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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).

Usage
-----

Configure the kernel with::

        CONFIG_KCOV=y

CONFIG_KCOV requires gcc built on revision 231296 or later.
Profiling data will only become accessible once debugfs has been mounted::

        mount -t debugfs none /sys/kernel/debug

The following program demonstrates kcov usage from within a test program:

.. 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)

    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, 0))
		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).