Documentation/dev-tools/kcov.rst - linux - Git at Google

 kcov: code coverage for fuzzing
 ===============================

 kcov exposes kernel code coverage information in a form suitable for coverage-
 guided fuzzing (randomized testing). Coverage data of a running kernel is
 exported via the "kcov" debugfs file. Coverage collection is enabled on a task
 basis, and thus it can capture precise coverage of a single system call.

 Note that kcov does not aim to collect as much coverage as possible. It aims
 to collect more or less stable coverage that is function of syscall inputs.
 To achieve this goal it does not collect coverage in soft/hard interrupts
 and instrumentation of some inherently non-deterministic parts of kernel is
 disabled (e.g. scheduler, locking).

 kcov is also able to collect comparison operands from the instrumented code
 (this feature currently requires that the kernel is compiled with clang).

 Prerequisites
 -------------

 Configure the kernel with::

         CONFIG_KCOV=y

 CONFIG_KCOV requires gcc 6.1.0 or later.

 If the comparison operands need to be collected, set::

 	CONFIG_KCOV_ENABLE_COMPARISONS=y

 Profiling data will only become accessible once debugfs has been mounted::

         mount -t debugfs none /sys/kernel/debug

 Coverage collection
 -------------------

 The following program demonstrates coverage collection from within a test
 program using kcov:

 .. code-block:: c

     #include <stdio.h>
     #include <stddef.h>
     #include <stdint.h>
     #include <stdlib.h>
     #include <sys/types.h>
     #include <sys/stat.h>
     #include <sys/ioctl.h>
     #include <sys/mman.h>
     #include <unistd.h>
     #include <fcntl.h>

     #define KCOV_INIT_TRACE			_IOR('c', 1, unsigned long)
     #define KCOV_ENABLE			_IO('c', 100)
     #define KCOV_DISABLE			_IO('c', 101)
     #define COVER_SIZE			(64<<10)

     #define KCOV_TRACE_PC  0
     #define KCOV_TRACE_CMP 1

     int main(int argc, char **argv)
     {
 	int fd;
 	unsigned long *cover, n, i;

 	/* A single fd descriptor allows coverage collection on a single
 	 * thread.
 	 */
 	fd = open("/sys/kernel/debug/kcov", O_RDWR);
 	if (fd == -1)
 		perror("open"), exit(1);
 	/* Setup trace mode and trace size. */
 	if (ioctl(fd, KCOV_INIT_TRACE, COVER_SIZE))
 		perror("ioctl"), exit(1);
 	/* Mmap buffer shared between kernel- and user-space. */
 	cover = (unsigned long*)mmap(NULL, COVER_SIZE * sizeof(unsigned long),
 				     PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
 	if ((void*)cover == MAP_FAILED)
 		perror("mmap"), exit(1);
 	/* Enable coverage collection on the current thread. */
 	if (ioctl(fd, KCOV_ENABLE, KCOV_TRACE_PC))
 		perror("ioctl"), exit(1);
 	/* Reset coverage from the tail of the ioctl() call. */
 	__atomic_store_n(&cover[0], 0, __ATOMIC_RELAXED);
 	/* That's the target syscal call. */
 	read(-1, NULL, 0);
 	/* Read number of PCs collected. */
 	n = __atomic_load_n(&cover[0], __ATOMIC_RELAXED);
 	for (i = 0; i < n; i++)
 		printf("0x%lx\n", cover[i + 1]);
 	/* Disable coverage collection for the current thread. After this call
 	 * coverage can be enabled for a different thread.
 	 */
 	if (ioctl(fd, KCOV_DISABLE, 0))
 		perror("ioctl"), exit(1);
 	/* Free resources. */
 	if (munmap(cover, COVER_SIZE * sizeof(unsigned long)))
 		perror("munmap"), exit(1);
 	if (close(fd))
 		perror("close"), exit(1);
 	return 0;
     }

 After piping through addr2line output of the program looks as follows::

     SyS_read
     fs/read_write.c:562
     __fdget_pos
     fs/file.c:774
     __fget_light
     fs/file.c:746
     __fget_light
     fs/file.c:750
     __fget_light
     fs/file.c:760
     __fdget_pos
     fs/file.c:784
     SyS_read
     fs/read_write.c:562

 If a program needs to collect coverage from several threads (independently),
 it needs to open /sys/kernel/debug/kcov in each thread separately.

 The interface is fine-grained to allow efficient forking of test processes.
 That is, a parent process opens /sys/kernel/debug/kcov, enables trace mode,
 mmaps coverage buffer and then forks child processes in a loop. Child processes
 only need to enable coverage (disable happens automatically on thread end).

 Comparison operands collection
 ------------------------------

 Comparison operands collection is similar to coverage collection:

 .. code-block:: c

     /* Same includes and defines as above. */

     /* Number of 64-bit words per record. */
     #define KCOV_WORDS_PER_CMP 4

     /*
      * The format for the types of collected comparisons.
      *
      * Bit 0 shows whether one of the arguments is a compile-time constant.
      * Bits 1 & 2 contain log2 of the argument size, up to 8 bytes.
      */

     #define KCOV_CMP_CONST          (1 << 0)
     #define KCOV_CMP_SIZE(n)        ((n) << 1)
     #define KCOV_CMP_MASK           KCOV_CMP_SIZE(3)

     int main(int argc, char **argv)
     {
 	int fd;
 	uint64_t *cover, type, arg1, arg2, is_const, size;
 	unsigned long n, i;

 	fd = open("/sys/kernel/debug/kcov", O_RDWR);
 	if (fd == -1)
 		perror("open"), exit(1);
 	if (ioctl(fd, KCOV_INIT_TRACE, COVER_SIZE))
 		perror("ioctl"), exit(1);
 	/*
 	* Note that the buffer pointer is of type uint64_t*, because all
 	* the comparison operands are promoted to uint64_t.
 	*/
 	cover = (uint64_t *)mmap(NULL, COVER_SIZE * sizeof(unsigned long),
 				     PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
 	if ((void*)cover == MAP_FAILED)
 		perror("mmap"), exit(1);
 	/* Note KCOV_TRACE_CMP instead of KCOV_TRACE_PC. */
 	if (ioctl(fd, KCOV_ENABLE, KCOV_TRACE_CMP))
 		perror("ioctl"), exit(1);
 	__atomic_store_n(&cover[0], 0, __ATOMIC_RELAXED);
 	read(-1, NULL, 0);
 	/* Read number of comparisons collected. */
 	n = __atomic_load_n(&cover[0], __ATOMIC_RELAXED);
 	for (i = 0; i < n; i++) {
 		type = cover[i * KCOV_WORDS_PER_CMP + 1];
 		/* arg1 and arg2 - operands of the comparison. */
 		arg1 = cover[i * KCOV_WORDS_PER_CMP + 2];
 		arg2 = cover[i * KCOV_WORDS_PER_CMP + 3];
 		/* ip - caller address. */
 		ip = cover[i * KCOV_WORDS_PER_CMP + 4];
 		/* size of the operands. */
 		size = 1 << ((type & KCOV_CMP_MASK) >> 1);
 		/* is_const - true if either operand is a compile-time constant.*/
 		is_const = type & KCOV_CMP_CONST;
 		printf("ip: 0x%lx type: 0x%lx, arg1: 0x%lx, arg2: 0x%lx, "
 			"size: %lu, %s\n",
 			ip, type, arg1, arg2, size,
 		is_const ? "const" : "non-const");
 	}
 	if (ioctl(fd, KCOV_DISABLE, 0))
 		perror("ioctl"), exit(1);
 	/* Free resources. */
 	if (munmap(cover, COVER_SIZE * sizeof(unsigned long)))
 		perror("munmap"), exit(1);
 	if (close(fd))
 		perror("close"), exit(1);
 	return 0;
     }

 Note that the kcov modes (coverage collection or comparison operands) are
 mutually exclusive.

 Remote coverage collection
 --------------------------

 With KCOV_ENABLE coverage is collected only for syscalls that are issued
 from the current process. With KCOV_REMOTE_ENABLE it's possible to collect
 coverage for arbitrary parts of the kernel code, provided that those parts
 are annotated with kcov_remote_start()/kcov_remote_stop().

 This allows to collect coverage from two types of kernel background
 threads: the global ones, that are spawned during kernel boot in a limited
 number of instances (e.g. one USB hub_event() worker thread is spawned per
 USB HCD); and the local ones, that are spawned when a user interacts with
 some kernel interface (e.g. vhost workers).

 To enable collecting coverage from a global background thread, a unique
 global handle must be assigned and passed to the corresponding
 kcov_remote_start() call. Then a userspace process can pass a list of such
 handles to the KCOV_REMOTE_ENABLE ioctl in the handles array field of the
 kcov_remote_arg struct. This will attach the used kcov device to the code
 sections, that are referenced by those handles.

 Since there might be many local background threads spawned from different
 userspace processes, we can't use a single global handle per annotation.
 Instead, the userspace process passes a non-zero handle through the
 common_handle field of the kcov_remote_arg struct. This common handle gets
 saved to the kcov_handle field in the current task_struct and needs to be
 passed to the newly spawned threads via custom annotations. Those threads
 should in turn be annotated with kcov_remote_start()/kcov_remote_stop().

 Internally kcov stores handles as u64 integers. The top byte of a handle
 is used to denote the id of a subsystem that this handle belongs to, and
 the lower 4 bytes are used to denote the id of a thread instance within
 that subsystem. A reserved value 0 is used as a subsystem id for common
 handles as they don't belong to a particular subsystem. The bytes 4-7 are
 currently reserved and must be zero. In the future the number of bytes
 used for the subsystem or handle ids might be increased.

 When a particular userspace proccess collects coverage by via a common
 handle, kcov will collect coverage for each code section that is annotated
 to use the common handle obtained as kcov_handle from the current
 task_struct. However non common handles allow to collect coverage
 selectively from different subsystems.

 .. code-block:: c

     struct kcov_remote_arg {
 	unsigned	trace_mode;
 	unsigned	area_size;
 	unsigned	num_handles;
 	uint64_t	common_handle;
 	uint64_t	handles[0];
     };

     #define KCOV_INIT_TRACE			_IOR('c', 1, unsigned long)
     #define KCOV_DISABLE			_IO('c', 101)
     #define KCOV_REMOTE_ENABLE		_IOW('c', 102, struct kcov_remote_arg)

     #define COVER_SIZE	(64 << 10)

     #define KCOV_TRACE_PC	0

     #define KCOV_SUBSYSTEM_COMMON	(0x00ull << 56)
     #define KCOV_SUBSYSTEM_USB	(0x01ull << 56)

     #define KCOV_SUBSYSTEM_MASK	(0xffull << 56)
     #define KCOV_INSTANCE_MASK	(0xffffffffull)

     static inline __u64 kcov_remote_handle(__u64 subsys, __u64 inst)
     {
 	if (subsys & ~KCOV_SUBSYSTEM_MASK || inst & ~KCOV_INSTANCE_MASK)
 		return 0;
 	return subsys | inst;
     }

     #define KCOV_COMMON_ID	0x42
     #define KCOV_USB_BUS_NUM	1

     int main(int argc, char **argv)
     {
 	int fd;
 	unsigned long *cover, n, i;
 	struct kcov_remote_arg *arg;

 	fd = open("/sys/kernel/debug/kcov", O_RDWR);
 	if (fd == -1)
 		perror("open"), exit(1);
 	if (ioctl(fd, KCOV_INIT_TRACE, COVER_SIZE))
 		perror("ioctl"), exit(1);
 	cover = (unsigned long*)mmap(NULL, COVER_SIZE * sizeof(unsigned long),
 				     PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
 	if ((void*)cover == MAP_FAILED)
 		perror("mmap"), exit(1);

 	/* Enable coverage collection via common handle and from USB bus #1. */
 	arg = calloc(1, sizeof(*arg) + sizeof(uint64_t));
 	if (!arg)
 		perror("calloc"), exit(1);
 	arg->trace_mode = KCOV_TRACE_PC;
 	arg->area_size = COVER_SIZE;
 	arg->num_handles = 1;
 	arg->common_handle = kcov_remote_handle(KCOV_SUBSYSTEM_COMMON,
 							KCOV_COMMON_ID);
 	arg->handles[0] = kcov_remote_handle(KCOV_SUBSYSTEM_USB,
 						KCOV_USB_BUS_NUM);
 	if (ioctl(fd, KCOV_REMOTE_ENABLE, arg))
 		perror("ioctl"), free(arg), exit(1);
 	free(arg);

 	/*
 	 * Here the user needs to trigger execution of a kernel code section
 	 * that is either annotated with the common handle, or to trigger some
 	 * activity on USB bus #1.
 	 */
 	sleep(2);

 	n = __atomic_load_n(&cover[0], __ATOMIC_RELAXED);
 	for (i = 0; i < n; i++)
 		printf("0x%lx\n", cover[i + 1]);
 	if (ioctl(fd, KCOV_DISABLE, 0))
 		perror("ioctl"), exit(1);
 	if (munmap(cover, COVER_SIZE * sizeof(unsigned long)))
 		perror("munmap"), exit(1);
 	if (close(fd))
 		perror("close"), exit(1);
 	return 0;
     }
	kcov: code coverage for fuzzing
	===============================

	kcov exposes kernel code coverage information in a form suitable for coverage-
	guided fuzzing (randomized testing). Coverage data of a running kernel is
	exported via the "kcov" debugfs file. Coverage collection is enabled on a task
	basis, and thus it can capture precise coverage of a single system call.

	Note that kcov does not aim to collect as much coverage as possible. It aims
	to collect more or less stable coverage that is function of syscall inputs.
	To achieve this goal it does not collect coverage in soft/hard interrupts
	and instrumentation of some inherently non-deterministic parts of kernel is
	disabled (e.g. scheduler, locking).

	kcov is also able to collect comparison operands from the instrumented code
	(this feature currently requires that the kernel is compiled with clang).

	Prerequisites
	-------------

	Configure the kernel with::

	CONFIG_KCOV=y

	CONFIG_KCOV requires gcc 6.1.0 or later.

	If the comparison operands need to be collected, set::

	CONFIG_KCOV_ENABLE_COMPARISONS=y

	Profiling data will only become accessible once debugfs has been mounted::

	mount -t debugfs none /sys/kernel/debug

	Coverage collection
	-------------------

	The following program demonstrates coverage collection from within a test
	program using kcov:

	.. code-block:: c

	#include <stdio.h>
	#include <stddef.h>
	#include <stdint.h>
	#include <stdlib.h>
	#include <sys/types.h>
	#include <sys/stat.h>
	#include <sys/ioctl.h>
	#include <sys/mman.h>
	#include <unistd.h>
	#include <fcntl.h>

	#define KCOV_INIT_TRACE _IOR('c', 1, unsigned long)
	#define KCOV_ENABLE _IO('c', 100)
	#define KCOV_DISABLE _IO('c', 101)
	#define COVER_SIZE (64<<10)

	#define KCOV_TRACE_PC 0
	#define KCOV_TRACE_CMP 1

	int main(int argc, char **argv)
	{
	int fd;
	unsigned long *cover, n, i;

	/* A single fd descriptor allows coverage collection on a single
	* thread.
	*/
	fd = open("/sys/kernel/debug/kcov", O_RDWR);
	if (fd == -1)
	perror("open"), exit(1);
	/* Setup trace mode and trace size. */
	if (ioctl(fd, KCOV_INIT_TRACE, COVER_SIZE))
	perror("ioctl"), exit(1);
	/* Mmap buffer shared between kernel- and user-space. */
	cover = (unsigned long)mmap(NULL, COVER_SIZE sizeof(unsigned long),
	PROT_READ \| PROT_WRITE, MAP_SHARED, fd, 0);
	if ((void*)cover == MAP_FAILED)
	perror("mmap"), exit(1);
	/* Enable coverage collection on the current thread. */
	if (ioctl(fd, KCOV_ENABLE, KCOV_TRACE_PC))
	perror("ioctl"), exit(1);
	/* Reset coverage from the tail of the ioctl() call. */
	__atomic_store_n(&cover[0], 0, __ATOMIC_RELAXED);
	/* That's the target syscal call. */
	read(-1, NULL, 0);
	/* Read number of PCs collected. */
	n = __atomic_load_n(&cover[0], __ATOMIC_RELAXED);
	for (i = 0; i < n; i++)
	printf("0x%lx\n", cover[i + 1]);
	/* Disable coverage collection for the current thread. After this call
	* coverage can be enabled for a different thread.
	*/
	if (ioctl(fd, KCOV_DISABLE, 0))
	perror("ioctl"), exit(1);
	/* Free resources. */
	if (munmap(cover, COVER_SIZE * sizeof(unsigned long)))
	perror("munmap"), exit(1);
	if (close(fd))
	perror("close"), exit(1);
	return 0;
	}

	After piping through addr2line output of the program looks as follows::

	SyS_read
	fs/read_write.c:562
	__fdget_pos
	fs/file.c:774
	__fget_light
	fs/file.c:746
	__fget_light
	fs/file.c:750
	__fget_light
	fs/file.c:760
	__fdget_pos
	fs/file.c:784
	SyS_read
	fs/read_write.c:562

	If a program needs to collect coverage from several threads (independently),
	it needs to open /sys/kernel/debug/kcov in each thread separately.

	The interface is fine-grained to allow efficient forking of test processes.
	That is, a parent process opens /sys/kernel/debug/kcov, enables trace mode,
	mmaps coverage buffer and then forks child processes in a loop. Child processes
	only need to enable coverage (disable happens automatically on thread end).

	Comparison operands collection
	------------------------------

	Comparison operands collection is similar to coverage collection:

	.. code-block:: c

	/* Same includes and defines as above. */

	/* Number of 64-bit words per record. */
	#define KCOV_WORDS_PER_CMP 4

	/*
	* The format for the types of collected comparisons.
	*
	* Bit 0 shows whether one of the arguments is a compile-time constant.
	* Bits 1 & 2 contain log2 of the argument size, up to 8 bytes.
	*/

	#define KCOV_CMP_CONST (1 << 0)
	#define KCOV_CMP_SIZE(n) ((n) << 1)
	#define KCOV_CMP_MASK KCOV_CMP_SIZE(3)

	int main(int argc, char **argv)
	{
	int fd;
	uint64_t *cover, type, arg1, arg2, is_const, size;
	unsigned long n, i;

	fd = open("/sys/kernel/debug/kcov", O_RDWR);
	if (fd == -1)
	perror("open"), exit(1);
	if (ioctl(fd, KCOV_INIT_TRACE, COVER_SIZE))
	perror("ioctl"), exit(1);
	/*
	* Note that the buffer pointer is of type uint64_t*, because all
	* the comparison operands are promoted to uint64_t.
	*/
	cover = (uint64_t )mmap(NULL, COVER_SIZE sizeof(unsigned long),
	PROT_READ \| PROT_WRITE, MAP_SHARED, fd, 0);
	if ((void*)cover == MAP_FAILED)
	perror("mmap"), exit(1);
	/* Note KCOV_TRACE_CMP instead of KCOV_TRACE_PC. */
	if (ioctl(fd, KCOV_ENABLE, KCOV_TRACE_CMP))
	perror("ioctl"), exit(1);
	__atomic_store_n(&cover[0], 0, __ATOMIC_RELAXED);
	read(-1, NULL, 0);
	/* Read number of comparisons collected. */
	n = __atomic_load_n(&cover[0], __ATOMIC_RELAXED);
	for (i = 0; i < n; i++) {
	type = cover[i * KCOV_WORDS_PER_CMP + 1];
	/* arg1 and arg2 - operands of the comparison. */
	arg1 = cover[i * KCOV_WORDS_PER_CMP + 2];
	arg2 = cover[i * KCOV_WORDS_PER_CMP + 3];
	/* ip - caller address. */
	ip = cover[i * KCOV_WORDS_PER_CMP + 4];
	/* size of the operands. */
	size = 1 << ((type & KCOV_CMP_MASK) >> 1);
	/* is_const - true if either operand is a compile-time constant.*/
	is_const = type & KCOV_CMP_CONST;
	printf("ip: 0x%lx type: 0x%lx, arg1: 0x%lx, arg2: 0x%lx, "
	"size: %lu, %s\n",
	ip, type, arg1, arg2, size,
	is_const ? "const" : "non-const");
	}
	if (ioctl(fd, KCOV_DISABLE, 0))
	perror("ioctl"), exit(1);
	/* Free resources. */
	if (munmap(cover, COVER_SIZE * sizeof(unsigned long)))
	perror("munmap"), exit(1);
	if (close(fd))
	perror("close"), exit(1);
	return 0;
	}

	Note that the kcov modes (coverage collection or comparison operands) are
	mutually exclusive.

	Remote coverage collection
	--------------------------

	With KCOV_ENABLE coverage is collected only for syscalls that are issued
	from the current process. With KCOV_REMOTE_ENABLE it's possible to collect
	coverage for arbitrary parts of the kernel code, provided that those parts
	are annotated with kcov_remote_start()/kcov_remote_stop().

	This allows to collect coverage from two types of kernel background
	threads: the global ones, that are spawned during kernel boot in a limited
	number of instances (e.g. one USB hub_event() worker thread is spawned per
	USB HCD); and the local ones, that are spawned when a user interacts with
	some kernel interface (e.g. vhost workers).

	To enable collecting coverage from a global background thread, a unique
	global handle must be assigned and passed to the corresponding
	kcov_remote_start() call. Then a userspace process can pass a list of such
	handles to the KCOV_REMOTE_ENABLE ioctl in the handles array field of the
	kcov_remote_arg struct. This will attach the used kcov device to the code
	sections, that are referenced by those handles.

	Since there might be many local background threads spawned from different
	userspace processes, we can't use a single global handle per annotation.
	Instead, the userspace process passes a non-zero handle through the
	common_handle field of the kcov_remote_arg struct. This common handle gets
	saved to the kcov_handle field in the current task_struct and needs to be
	passed to the newly spawned threads via custom annotations. Those threads
	should in turn be annotated with kcov_remote_start()/kcov_remote_stop().

	Internally kcov stores handles as u64 integers. The top byte of a handle
	is used to denote the id of a subsystem that this handle belongs to, and
	the lower 4 bytes are used to denote the id of a thread instance within
	that subsystem. A reserved value 0 is used as a subsystem id for common
	handles as they don't belong to a particular subsystem. The bytes 4-7 are
	currently reserved and must be zero. In the future the number of bytes
	used for the subsystem or handle ids might be increased.

	When a particular userspace proccess collects coverage by via a common
	handle, kcov will collect coverage for each code section that is annotated
	to use the common handle obtained as kcov_handle from the current
	task_struct. However non common handles allow to collect coverage
	selectively from different subsystems.

	.. code-block:: c

	struct kcov_remote_arg {
	unsigned trace_mode;
	unsigned area_size;
	unsigned num_handles;
	uint64_t common_handle;
	uint64_t handles[0];
	};

	#define KCOV_INIT_TRACE _IOR('c', 1, unsigned long)
	#define KCOV_DISABLE _IO('c', 101)
	#define KCOV_REMOTE_ENABLE _IOW('c', 102, struct kcov_remote_arg)

	#define COVER_SIZE (64 << 10)

	#define KCOV_TRACE_PC 0

	#define KCOV_SUBSYSTEM_COMMON (0x00ull << 56)
	#define KCOV_SUBSYSTEM_USB (0x01ull << 56)

	#define KCOV_SUBSYSTEM_MASK (0xffull << 56)
	#define KCOV_INSTANCE_MASK (0xffffffffull)

	static inline __u64 kcov_remote_handle(__u64 subsys, __u64 inst)
	{
	if (subsys & ~KCOV_SUBSYSTEM_MASK \|\| inst & ~KCOV_INSTANCE_MASK)
	return 0;
	return subsys \| inst;
	}

	#define KCOV_COMMON_ID 0x42
	#define KCOV_USB_BUS_NUM 1

	int main(int argc, char **argv)
	{
	int fd;
	unsigned long *cover, n, i;
	struct kcov_remote_arg *arg;

	fd = open("/sys/kernel/debug/kcov", O_RDWR);
	if (fd == -1)
	perror("open"), exit(1);
	if (ioctl(fd, KCOV_INIT_TRACE, COVER_SIZE))
	perror("ioctl"), exit(1);
	cover = (unsigned long)mmap(NULL, COVER_SIZE sizeof(unsigned long),
	PROT_READ \| PROT_WRITE, MAP_SHARED, fd, 0);
	if ((void*)cover == MAP_FAILED)
	perror("mmap"), exit(1);

	/* Enable coverage collection via common handle and from USB bus #1. */
	arg = calloc(1, sizeof(*arg) + sizeof(uint64_t));
	if (!arg)
	perror("calloc"), exit(1);
	arg->trace_mode = KCOV_TRACE_PC;
	arg->area_size = COVER_SIZE;
	arg->num_handles = 1;
	arg->common_handle = kcov_remote_handle(KCOV_SUBSYSTEM_COMMON,
	KCOV_COMMON_ID);
	arg->handles[0] = kcov_remote_handle(KCOV_SUBSYSTEM_USB,
	KCOV_USB_BUS_NUM);
	if (ioctl(fd, KCOV_REMOTE_ENABLE, arg))
	perror("ioctl"), free(arg), exit(1);
	free(arg);

	/*
	* Here the user needs to trigger execution of a kernel code section
	* that is either annotated with the common handle, or to trigger some
	* activity on USB bus #1.
	*/
	sleep(2);

	n = __atomic_load_n(&cover[0], __ATOMIC_RELAXED);
	for (i = 0; i < n; i++)
	printf("0x%lx\n", cover[i + 1]);
	if (ioctl(fd, KCOV_DISABLE, 0))
	perror("ioctl"), exit(1);
	if (munmap(cover, COVER_SIZE * sizeof(unsigned long)))
	perror("munmap"), exit(1);
	if (close(fd))
	perror("close"), exit(1);
	return 0;
	}