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).
Configure the kernel with:
CONFIG_KCOV=yCONFIG_KCOV requires gcc 6.1.0 or later.
If the comparison operands need to be collected, set:
CONFIG_KCOV_ENABLE_COMPARISONS=yProfiling data will only become accessible once debugfs has been mounted:
mount -t debugfs none /sys/kernel/debugThe following program demonstrates coverage collection from within a test program using kcov:
#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:562If 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 is similar to coverage collection:
/* 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.
With KCOV_REMOTE_ENABLE it's possible to collect coverage from arbitrary kernel threads. For that the targeted section of kernel code needs to be annotated with kcov_remote_start(unique_id) and kcov_remote_stop(). Then ioctl(kcov, KCOV_REMOTE_ENABLE) can be used to make kcov start collecting coverage from that code section. Multiple ids can be targeted with the same kcov device simultaneously.
This allows to collect coverage from both types of kernel background threads: the global ones, that are spawned during kernel boot and are always running (e.g. USB hub_event); and the local ones, that are spawned when a user interacts with some kernel interfaces (e.g. vhost). Collecting coverage from both types of kernel threads requires custom annotations.
To collect coverage from a global background thread add kcov_remote_start/ kcov_remote_stop annotation with a unique id to that thread's code and then pass this id in the handles array field of the kcov_remote_arg struct.
A tracking id for local background threads is passed through the common_handle field of the kcov_remote_arg struct. This id gets saved to the kcov_handle field in the thread that created kcov and needs to be passed to the newly spawned threads via custom annotations. Those threads should be in turn annotated with kcov_remote_start/kcov_remote_stop.
struct kcov_remote_arg {
unsigned trace_mode;
unsigned area_size;
unsigned num_handles;
uint64_t common_handle;
uint64_t handles[0];
};
#define KCOV_REMOTE_MAX_HANDLES 0x10000
#define KCOV_INIT_TRACE _IOR('c', 1, unsigned long)
#define KCOV_ENABLE _IO('c', 100)
#define KCOV_DISABLE _IO('c', 101)
#define KCOV_REMOTE_ENABLE _IOW('c', 102, struct kcov_remote_arg)
#define KCOV_REMOTE_HANDLE_USB 0x4242000000000000ull
static inline __u64 kcov_remote_handle_usb(int bus)
{
return KCOV_REMOTE_HANDLE_USB + (__u64)bus;
}
#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;
uint64_t handle;
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 from the 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->handles[0] = kcov_remote_handle_usb(1);
if (ioctl(fd, KCOV_REMOTE_ENABLE, arg))
perror("ioctl"), free(arg), exit(1);
free(arg);
/* Sleep. The user needs to trigger some activity on the 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;
}