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| 1 | +kcov: code coverage for fuzzing |
| 2 | +=============================== |
| 3 | + |
| 4 | +kcov exposes kernel code coverage information in a form suitable for coverage- |
| 5 | +guided fuzzing (randomized testing). Coverage data of a running kernel is |
| 6 | +exported via the "kcov" debugfs file. Coverage collection is enabled on a task |
| 7 | +basis, and thus it can capture precise coverage of a single system call. |
| 8 | + |
| 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 is function of syscall inputs. |
| 11 | +To achieve this goal it does not collect coverage in soft/hard interrupts |
| 12 | +and instrumentation of some inherently non-deterministic parts of kernel is |
| 13 | +disbled (e.g. scheduler, locking). |
| 14 | + |
| 15 | +Usage: |
| 16 | +====== |
| 17 | + |
| 18 | +Configure kernel with: |
| 19 | + |
| 20 | + CONFIG_KCOV=y |
| 21 | + |
| 22 | +CONFIG_KCOV requires gcc built on revision 231296 or later. |
| 23 | +Profiling data will only become accessible once debugfs has been mounted: |
| 24 | + |
| 25 | + mount -t debugfs none /sys/kernel/debug |
| 26 | + |
| 27 | +The following program demonstrates kcov usage from within a test program: |
| 28 | + |
| 29 | +#include <stdio.h> |
| 30 | +#include <stddef.h> |
| 31 | +#include <stdint.h> |
| 32 | +#include <stdlib.h> |
| 33 | +#include <sys/types.h> |
| 34 | +#include <sys/stat.h> |
| 35 | +#include <sys/ioctl.h> |
| 36 | +#include <sys/mman.h> |
| 37 | +#include <unistd.h> |
| 38 | +#include <fcntl.h> |
| 39 | + |
| 40 | +#define KCOV_INIT_TRACE _IOR('c', 1, unsigned long) |
| 41 | +#define KCOV_ENABLE _IO('c', 100) |
| 42 | +#define KCOV_DISABLE _IO('c', 101) |
| 43 | +#define COVER_SIZE (64<<10) |
| 44 | + |
| 45 | +int main(int argc, char **argv) |
| 46 | +{ |
| 47 | + int fd; |
| 48 | + unsigned long *cover, n, i; |
| 49 | + |
| 50 | + /* A single fd descriptor allows coverage collection on a single |
| 51 | + * thread. |
| 52 | + */ |
| 53 | + fd = open("/sys/kernel/debug/kcov", O_RDWR); |
| 54 | + if (fd == -1) |
| 55 | + perror("open"), exit(1); |
| 56 | + /* Setup trace mode and trace size. */ |
| 57 | + if (ioctl(fd, KCOV_INIT_TRACE, COVER_SIZE)) |
| 58 | + perror("ioctl"), exit(1); |
| 59 | + /* Mmap buffer shared between kernel- and user-space. */ |
| 60 | + cover = (unsigned long*)mmap(NULL, COVER_SIZE * sizeof(unsigned long), |
| 61 | + PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0); |
| 62 | + if ((void*)cover == MAP_FAILED) |
| 63 | + perror("mmap"), exit(1); |
| 64 | + /* Enable coverage collection on the current thread. */ |
| 65 | + if (ioctl(fd, KCOV_ENABLE, 0)) |
| 66 | + perror("ioctl"), exit(1); |
| 67 | + /* Reset coverage from the tail of the ioctl() call. */ |
| 68 | + __atomic_store_n(&cover[0], 0, __ATOMIC_RELAXED); |
| 69 | + /* That's the target syscal call. */ |
| 70 | + read(-1, NULL, 0); |
| 71 | + /* Read number of PCs collected. */ |
| 72 | + n = __atomic_load_n(&cover[0], __ATOMIC_RELAXED); |
| 73 | + for (i = 0; i < n; i++) |
| 74 | + printf("0x%lx\n", cover[i + 1]); |
| 75 | + /* Disable coverage collection for the current thread. After this call |
| 76 | + * coverage can be enabled for a different thread. |
| 77 | + */ |
| 78 | + if (ioctl(fd, KCOV_DISABLE, 0)) |
| 79 | + perror("ioctl"), exit(1); |
| 80 | + /* Free resources. */ |
| 81 | + if (munmap(cover, COVER_SIZE * sizeof(unsigned long))) |
| 82 | + perror("munmap"), exit(1); |
| 83 | + if (close(fd)) |
| 84 | + perror("close"), exit(1); |
| 85 | + return 0; |
| 86 | +} |
| 87 | + |
| 88 | +After piping through addr2line output of the program looks as follows: |
| 89 | + |
| 90 | +SyS_read |
| 91 | +fs/read_write.c:562 |
| 92 | +__fdget_pos |
| 93 | +fs/file.c:774 |
| 94 | +__fget_light |
| 95 | +fs/file.c:746 |
| 96 | +__fget_light |
| 97 | +fs/file.c:750 |
| 98 | +__fget_light |
| 99 | +fs/file.c:760 |
| 100 | +__fdget_pos |
| 101 | +fs/file.c:784 |
| 102 | +SyS_read |
| 103 | +fs/read_write.c:562 |
| 104 | + |
| 105 | +If a program needs to collect coverage from several threads (independently), |
| 106 | +it needs to open /sys/kernel/debug/kcov in each thread separately. |
| 107 | + |
| 108 | +The interface is fine-grained to allow efficient forking of test processes. |
| 109 | +That is, a parent process opens /sys/kernel/debug/kcov, enables trace mode, |
| 110 | +mmaps coverage buffer and then forks child processes in a loop. Child processes |
| 111 | +only need to enable coverage (disable happens automatically on thread end). |
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