1kcov: 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
13disabled (e.g. scheduler, locking).
14
15kcov 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-------------
20
21Configure the kernel with::
22
23        CONFIG_KCOV=y
24
25CONFIG_KCOV requires gcc 6.1.0 or later.
26
27If the comparison operands need to be collected, set::
28
29	CONFIG_KCOV_ENABLE_COMPARISONS=y
30
31Profiling data will only become accessible once debugfs has been mounted::
32
33        mount -t debugfs none /sys/kernel/debug
34
35Coverage collection
36-------------------
37
38The following program demonstrates coverage collection from within a test
39program using kcov:
40
41.. code-block:: c
42
43    #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>
53
54    #define KCOV_INIT_TRACE			_IOR('c', 1, unsigned long)
55    #define KCOV_ENABLE			_IO('c', 100)
56    #define KCOV_DISABLE			_IO('c', 101)
57    #define COVER_SIZE			(64<<10)
58
59    #define KCOV_TRACE_PC  0
60    #define KCOV_TRACE_CMP 1
61
62    int main(int argc, char **argv)
63    {
64	int fd;
65	unsigned long *cover, n, i;
66
67	/* A single fd descriptor allows coverage collection on a single
68	 * thread.
69	 */
70	fd = open("/sys/kernel/debug/kcov", O_RDWR);
71	if (fd == -1)
72		perror("open"), exit(1);
73	/* Setup trace mode and trace size. */
74	if (ioctl(fd, KCOV_INIT_TRACE, COVER_SIZE))
75		perror("ioctl"), exit(1);
76	/* Mmap buffer shared between kernel- and user-space. */
77	cover = (unsigned long*)mmap(NULL, COVER_SIZE * sizeof(unsigned long),
78				     PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
79	if ((void*)cover == MAP_FAILED)
80		perror("mmap"), exit(1);
81	/* Enable coverage collection on the current thread. */
82	if (ioctl(fd, KCOV_ENABLE, KCOV_TRACE_PC))
83		perror("ioctl"), exit(1);
84	/* Reset coverage from the tail of the ioctl() call. */
85	__atomic_store_n(&cover[0], 0, __ATOMIC_RELAXED);
86	/* That's the target syscal call. */
87	read(-1, NULL, 0);
88	/* Read number of PCs collected. */
89	n = __atomic_load_n(&cover[0], __ATOMIC_RELAXED);
90	for (i = 0; i < n; i++)
91		printf("0x%lx\n", cover[i + 1]);
92	/* Disable coverage collection for the current thread. After this call
93	 * coverage can be enabled for a different thread.
94	 */
95	if (ioctl(fd, KCOV_DISABLE, 0))
96		perror("ioctl"), exit(1);
97	/* Free resources. */
98	if (munmap(cover, COVER_SIZE * sizeof(unsigned long)))
99		perror("munmap"), exit(1);
100	if (close(fd))
101		perror("close"), exit(1);
102	return 0;
103    }
104
105After piping through addr2line output of the program looks as follows::
106
107    SyS_read
108    fs/read_write.c:562
109    __fdget_pos
110    fs/file.c:774
111    __fget_light
112    fs/file.c:746
113    __fget_light
114    fs/file.c:750
115    __fget_light
116    fs/file.c:760
117    __fdget_pos
118    fs/file.c:784
119    SyS_read
120    fs/read_write.c:562
121
122If a program needs to collect coverage from several threads (independently),
123it needs to open /sys/kernel/debug/kcov in each thread separately.
124
125The interface is fine-grained to allow efficient forking of test processes.
126That is, a parent process opens /sys/kernel/debug/kcov, enables trace mode,
127mmaps coverage buffer and then forks child processes in a loop. Child processes
128only need to enable coverage (disable happens automatically on thread end).
129
130Comparison operands collection
131------------------------------
132
133Comparison operands collection is similar to coverage collection:
134
135.. code-block:: c
136
137    /* Same includes and defines as above. */
138
139    /* Number of 64-bit words per record. */
140    #define KCOV_WORDS_PER_CMP 4
141
142    /*
143     * The format for the types of collected comparisons.
144     *
145     * Bit 0 shows whether one of the arguments is a compile-time constant.
146     * Bits 1 & 2 contain log2 of the argument size, up to 8 bytes.
147     */
148
149    #define KCOV_CMP_CONST          (1 << 0)
150    #define KCOV_CMP_SIZE(n)        ((n) << 1)
151    #define KCOV_CMP_MASK           KCOV_CMP_SIZE(3)
152
153    int main(int argc, char **argv)
154    {
155	int fd;
156	uint64_t *cover, type, arg1, arg2, is_const, size;
157	unsigned long n, i;
158
159	fd = open("/sys/kernel/debug/kcov", O_RDWR);
160	if (fd == -1)
161		perror("open"), exit(1);
162	if (ioctl(fd, KCOV_INIT_TRACE, COVER_SIZE))
163		perror("ioctl"), exit(1);
164	/*
165	* Note that the buffer pointer is of type uint64_t*, because all
166	* the comparison operands are promoted to uint64_t.
167	*/
168	cover = (uint64_t *)mmap(NULL, COVER_SIZE * sizeof(unsigned long),
169				     PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
170	if ((void*)cover == MAP_FAILED)
171		perror("mmap"), exit(1);
172	/* Note KCOV_TRACE_CMP instead of KCOV_TRACE_PC. */
173	if (ioctl(fd, KCOV_ENABLE, KCOV_TRACE_CMP))
174		perror("ioctl"), exit(1);
175	__atomic_store_n(&cover[0], 0, __ATOMIC_RELAXED);
176	read(-1, NULL, 0);
177	/* Read number of comparisons collected. */
178	n = __atomic_load_n(&cover[0], __ATOMIC_RELAXED);
179	for (i = 0; i < n; i++) {
180		type = cover[i * KCOV_WORDS_PER_CMP + 1];
181		/* arg1 and arg2 - operands of the comparison. */
182		arg1 = cover[i * KCOV_WORDS_PER_CMP + 2];
183		arg2 = cover[i * KCOV_WORDS_PER_CMP + 3];
184		/* ip - caller address. */
185		ip = cover[i * KCOV_WORDS_PER_CMP + 4];
186		/* size of the operands. */
187		size = 1 << ((type & KCOV_CMP_MASK) >> 1);
188		/* is_const - true if either operand is a compile-time constant.*/
189		is_const = type & KCOV_CMP_CONST;
190		printf("ip: 0x%lx type: 0x%lx, arg1: 0x%lx, arg2: 0x%lx, "
191			"size: %lu, %s\n",
192			ip, type, arg1, arg2, size,
193		is_const ? "const" : "non-const");
194	}
195	if (ioctl(fd, KCOV_DISABLE, 0))
196		perror("ioctl"), exit(1);
197	/* Free resources. */
198	if (munmap(cover, COVER_SIZE * sizeof(unsigned long)))
199		perror("munmap"), exit(1);
200	if (close(fd))
201		perror("close"), exit(1);
202	return 0;
203    }
204
205Note that the kcov modes (coverage collection or comparison operands) are
206mutually exclusive.
207
208Remote coverage collection
209--------------------------
210
211With KCOV_ENABLE coverage is collected only for syscalls that are issued
212from the current process. With KCOV_REMOTE_ENABLE it's possible to collect
213coverage for arbitrary parts of the kernel code, provided that those parts
214are annotated with kcov_remote_start()/kcov_remote_stop().
215
216This allows to collect coverage from two types of kernel background
217threads: the global ones, that are spawned during kernel boot in a limited
218number of instances (e.g. one USB hub_event() worker thread is spawned per
219USB HCD); and the local ones, that are spawned when a user interacts with
220some kernel interface (e.g. vhost workers).
221
222To enable collecting coverage from a global background thread, a unique
223global handle must be assigned and passed to the corresponding
224kcov_remote_start() call. Then a userspace process can pass a list of such
225handles to the KCOV_REMOTE_ENABLE ioctl in the handles array field of the
226kcov_remote_arg struct. This will attach the used kcov device to the code
227sections, that are referenced by those handles.
228
229Since there might be many local background threads spawned from different
230userspace processes, we can't use a single global handle per annotation.
231Instead, the userspace process passes a non-zero handle through the
232common_handle field of the kcov_remote_arg struct. This common handle gets
233saved to the kcov_handle field in the current task_struct and needs to be
234passed to the newly spawned threads via custom annotations. Those threads
235should in turn be annotated with kcov_remote_start()/kcov_remote_stop().
236
237Internally kcov stores handles as u64 integers. The top byte of a handle
238is used to denote the id of a subsystem that this handle belongs to, and
239the lower 4 bytes are used to denote the id of a thread instance within
240that subsystem. A reserved value 0 is used as a subsystem id for common
241handles as they don't belong to a particular subsystem. The bytes 4-7 are
242currently reserved and must be zero. In the future the number of bytes
243used for the subsystem or handle ids might be increased.
244
245When a particular userspace proccess collects coverage by via a common
246handle, kcov will collect coverage for each code section that is annotated
247to use the common handle obtained as kcov_handle from the current
248task_struct. However non common handles allow to collect coverage
249selectively from different subsystems.
250
251.. code-block:: c
252
253    struct kcov_remote_arg {
254	__u32		trace_mode;
255	__u32		area_size;
256	__u32		num_handles;
257	__aligned_u64	common_handle;
258	__aligned_u64	handles[0];
259    };
260
261    #define KCOV_INIT_TRACE			_IOR('c', 1, unsigned long)
262    #define KCOV_DISABLE			_IO('c', 101)
263    #define KCOV_REMOTE_ENABLE		_IOW('c', 102, struct kcov_remote_arg)
264
265    #define COVER_SIZE	(64 << 10)
266
267    #define KCOV_TRACE_PC	0
268
269    #define KCOV_SUBSYSTEM_COMMON	(0x00ull << 56)
270    #define KCOV_SUBSYSTEM_USB	(0x01ull << 56)
271
272    #define KCOV_SUBSYSTEM_MASK	(0xffull << 56)
273    #define KCOV_INSTANCE_MASK	(0xffffffffull)
274
275    static inline __u64 kcov_remote_handle(__u64 subsys, __u64 inst)
276    {
277	if (subsys & ~KCOV_SUBSYSTEM_MASK || inst & ~KCOV_INSTANCE_MASK)
278		return 0;
279	return subsys | inst;
280    }
281
282    #define KCOV_COMMON_ID	0x42
283    #define KCOV_USB_BUS_NUM	1
284
285    int main(int argc, char **argv)
286    {
287	int fd;
288	unsigned long *cover, n, i;
289	struct kcov_remote_arg *arg;
290
291	fd = open("/sys/kernel/debug/kcov", O_RDWR);
292	if (fd == -1)
293		perror("open"), exit(1);
294	if (ioctl(fd, KCOV_INIT_TRACE, COVER_SIZE))
295		perror("ioctl"), exit(1);
296	cover = (unsigned long*)mmap(NULL, COVER_SIZE * sizeof(unsigned long),
297				     PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
298	if ((void*)cover == MAP_FAILED)
299		perror("mmap"), exit(1);
300
301	/* Enable coverage collection via common handle and from USB bus #1. */
302	arg = calloc(1, sizeof(*arg) + sizeof(uint64_t));
303	if (!arg)
304		perror("calloc"), exit(1);
305	arg->trace_mode = KCOV_TRACE_PC;
306	arg->area_size = COVER_SIZE;
307	arg->num_handles = 1;
308	arg->common_handle = kcov_remote_handle(KCOV_SUBSYSTEM_COMMON,
309							KCOV_COMMON_ID);
310	arg->handles[0] = kcov_remote_handle(KCOV_SUBSYSTEM_USB,
311						KCOV_USB_BUS_NUM);
312	if (ioctl(fd, KCOV_REMOTE_ENABLE, arg))
313		perror("ioctl"), free(arg), exit(1);
314	free(arg);
315
316	/*
317	 * Here the user needs to trigger execution of a kernel code section
318	 * that is either annotated with the common handle, or to trigger some
319	 * activity on USB bus #1.
320	 */
321	sleep(2);
322
323	n = __atomic_load_n(&cover[0], __ATOMIC_RELAXED);
324	for (i = 0; i < n; i++)
325		printf("0x%lx\n", cover[i + 1]);
326	if (ioctl(fd, KCOV_DISABLE, 0))
327		perror("ioctl"), exit(1);
328	if (munmap(cover, COVER_SIZE * sizeof(unsigned long)))
329		perror("munmap"), exit(1);
330	if (close(fd))
331		perror("close"), exit(1);
332	return 0;
333    }
334