coverage collection: 134 135.. code-block:: c 136 137 /* Same includes and defines as above. */ 138 139 /* Number of 64-bit words per record. */ 140 #define KCOV_WORDS_PER_CMP 4 141 142 /* 143 * The format for the types of collected comparisons. 144 * 145 * Bit 0 shows whether one of the arguments is a compile-time constant. 146 * Bits 1 & 2 contain log2 of the argument size, up to 8 bytes. 147 */ 148 149 #define KCOV_CMP_CONST (1 << 0) 150 #define KCOV_CMP_SIZE(n) ((n) << 1) 151 #define KCOV_CMP_MASK KCOV_CMP_SIZE(3) 152 153 int main(int argc, char **argv) 154 { 155 int fd; 156 uint64_t *cover, type, arg1, arg2, is_const, size; 157 unsigned long n, i; 158 159 fd = open("/sys/kernel/debug/kcov", O_RDWR); 160 if (fd == -1) 161 perror("open"), exit(1); 162 if (ioctl(fd, KCOV_INIT_TRACE, COVER_SIZE)) 163 perror("ioctl"), exit(1); 164 /* 165 * Note that the buffer pointer is of type uint64_t*, because all 166 * the comparison operands are promoted to uint64_t. 167 */ 168 cover = (uint64_t *)mmap(NULL, COVER_SIZE * sizeof(unsigned long), 169 PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0); 170 if ((void*)cover == MAP_FAILED) 171 perror("mmap"), exit(1); 172 /* Note KCOV_TRACE_CMP instead of KCOV_TRACE_PC. */ 173 if (ioctl(fd, KCOV_ENABLE, KCOV_TRACE_CMP)) 174 perror("ioctl"), exit(1); 175 __atomic_store_n(&cover[0], 0, __ATOMIC_RELAXED); 176 read(-1, NULL, 0); 177 /* Read number of comparisons collected. */ 178 n = __atomic_load_n(&cover[0], __ATOMIC_RELAXED); 179 for (i = 0; i < n; i++) { 180 type = cover[i * KCOV_WORDS_PER_CMP + 1]; 181 /* arg1 and arg2 - operands of the comparison. */ 182 arg1 = cover[i * KCOV_WORDS_PER_CMP + 2]; 183 arg2 = cover[i * KCOV_WORDS_PER_CMP + 3]; 184 /* ip - caller address. */ 185 ip = cover[i * KCOV_WORDS_PER_CMP + 4]; 186 /* size of the operands. */ 187 size = 1 << ((type & KCOV_CMP_MASK) >> 1); 188 /* is_const - true if either operand is a compile-time constant.*/ 189 is_const = type & KCOV_CMP_CONST; 190 printf("ip: 0x%lx type: 0x%lx, arg1: 0x%lx, arg2: 0x%lx, " 191 "size: %lu, %s\n", 192 ip, type, arg1, arg2, size, 193 is_const ? "const" : "non-const"); 194 } 195 if (ioctl(fd, KCOV_DISABLE, 0)) 196 perror("ioctl"), exit(1); 197 /* Free resources. */ 198 if (munmap(cover, COVER_SIZE * sizeof(unsigned long))) 199 perror("munmap"), exit(1); 200 if (close(fd)) 201 perror("close"), exit(1); 202 return 0; 203 } 204 205Note that the kcov modes (coverage collection or comparison operands) are 206mutually exclusive. 207 208Remote coverage collection 209-------------------------- 210 211With KCOV_ENABLE coverage is collected only for syscalls that are issued 212from the current process. With KCOV_REMOTE_ENABLE it's possible to collect 213coverage for arbitrary parts of the kernel code, provided that those parts 214are annotated with kcov_remote_start()/kcov_remote_stop(). 215 216This allows to collect coverage from two types of kernel background 217threads: the global ones, that are spawned during kernel boot in a limited 218number of instances (e.g. one USB hub_event() worker thread is spawned per 219USB HCD); and the local ones, that are spawned when a user interacts with 220some kernel interface (e.g. vhost workers). 221 222To enable collecting coverage from a global background thread, a unique 223global handle must be assigned and passed to the corresponding 224kcov_remote_start() call. Then a userspace process can pass a list of such 225handles to the KCOV_REMOTE_ENABLE ioctl in the handles array field of the 226kcov_remote_arg struct. This will attach the used kcov device to the code 227sections, that are referenced by those handles. 228 229Since there might be many local background threads spawned from different 230userspace processes, we can't use a single global handle per annotation. 231Instead, the userspace process passes a non-zero handle through the 232common_handle field of the kcov_remote_arg struct. This common handle gets 233saved to the kcov_handle field in the current task_struct and needs to be 234passed to the newly spawned threads via custom annotations. Those threads 235should in turn be annotated with kcov_remote_start()/kcov_remote_stop(). 236 237Internally kcov stores handles as u64 integers. The top byte of a handle 238is used to denote the id of a subsystem that this handle belongs to, and 239the lower 4 bytes are used to denote the id of a thread instance within 240that subsystem. A reserved value 0 is used as a subsystem id for common 241handles as they don't belong to a particular subsystem. The bytes 4-7 are 242currently reserved and must be zero. In the future the number of bytes 243used for the subsystem or handle ids might be increased. 244 245When a particular userspace proccess collects coverage by via a common 246handle, kcov will collect coverage for each code section that is annotated 247to use the common handle obtained as kcov_handle from the current 248task_struct. However non common handles allow to collect coverage 249selectively from different subsystems. 250 251.. code-block:: c 252 253 struct kcov_remote_arg { 254 __u32 trace_mode; 255 __u32 area_size; 256 __u32 num_handles; 257 __aligned_u64 common_handle; 258 __aligned_u64 handles[0]; 259 }; 260 261 #define KCOV_INIT_TRACE _IOR('c', 1, unsigned long) 262 #define KCOV_DISABLE _IO('c', 101) 263 #define KCOV_REMOTE_ENABLE _IOW('c', 102, struct kcov_remote_arg) 264 265 #define COVER_SIZE (64 << 10) 266 267 #define KCOV_TRACE_PC 0 268 269 #define KCOV_SUBSYSTEM_COMMON (0x00ull << 56) 270 #define KCOV_SUBSYSTEM_USB (0x01ull << 56) 271 272 #define KCOV_SUBSYSTEM_MASK (0xffull << 56) 273 #define KCOV_INSTANCE_MASK (0xffffffffull) 274 275 static inline __u64 kcov_remote_handle(__u64 subsys, __u64 inst) 276 { 277 if (subsys & ~KCOV_SUBSYSTEM_MASK || inst & ~KCOV_INSTANCE_MASK) 278 return 0; 279 return subsys | inst; 280 } 281 282 #define KCOV_COMMON_ID 0x42 283 #define KCOV_USB_BUS_NUM 1 284 285 int main(int argc, char **argv) 286 { 287 int fd; 288 unsigned long *cover, n, i; 289 struct kcov_remote_arg *arg; 290 291 fd = open("/sys/kernel/debug/kcov", O_RDWR); 292 if (fd == -1) 293 perror("open"), exit(1); 294 if (ioctl(fd, KCOV_INIT_TRACE, COVER_SIZE)) 295 perror("ioctl"), exit(1); 296 cover = (unsigned long*)mmap(NULL, COVER_SIZE * sizeof(unsigned long), 297 PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0); 298 if ((void*)cover == MAP_FAILED) 299 perror("mmap"), exit(1); 300 301 /* Enable coverage collection via common handle and from USB bus #1. */ 302 arg = calloc(1, sizeof(*arg) + sizeof(uint64_t)); 303 if (!arg) 304 perror("calloc"), exit(1); 305 arg->trace_mode = KCOV_TRACE_PC; 306 arg->area_size = COVER_SIZE; 307 arg->num_handles = 1; 308 arg->common_handle = kcov_remote_handle(KCOV_SUBSYSTEM_COMMON, 309 KCOV_COMMON_ID); 310 arg->handles[0] = kcov_remote_handle(KCOV_SUBSYSTEM_USB, 311 KCOV_USB_BUS_NUM); 312 if (ioctl(fd, KCOV_REMOTE_ENABLE, arg)) 313 perror("ioctl"), free(arg), exit(1); 314 free(arg); 315 316 /* 317 * Here the user needs to trigger execution of a kernel code section 318 * that is either annotated with the common handle, or to trigger some 319 * activity on USB bus #1. 320 */ 321 sleep(2); 322 323 n = __atomic_load_n(&cover[0], __ATOMIC_RELAXED); 324 for (i = 0; i < n; i++) 325 printf("0x%lx\n", cover[i + 1]); 326 if (ioctl(fd, KCOV_DISABLE, 0)) 327 perror("ioctl"), exit(1); 328 if (munmap(cover, COVER_SIZE * sizeof(unsigned long))) 329 perror("munmap"), exit(1); 330 if (close(fd)) 331 perror("close"), exit(1); 332 return 0; 333 } 334