projects / linux.git / blame
| Commit | Line | Data |
|---|---|---|
| 5c9a8750 DV |
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). |