kernel: add kcov code coverage

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

disbled (e.g. scheduler, locking).

Usage:

======

Configure 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:

#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).

CFLAGS_KERNEL	=

AFLAGS_KERNEL	=

CFLAGS_GCOV	= -fprofile-arcs -ftest-coverage

CFLAGS_KCOV	= -fsanitize-coverage=trace-pc

# Use USERINCLUDE when you must reference the UAPI directories only.

export HOSTCXX HOSTCXXFLAGS LDFLAGS_MODULE CHECK CHECKFLAGS

export KBUILD_CPPFLAGS NOSTDINC_FLAGS LINUXINCLUDE OBJCOPYFLAGS LDFLAGS

export KBUILD_CFLAGS CFLAGS_KERNEL CFLAGS_MODULE CFLAGS_GCOV CFLAGS_KASAN CFLAGS_UBSAN

export KBUILD_CFLAGS CFLAGS_KERNEL CFLAGS_MODULE CFLAGS_GCOV CFLAGS_KCOV CFLAGS_KASAN CFLAGS_UBSAN

export KBUILD_AFLAGS AFLAGS_KERNEL AFLAGS_MODULE

export KBUILD_AFLAGS_MODULE KBUILD_CFLAGS_MODULE KBUILD_LDFLAGS_MODULE

export KBUILD_AFLAGS_KERNEL KBUILD_CFLAGS_KERNEL

endif

KBUILD_CFLAGS += $(stackp-flag)

ifdef CONFIG_KCOV

  ifeq ($(call cc-option, $(CFLAGS_KCOV)),)

    $(warning Cannot use CONFIG_KCOV: \

             -fsanitize-coverage=trace-pc is not supported by compiler)

    CFLAGS_KCOV =

  endif

endif

ifeq ($(cc-name),clang)

KBUILD_CPPFLAGS += $(call cc-option,-Qunused-arguments,)

KBUILD_CPPFLAGS += $(call cc-option,-Wno-unknown-warning-option,)

	select ARCH_HAS_ELF_RANDOMIZE

	select ARCH_HAS_FAST_MULTIPLIER

	select ARCH_HAS_GCOV_PROFILE_ALL

	select ARCH_HAS_KCOV			if X86_64

	select ARCH_HAS_PMEM_API		if X86_64

	select ARCH_HAS_MMIO_FLUSH

	select ARCH_HAS_SG_CHAIN

KASAN_SANITIZE			:= n

OBJECT_FILES_NON_STANDARD	:= y

# Kernel does not boot with kcov instrumentation here.

# One of the problems observed was insertion of __sanitizer_cov_trace_pc()

# callback into middle of per-cpu data enabling code. Thus the callback observed

# inconsistent state and crashed. We are interested mostly in syscall coverage,

# so boot code is not interesting anyway.

KCOV_INSTRUMENT		:= n

# If you want to preset the SVGA mode, uncomment the next line and

# set SVGA_MODE to whatever number you want.

# Set it to -DSVGA_MODE=NORMAL_VGA if you just want the EGA/VGA mode.

KASAN_SANITIZE			:= n

OBJECT_FILES_NON_STANDARD	:= y

# Prevents link failures: __sanitizer_cov_trace_pc() is not linked in.

KCOV_INSTRUMENT		:= n

targets := vmlinux vmlinux.bin vmlinux.bin.gz vmlinux.bin.bz2 vmlinux.bin.lzma \

	vmlinux.bin.xz vmlinux.bin.lzo vmlinux.bin.lz4