Documentation/dev-tools/kcov.rst

   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 disabled (e.g. scheduler, locking).
  14
  15 kcov is also able to collect comparison operands from the instrumented code
  16 (this feature currently requires that the kernel is compiled with clang).
  17
  18 Prerequisites
  19 -------------
  20
  21 Configure the kernel with::
  22
  23         CONFIG_KCOV=y
  24
  25 CONFIG_KCOV requires gcc built on revision 231296 or later.
  26
  27 If the comparison operands need to be collected, set::
  28
  29         CONFIG_KCOV_ENABLE_COMPARISONS=y
  30
  31 Profiling data will only become accessible once debugfs has been mounted::
  32
  33         mount -t debugfs none /sys/kernel/debug
  34
  35 Coverage collection
  36 -------------------
  37 The following program demonstrates coverage collection from within a test
  38 program 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
 104 After 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
 121 If a program needs to collect coverage from several threads (independently),
 122 it needs to open /sys/kernel/debug/kcov in each thread separately.
 123
 124 The interface is fine-grained to allow efficient forking of test processes.
 125 That is, a parent process opens /sys/kernel/debug/kcov, enables trace mode,
 126 mmaps coverage buffer and then forks child processes in a loop. Child processes
 127 only need to enable coverage (disable happens automatically on thread end).
 128
 129 Comparison operands collection
 130 ------------------------------
 131 Comparison 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
 203 Note that the kcov modes (coverage collection or comparison operands) are
 204 mutually exclusive.