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Dmitry Vyukov5c9a8752016-03-22 14:27:30 -07001kcov: 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
Masahiro Yamada8a1115f2017-03-09 16:16:31 -080013disabled (e.g. scheduler, locking).
Dmitry Vyukov5c9a8752016-03-22 14:27:30 -070014
Victor Chibotaruc512ac02017-11-17 15:30:53 -080015kcov 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-------------
Dmitry Vyukov5c9a8752016-03-22 14:27:30 -070020
Jonathan Corbet758f7262016-08-07 15:13:00 -060021Configure the kernel with::
Dmitry Vyukov5c9a8752016-03-22 14:27:30 -070022
23 CONFIG_KCOV=y
24
Sebastian Andrzej Siewior58f4df32019-01-14 11:08:07 +010025CONFIG_KCOV requires gcc 6.1.0 or later.
Victor Chibotaruc512ac02017-11-17 15:30:53 -080026
27If the comparison operands need to be collected, set::
28
29 CONFIG_KCOV_ENABLE_COMPARISONS=y
30
Jonathan Corbet758f7262016-08-07 15:13:00 -060031Profiling data will only become accessible once debugfs has been mounted::
Dmitry Vyukov5c9a8752016-03-22 14:27:30 -070032
33 mount -t debugfs none /sys/kernel/debug
34
Victor Chibotaruc512ac02017-11-17 15:30:53 -080035Coverage collection
36-------------------
Andrey Konovaloveec028c2019-12-04 16:52:43 -080037
Victor Chibotaruc512ac02017-11-17 15:30:53 -080038The following program demonstrates coverage collection from within a test
39program using kcov:
Jani Nikula57131dd2016-11-03 11:44:04 +020040
41.. code-block:: c
Dmitry Vyukov5c9a8752016-03-22 14:27:30 -070042
Jonathan Corbet758f7262016-08-07 15:13:00 -060043 #include <stdio.h>
44 #include <stddef.h>
45 #include <stdint.h>
46 #include <stdlib.h>
47 #include <sys/types.h>
48 #include <sys/stat.h>
49 #include <sys/ioctl.h>
50 #include <sys/mman.h>
51 #include <unistd.h>
52 #include <fcntl.h>
Sebastian Andrzej Siewiord687a9c2021-11-08 18:35:28 -080053 #include <linux/types.h>
Dmitry Vyukov5c9a8752016-03-22 14:27:30 -070054
Jonathan Corbet758f7262016-08-07 15:13:00 -060055 #define KCOV_INIT_TRACE _IOR('c', 1, unsigned long)
56 #define KCOV_ENABLE _IO('c', 100)
57 #define KCOV_DISABLE _IO('c', 101)
58 #define COVER_SIZE (64<<10)
Dmitry Vyukov5c9a8752016-03-22 14:27:30 -070059
Victor Chibotaruc512ac02017-11-17 15:30:53 -080060 #define KCOV_TRACE_PC 0
61 #define KCOV_TRACE_CMP 1
62
Jonathan Corbet758f7262016-08-07 15:13:00 -060063 int main(int argc, char **argv)
64 {
Dmitry Vyukov5c9a8752016-03-22 14:27:30 -070065 int fd;
66 unsigned long *cover, n, i;
67
68 /* A single fd descriptor allows coverage collection on a single
69 * thread.
70 */
71 fd = open("/sys/kernel/debug/kcov", O_RDWR);
72 if (fd == -1)
73 perror("open"), exit(1);
74 /* Setup trace mode and trace size. */
75 if (ioctl(fd, KCOV_INIT_TRACE, COVER_SIZE))
76 perror("ioctl"), exit(1);
77 /* Mmap buffer shared between kernel- and user-space. */
78 cover = (unsigned long*)mmap(NULL, COVER_SIZE * sizeof(unsigned long),
79 PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
80 if ((void*)cover == MAP_FAILED)
81 perror("mmap"), exit(1);
82 /* Enable coverage collection on the current thread. */
Victor Chibotaruc512ac02017-11-17 15:30:53 -080083 if (ioctl(fd, KCOV_ENABLE, KCOV_TRACE_PC))
Dmitry Vyukov5c9a8752016-03-22 14:27:30 -070084 perror("ioctl"), exit(1);
85 /* Reset coverage from the tail of the ioctl() call. */
86 __atomic_store_n(&cover[0], 0, __ATOMIC_RELAXED);
87 /* That's the target syscal call. */
88 read(-1, NULL, 0);
89 /* Read number of PCs collected. */
90 n = __atomic_load_n(&cover[0], __ATOMIC_RELAXED);
91 for (i = 0; i < n; i++)
92 printf("0x%lx\n", cover[i + 1]);
93 /* Disable coverage collection for the current thread. After this call
94 * coverage can be enabled for a different thread.
95 */
96 if (ioctl(fd, KCOV_DISABLE, 0))
97 perror("ioctl"), exit(1);
98 /* Free resources. */
99 if (munmap(cover, COVER_SIZE * sizeof(unsigned long)))
100 perror("munmap"), exit(1);
101 if (close(fd))
102 perror("close"), exit(1);
103 return 0;
Jonathan Corbet758f7262016-08-07 15:13:00 -0600104 }
Dmitry Vyukov5c9a8752016-03-22 14:27:30 -0700105
Jonathan Corbet758f7262016-08-07 15:13:00 -0600106After piping through addr2line output of the program looks as follows::
Dmitry Vyukov5c9a8752016-03-22 14:27:30 -0700107
Jonathan Corbet758f7262016-08-07 15:13:00 -0600108 SyS_read
109 fs/read_write.c:562
110 __fdget_pos
111 fs/file.c:774
112 __fget_light
113 fs/file.c:746
114 __fget_light
115 fs/file.c:750
116 __fget_light
117 fs/file.c:760
118 __fdget_pos
119 fs/file.c:784
120 SyS_read
121 fs/read_write.c:562
Dmitry Vyukov5c9a8752016-03-22 14:27:30 -0700122
123If a program needs to collect coverage from several threads (independently),
124it needs to open /sys/kernel/debug/kcov in each thread separately.
125
126The interface is fine-grained to allow efficient forking of test processes.
127That is, a parent process opens /sys/kernel/debug/kcov, enables trace mode,
128mmaps coverage buffer and then forks child processes in a loop. Child processes
129only need to enable coverage (disable happens automatically on thread end).
Victor Chibotaruc512ac02017-11-17 15:30:53 -0800130
131Comparison operands collection
132------------------------------
Andrey Konovaloveec028c2019-12-04 16:52:43 -0800133
Victor Chibotaruc512ac02017-11-17 15:30:53 -0800134Comparison operands collection is similar to coverage collection:
135
136.. code-block:: c
137
138 /* Same includes and defines as above. */
139
140 /* Number of 64-bit words per record. */
141 #define KCOV_WORDS_PER_CMP 4
142
143 /*
144 * The format for the types of collected comparisons.
145 *
146 * Bit 0 shows whether one of the arguments is a compile-time constant.
147 * Bits 1 & 2 contain log2 of the argument size, up to 8 bytes.
148 */
149
150 #define KCOV_CMP_CONST (1 << 0)
151 #define KCOV_CMP_SIZE(n) ((n) << 1)
152 #define KCOV_CMP_MASK KCOV_CMP_SIZE(3)
153
154 int main(int argc, char **argv)
155 {
156 int fd;
157 uint64_t *cover, type, arg1, arg2, is_const, size;
158 unsigned long n, i;
159
160 fd = open("/sys/kernel/debug/kcov", O_RDWR);
161 if (fd == -1)
162 perror("open"), exit(1);
163 if (ioctl(fd, KCOV_INIT_TRACE, COVER_SIZE))
164 perror("ioctl"), exit(1);
165 /*
166 * Note that the buffer pointer is of type uint64_t*, because all
167 * the comparison operands are promoted to uint64_t.
168 */
169 cover = (uint64_t *)mmap(NULL, COVER_SIZE * sizeof(unsigned long),
170 PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
171 if ((void*)cover == MAP_FAILED)
172 perror("mmap"), exit(1);
173 /* Note KCOV_TRACE_CMP instead of KCOV_TRACE_PC. */
174 if (ioctl(fd, KCOV_ENABLE, KCOV_TRACE_CMP))
175 perror("ioctl"), exit(1);
176 __atomic_store_n(&cover[0], 0, __ATOMIC_RELAXED);
177 read(-1, NULL, 0);
178 /* Read number of comparisons collected. */
179 n = __atomic_load_n(&cover[0], __ATOMIC_RELAXED);
180 for (i = 0; i < n; i++) {
Sebastian Andrzej Siewior6f1d34b2021-11-08 18:35:31 -0800181 uint64_t ip;
182
Victor Chibotaruc512ac02017-11-17 15:30:53 -0800183 type = cover[i * KCOV_WORDS_PER_CMP + 1];
184 /* arg1 and arg2 - operands of the comparison. */
185 arg1 = cover[i * KCOV_WORDS_PER_CMP + 2];
186 arg2 = cover[i * KCOV_WORDS_PER_CMP + 3];
187 /* ip - caller address. */
188 ip = cover[i * KCOV_WORDS_PER_CMP + 4];
189 /* size of the operands. */
190 size = 1 << ((type & KCOV_CMP_MASK) >> 1);
191 /* is_const - true if either operand is a compile-time constant.*/
192 is_const = type & KCOV_CMP_CONST;
193 printf("ip: 0x%lx type: 0x%lx, arg1: 0x%lx, arg2: 0x%lx, "
194 "size: %lu, %s\n",
195 ip, type, arg1, arg2, size,
196 is_const ? "const" : "non-const");
197 }
198 if (ioctl(fd, KCOV_DISABLE, 0))
199 perror("ioctl"), exit(1);
200 /* Free resources. */
201 if (munmap(cover, COVER_SIZE * sizeof(unsigned long)))
202 perror("munmap"), exit(1);
203 if (close(fd))
204 perror("close"), exit(1);
205 return 0;
206 }
207
208Note that the kcov modes (coverage collection or comparison operands) are
209mutually exclusive.
Andrey Konovaloveec028c2019-12-04 16:52:43 -0800210
211Remote coverage collection
212--------------------------
213
214With KCOV_ENABLE coverage is collected only for syscalls that are issued
215from the current process. With KCOV_REMOTE_ENABLE it's possible to collect
216coverage for arbitrary parts of the kernel code, provided that those parts
217are annotated with kcov_remote_start()/kcov_remote_stop().
218
219This allows to collect coverage from two types of kernel background
220threads: the global ones, that are spawned during kernel boot in a limited
221number of instances (e.g. one USB hub_event() worker thread is spawned per
222USB HCD); and the local ones, that are spawned when a user interacts with
Andrey Konovalov5ff3b302020-06-04 16:46:04 -0700223some kernel interface (e.g. vhost workers); as well as from soft
224interrupts.
Andrey Konovaloveec028c2019-12-04 16:52:43 -0800225
Andrey Konovalov5ff3b302020-06-04 16:46:04 -0700226To enable collecting coverage from a global background thread or from a
227softirq, a unique global handle must be assigned and passed to the
228corresponding kcov_remote_start() call. Then a userspace process can pass
229a list of such handles to the KCOV_REMOTE_ENABLE ioctl in the handles
230array field of the kcov_remote_arg struct. This will attach the used kcov
231device to the code sections, that are referenced by those handles.
Andrey Konovaloveec028c2019-12-04 16:52:43 -0800232
233Since there might be many local background threads spawned from different
234userspace processes, we can't use a single global handle per annotation.
235Instead, the userspace process passes a non-zero handle through the
236common_handle field of the kcov_remote_arg struct. This common handle gets
237saved to the kcov_handle field in the current task_struct and needs to be
238passed to the newly spawned threads via custom annotations. Those threads
239should in turn be annotated with kcov_remote_start()/kcov_remote_stop().
240
241Internally kcov stores handles as u64 integers. The top byte of a handle
242is used to denote the id of a subsystem that this handle belongs to, and
243the lower 4 bytes are used to denote the id of a thread instance within
244that subsystem. A reserved value 0 is used as a subsystem id for common
245handles as they don't belong to a particular subsystem. The bytes 4-7 are
246currently reserved and must be zero. In the future the number of bytes
247used for the subsystem or handle ids might be increased.
248
Andrew Klychkove0a45cd2020-12-02 10:54:38 +0300249When a particular userspace process collects coverage via a common
Andrey Konovaloveec028c2019-12-04 16:52:43 -0800250handle, kcov will collect coverage for each code section that is annotated
251to use the common handle obtained as kcov_handle from the current
252task_struct. However non common handles allow to collect coverage
253selectively from different subsystems.
254
255.. code-block:: c
256
Sebastian Andrzej Siewiord687a9c2021-11-08 18:35:28 -0800257 /* Same includes and defines as above. */
258
Andrey Konovaloveec028c2019-12-04 16:52:43 -0800259 struct kcov_remote_arg {
Andrey Konovalova69b83e2020-01-04 12:59:39 -0800260 __u32 trace_mode;
261 __u32 area_size;
262 __u32 num_handles;
263 __aligned_u64 common_handle;
264 __aligned_u64 handles[0];
Andrey Konovaloveec028c2019-12-04 16:52:43 -0800265 };
266
267 #define KCOV_INIT_TRACE _IOR('c', 1, unsigned long)
268 #define KCOV_DISABLE _IO('c', 101)
269 #define KCOV_REMOTE_ENABLE _IOW('c', 102, struct kcov_remote_arg)
270
271 #define COVER_SIZE (64 << 10)
272
273 #define KCOV_TRACE_PC 0
274
275 #define KCOV_SUBSYSTEM_COMMON (0x00ull << 56)
276 #define KCOV_SUBSYSTEM_USB (0x01ull << 56)
277
278 #define KCOV_SUBSYSTEM_MASK (0xffull << 56)
279 #define KCOV_INSTANCE_MASK (0xffffffffull)
280
281 static inline __u64 kcov_remote_handle(__u64 subsys, __u64 inst)
282 {
283 if (subsys & ~KCOV_SUBSYSTEM_MASK || inst & ~KCOV_INSTANCE_MASK)
284 return 0;
285 return subsys | inst;
286 }
287
288 #define KCOV_COMMON_ID 0x42
289 #define KCOV_USB_BUS_NUM 1
290
291 int main(int argc, char **argv)
292 {
293 int fd;
294 unsigned long *cover, n, i;
295 struct kcov_remote_arg *arg;
296
297 fd = open("/sys/kernel/debug/kcov", O_RDWR);
298 if (fd == -1)
299 perror("open"), exit(1);
300 if (ioctl(fd, KCOV_INIT_TRACE, COVER_SIZE))
301 perror("ioctl"), exit(1);
302 cover = (unsigned long*)mmap(NULL, COVER_SIZE * sizeof(unsigned long),
303 PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
304 if ((void*)cover == MAP_FAILED)
305 perror("mmap"), exit(1);
306
307 /* Enable coverage collection via common handle and from USB bus #1. */
308 arg = calloc(1, sizeof(*arg) + sizeof(uint64_t));
309 if (!arg)
310 perror("calloc"), exit(1);
311 arg->trace_mode = KCOV_TRACE_PC;
312 arg->area_size = COVER_SIZE;
313 arg->num_handles = 1;
314 arg->common_handle = kcov_remote_handle(KCOV_SUBSYSTEM_COMMON,
315 KCOV_COMMON_ID);
316 arg->handles[0] = kcov_remote_handle(KCOV_SUBSYSTEM_USB,
317 KCOV_USB_BUS_NUM);
318 if (ioctl(fd, KCOV_REMOTE_ENABLE, arg))
319 perror("ioctl"), free(arg), exit(1);
320 free(arg);
321
322 /*
323 * Here the user needs to trigger execution of a kernel code section
324 * that is either annotated with the common handle, or to trigger some
325 * activity on USB bus #1.
326 */
327 sleep(2);
328
329 n = __atomic_load_n(&cover[0], __ATOMIC_RELAXED);
330 for (i = 0; i < n; i++)
331 printf("0x%lx\n", cover[i + 1]);
332 if (ioctl(fd, KCOV_DISABLE, 0))
333 perror("ioctl"), exit(1);
334 if (munmap(cover, COVER_SIZE * sizeof(unsigned long)))
335 perror("munmap"), exit(1);
336 if (close(fd))
337 perror("close"), exit(1);
338 return 0;
339 }