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------------------- 37The following program demonstrates coverage collection from within a test 38program using kcov: 39 40.. code-block:: c 41 42 #include <stdio.h> 43 #include <stddef.h> 44 #include <stdint.h> 45 #include <stdlib.h> 46 #include <sys/types.h> 47 #include <sys/stat.h> 48 #include <sys/ioctl.h> 49 #include <sys/mman.h> 50 #include <unistd.h> 51 #include <fcntl.h> 52 53 #define KCOV_INIT_TRACE _IOR('c', 1, unsigned long) 54 #define KCOV_ENABLE _IO('c', 100) 55 #define KCOV_DISABLE _IO('c', 101) 56 #define COVER_SIZE (64<<10) 57 58 #define KCOV_TRACE_PC 0 59 #define KCOV_TRACE_CMP 1 60 61 int main(int argc, char **argv) 62 { 63 int fd; 64 unsigned long *cover, n, i; 65 66 /* A single fd descriptor allows coverage collection on a single 67 * thread. 68 */ 69 fd = open("/sys/kernel/debug/kcov", O_RDWR); 70 if (fd == -1) 71 perror("open"), exit(1); 72 /* Setup trace mode and trace size. */ 73 if (ioctl(fd, KCOV_INIT_TRACE, COVER_SIZE)) 74 perror("ioctl"), exit(1); 75 /* Mmap buffer shared between kernel- and user-space. */ 76 cover = (unsigned long*)mmap(NULL, COVER_SIZE * sizeof(unsigned long), 77 PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0); 78 if ((void*)cover == MAP_FAILED) 79 perror("mmap"), exit(1); 80 /* Enable coverage collection on the current thread. */ 81 if (ioctl(fd, KCOV_ENABLE, KCOV_TRACE_PC)) 82 perror("ioctl"), exit(1); 83 /* Reset coverage from the tail of the ioctl() call. */ 84 __atomic_store_n(&cover[0], 0, __ATOMIC_RELAXED); 85 /* That's the target syscal call. */ 86 read(-1, NULL, 0); 87 /* Read number of PCs collected. */ 88 n = __atomic_load_n(&cover[0], __ATOMIC_RELAXED); 89 for (i = 0; i < n; i++) 90 printf("0x%lx\n", cover[i + 1]); 91 /* Disable coverage collection for the current thread. After this call 92 * coverage can be enabled for a different thread. 93 */ 94 if (ioctl(fd, KCOV_DISABLE, 0)) 95 perror("ioctl"), exit(1); 96 /* Free resources. */ 97 if (munmap(cover, COVER_SIZE * sizeof(unsigned long))) 98 perror("munmap"), exit(1); 99 if (close(fd)) 100 perror("close"), exit(1); 101 return 0; 102 } 103 104After piping through addr2line output of the program looks as follows:: 105 106 SyS_read 107 fs/read_write.c:562 108 __fdget_pos 109 fs/file.c:774 110 __fget_light 111 fs/file.c:746 112 __fget_light 113 fs/file.c:750 114 __fget_light 115 fs/file.c:760 116 __fdget_pos 117 fs/file.c:784 118 SyS_read 119 fs/read_write.c:562 120 121If a program needs to collect coverage from several threads (independently), 122it needs to open /sys/kernel/debug/kcov in each thread separately. 123 124The interface is fine-grained to allow efficient forking of test processes. 125That is, a parent process opens /sys/kernel/debug/kcov, enables trace mode, 126mmaps coverage buffer and then forks child processes in a loop. Child processes 127only need to enable coverage (disable happens automatically on thread end). 128 129Comparison operands collection 130------------------------------ 131Comparison operands collection is similar to coverage collection: 132 133.. code-block:: c 134 135 /* Same includes and defines as above. */ 136 137 /* Number of 64-bit words per record. */ 138 #define KCOV_WORDS_PER_CMP 4 139 140 /* 141 * The format for the types of collected comparisons. 142 * 143 * Bit 0 shows whether one of the arguments is a compile-time constant. 144 * Bits 1 & 2 contain log2 of the argument size, up to 8 bytes. 145 */ 146 147 #define KCOV_CMP_CONST (1 << 0) 148 #define KCOV_CMP_SIZE(n) ((n) << 1) 149 #define KCOV_CMP_MASK KCOV_CMP_SIZE(3) 150 151 int main(int argc, char **argv) 152 { 153 int fd; 154 uint64_t *cover, type, arg1, arg2, is_const, size; 155 unsigned long n, i; 156 157 fd = open("/sys/kernel/debug/kcov", O_RDWR); 158 if (fd == -1) 159 perror("open"), exit(1); 160 if (ioctl(fd, KCOV_INIT_TRACE, COVER_SIZE)) 161 perror("ioctl"), exit(1); 162 /* 163 * Note that the buffer pointer is of type uint64_t*, because all 164 * the comparison operands are promoted to uint64_t. 165 */ 166 cover = (uint64_t *)mmap(NULL, COVER_SIZE * sizeof(unsigned long), 167 PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0); 168 if ((void*)cover == MAP_FAILED) 169 perror("mmap"), exit(1); 170 /* Note KCOV_TRACE_CMP instead of KCOV_TRACE_PC. */ 171 if (ioctl(fd, KCOV_ENABLE, KCOV_TRACE_CMP)) 172 perror("ioctl"), exit(1); 173 __atomic_store_n(&cover[0], 0, __ATOMIC_RELAXED); 174 read(-1, NULL, 0); 175 /* Read number of comparisons collected. */ 176 n = __atomic_load_n(&cover[0], __ATOMIC_RELAXED); 177 for (i = 0; i < n; i++) { 178 type = cover[i * KCOV_WORDS_PER_CMP + 1]; 179 /* arg1 and arg2 - operands of the comparison. */ 180 arg1 = cover[i * KCOV_WORDS_PER_CMP + 2]; 181 arg2 = cover[i * KCOV_WORDS_PER_CMP + 3]; 182 /* ip - caller address. */ 183 ip = cover[i * KCOV_WORDS_PER_CMP + 4]; 184 /* size of the operands. */ 185 size = 1 << ((type & KCOV_CMP_MASK) >> 1); 186 /* is_const - true if either operand is a compile-time constant.*/ 187 is_const = type & KCOV_CMP_CONST; 188 printf("ip: 0x%lx type: 0x%lx, arg1: 0x%lx, arg2: 0x%lx, " 189 "size: %lu, %s\n", 190 ip, type, arg1, arg2, size, 191 is_const ? "const" : "non-const"); 192 } 193 if (ioctl(fd, KCOV_DISABLE, 0)) 194 perror("ioctl"), exit(1); 195 /* Free resources. */ 196 if (munmap(cover, COVER_SIZE * sizeof(unsigned long))) 197 perror("munmap"), exit(1); 198 if (close(fd)) 199 perror("close"), exit(1); 200 return 0; 201 } 202 203Note that the kcov modes (coverage collection or comparison operands) are 204mutually exclusive. 205