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-------------------
37The following program demonstrates coverage collection from within a test
38program 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
104After 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
121If a program needs to collect coverage from several threads (independently),
122it needs to open /sys/kernel/debug/kcov in each thread separately.
123
124The interface is fine-grained to allow efficient forking of test processes.
125That is, a parent process opens /sys/kernel/debug/kcov, enables trace mode,
126mmaps coverage buffer and then forks child processes in a loop. Child processes
127only need to enable coverage (disable happens automatically on thread end).
128
129Comparison operands collection
130------------------------------
131Comparison 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
203Note that the kcov modes (coverage collection or comparison operands) are
204mutually exclusive.
205