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 #include <linux/types.h> 54 55 #define KCOV_INIT_TRACE _IOR('c', 1, unsigned long) 56 #define KCOV_ENABLE _IO('c', 100) 57 #define KCOV_DISABLE _IO('c', 101) 58 #define COVER_SIZE (64<<10) 59 60 #define KCOV_TRACE_PC 0 61 #define KCOV_TRACE_CMP 1 62 63 int main(int argc, char **argv) 64 { 65 int fd; 66 unsigned long *cover, n, i; 67 68 /* A single fd descriptor allows coverage collection on a single 69 * thread. 70 */ 71 fd = open("/sys/kernel/debug/kcov", O_RDWR); 72 if (fd == -1) 73 perror("open"), exit(1); 74 /* Setup trace mode and trace size. */ 75 if (ioctl(fd, KCOV_INIT_TRACE, COVER_SIZE)) 76 perror("ioctl"), exit(1); 77 /* Mmap buffer shared between kernel- and user-space. */ 78 cover = (unsigned long*)mmap(NULL, COVER_SIZE * sizeof(unsigned long), 79 PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0); 80 if ((void*)cover == MAP_FAILED) 81 perror("mmap"), exit(1); 82 /* Enable coverage collection on the current thread. */ 83 if (ioctl(fd, KCOV_ENABLE, KCOV_TRACE_PC)) 84 perror("ioctl"), exit(1); 85 /* Reset coverage from the tail of the ioctl() call. */ 86 __atomic_store_n(&cover[0], 0, __ATOMIC_RELAXED); 87 /* That's the target syscal call. */ 88 read(-1, NULL, 0); 89 /* Read number of PCs collected. */ 90 n = __atomic_load_n(&cover[0], __ATOMIC_RELAXED); 91 for (i = 0; i < n; i++) 92 printf("0x%lx\n", cover[i + 1]); 93 /* Disable coverage collection for the current thread. After this call 94 * coverage can be enabled for a different thread. 95 */ 96 if (ioctl(fd, KCOV_DISABLE, 0)) 97 perror("ioctl"), exit(1); 98 /* Free resources. */ 99 if (munmap(cover, COVER_SIZE * sizeof(unsigned long))) 100 perror("munmap"), exit(1); 101 if (close(fd)) 102 perror("close"), exit(1); 103 return 0; 104 } 105 106After piping through addr2line output of the program looks as follows:: 107 108 SyS_read 109 fs/read_write.c:562 110 __fdget_pos 111 fs/file.c:774 112 __fget_light 113 fs/file.c:746 114 __fget_light 115 fs/file.c:750 116 __fget_light 117 fs/file.c:760 118 __fdget_pos 119 fs/file.c:784 120 SyS_read 121 fs/read_write.c:562 122 123If a program needs to collect coverage from several threads (independently), 124it needs to open /sys/kernel/debug/kcov in each thread separately. 125 126The interface is fine-grained to allow efficient forking of test processes. 127That is, a parent process opens /sys/kernel/debug/kcov, enables trace mode, 128mmaps coverage buffer and then forks child processes in a loop. Child processes 129only need to enable coverage (disable happens automatically on thread end). 130 131Comparison operands collection 132------------------------------ 133 134Comparison operands collection is similar to coverage collection: 135 136.. code-block:: c 137 138 /* Same includes and defines as above. */ 139 140 /* Number of 64-bit words per record. */ 141 #define KCOV_WORDS_PER_CMP 4 142 143 /* 144 * The format for the types of collected comparisons. 145 * 146 * Bit 0 shows whether one of the arguments is a compile-time constant. 147 * Bits 1 & 2 contain log2 of the argument size, up to 8 bytes. 148 */ 149 150 #define KCOV_CMP_CONST (1 << 0) 151 #define KCOV_CMP_SIZE(n) ((n) << 1) 152 #define KCOV_CMP_MASK KCOV_CMP_SIZE(3) 153 154 int main(int argc, char **argv) 155 { 156 int fd; 157 uint64_t *cover, type, arg1, arg2, is_const, size; 158 unsigned long n, i; 159 160 fd = open("/sys/kernel/debug/kcov", O_RDWR); 161 if (fd == -1) 162 perror("open"), exit(1); 163 if (ioctl(fd, KCOV_INIT_TRACE, COVER_SIZE)) 164 perror("ioctl"), exit(1); 165 /* 166 * Note that the buffer pointer is of type uint64_t*, because all 167 * the comparison operands are promoted to uint64_t. 168 */ 169 cover = (uint64_t *)mmap(NULL, COVER_SIZE * sizeof(unsigned long), 170 PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0); 171 if ((void*)cover == MAP_FAILED) 172 perror("mmap"), exit(1); 173 /* Note KCOV_TRACE_CMP instead of KCOV_TRACE_PC. */ 174 if (ioctl(fd, KCOV_ENABLE, KCOV_TRACE_CMP)) 175 perror("ioctl"), exit(1); 176 __atomic_store_n(&cover[0], 0, __ATOMIC_RELAXED); 177 read(-1, NULL, 0); 178 /* Read number of comparisons collected. */ 179 n = __atomic_load_n(&cover[0], __ATOMIC_RELAXED); 180 for (i = 0; i < n; i++) { 181 uint64_t ip; 182 183 type = cover[i * KCOV_WORDS_PER_CMP + 1]; 184 /* arg1 and arg2 - operands of the comparison. */ 185 arg1 = cover[i * KCOV_WORDS_PER_CMP + 2]; 186 arg2 = cover[i * KCOV_WORDS_PER_CMP + 3]; 187 /* ip - caller address. */ 188 ip = cover[i * KCOV_WORDS_PER_CMP + 4]; 189 /* size of the operands. */ 190 size = 1 << ((type & KCOV_CMP_MASK) >> 1); 191 /* is_const - true if either operand is a compile-time constant.*/ 192 is_const = type & KCOV_CMP_CONST; 193 printf("ip: 0x%lx type: 0x%lx, arg1: 0x%lx, arg2: 0x%lx, " 194 "size: %lu, %s\n", 195 ip, type, arg1, arg2, size, 196 is_const ? "const" : "non-const"); 197 } 198 if (ioctl(fd, KCOV_DISABLE, 0)) 199 perror("ioctl"), exit(1); 200 /* Free resources. */ 201 if (munmap(cover, COVER_SIZE * sizeof(unsigned long))) 202 perror("munmap"), exit(1); 203 if (close(fd)) 204 perror("close"), exit(1); 205 return 0; 206 } 207 208Note that the kcov modes (coverage collection or comparison operands) are 209mutually exclusive. 210 211Remote coverage collection 212-------------------------- 213 214With KCOV_ENABLE coverage is collected only for syscalls that are issued 215from the current process. With KCOV_REMOTE_ENABLE it's possible to collect 216coverage for arbitrary parts of the kernel code, provided that those parts 217are annotated with kcov_remote_start()/kcov_remote_stop(). 218 219This allows to collect coverage from two types of kernel background 220threads: the global ones, that are spawned during kernel boot in a limited 221number of instances (e.g. one USB hub_event() worker thread is spawned per 222USB HCD); and the local ones, that are spawned when a user interacts with 223some kernel interface (e.g. vhost workers); as well as from soft 224interrupts. 225 226To enable collecting coverage from a global background thread or from a 227softirq, a unique global handle must be assigned and passed to the 228corresponding kcov_remote_start() call. Then a userspace process can pass 229a list of such handles to the KCOV_REMOTE_ENABLE ioctl in the handles 230array field of the kcov_remote_arg struct. This will attach the used kcov 231device to the code sections, that are referenced by those handles. 232 233Since there might be many local background threads spawned from different 234userspace processes, we can't use a single global handle per annotation. 235Instead, the userspace process passes a non-zero handle through the 236common_handle field of the kcov_remote_arg struct. This common handle gets 237saved to the kcov_handle field in the current task_struct and needs to be 238passed to the newly spawned threads via custom annotations. Those threads 239should in turn be annotated with kcov_remote_start()/kcov_remote_stop(). 240 241Internally kcov stores handles as u64 integers. The top byte of a handle 242is used to denote the id of a subsystem that this handle belongs to, and 243the lower 4 bytes are used to denote the id of a thread instance within 244that subsystem. A reserved value 0 is used as a subsystem id for common 245handles as they don't belong to a particular subsystem. The bytes 4-7 are 246currently reserved and must be zero. In the future the number of bytes 247used for the subsystem or handle ids might be increased. 248 249When a particular userspace process collects coverage via a common 250handle, kcov will collect coverage for each code section that is annotated 251to use the common handle obtained as kcov_handle from the current 252task_struct. However non common handles allow to collect coverage 253selectively from different subsystems. 254 255.. code-block:: c 256 257 /* Same includes and defines as above. */ 258 259 struct kcov_remote_arg { 260 __u32 trace_mode; 261 __u32 area_size; 262 __u32 num_handles; 263 __aligned_u64 common_handle; 264 __aligned_u64 handles[0]; 265 }; 266 267 #define KCOV_INIT_TRACE _IOR('c', 1, unsigned long) 268 #define KCOV_DISABLE _IO('c', 101) 269 #define KCOV_REMOTE_ENABLE _IOW('c', 102, struct kcov_remote_arg) 270 271 #define COVER_SIZE (64 << 10) 272 273 #define KCOV_TRACE_PC 0 274 275 #define KCOV_SUBSYSTEM_COMMON (0x00ull << 56) 276 #define KCOV_SUBSYSTEM_USB (0x01ull << 56) 277 278 #define KCOV_SUBSYSTEM_MASK (0xffull << 56) 279 #define KCOV_INSTANCE_MASK (0xffffffffull) 280 281 static inline __u64 kcov_remote_handle(__u64 subsys, __u64 inst) 282 { 283 if (subsys & ~KCOV_SUBSYSTEM_MASK || inst & ~KCOV_INSTANCE_MASK) 284 return 0; 285 return subsys | inst; 286 } 287 288 #define KCOV_COMMON_ID 0x42 289 #define KCOV_USB_BUS_NUM 1 290 291 int main(int argc, char **argv) 292 { 293 int fd; 294 unsigned long *cover, n, i; 295 struct kcov_remote_arg *arg; 296 297 fd = open("/sys/kernel/debug/kcov", O_RDWR); 298 if (fd == -1) 299 perror("open"), exit(1); 300 if (ioctl(fd, KCOV_INIT_TRACE, COVER_SIZE)) 301 perror("ioctl"), exit(1); 302 cover = (unsigned long*)mmap(NULL, COVER_SIZE * sizeof(unsigned long), 303 PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0); 304 if ((void*)cover == MAP_FAILED) 305 perror("mmap"), exit(1); 306 307 /* Enable coverage collection via common handle and from USB bus #1. */ 308 arg = calloc(1, sizeof(*arg) + sizeof(uint64_t)); 309 if (!arg) 310 perror("calloc"), exit(1); 311 arg->trace_mode = KCOV_TRACE_PC; 312 arg->area_size = COVER_SIZE; 313 arg->num_handles = 1; 314 arg->common_handle = kcov_remote_handle(KCOV_SUBSYSTEM_COMMON, 315 KCOV_COMMON_ID); 316 arg->handles[0] = kcov_remote_handle(KCOV_SUBSYSTEM_USB, 317 KCOV_USB_BUS_NUM); 318 if (ioctl(fd, KCOV_REMOTE_ENABLE, arg)) 319 perror("ioctl"), free(arg), exit(1); 320 free(arg); 321 322 /* 323 * Here the user needs to trigger execution of a kernel code section 324 * that is either annotated with the common handle, or to trigger some 325 * activity on USB bus #1. 326 */ 327 sleep(2); 328 329 n = __atomic_load_n(&cover[0], __ATOMIC_RELAXED); 330 for (i = 0; i < n; i++) 331 printf("0x%lx\n", cover[i + 1]); 332 if (ioctl(fd, KCOV_DISABLE, 0)) 333 perror("ioctl"), exit(1); 334 if (munmap(cover, COVER_SIZE * sizeof(unsigned long))) 335 perror("munmap"), exit(1); 336 if (close(fd)) 337 perror("close"), exit(1); 338 return 0; 339 } 340