Documentation/kcov.txt

   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).