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Based on kernel version 4.8. Page generated on 2016-10-06 23:16 EST.

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).
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