KCOV(4)

NAME
kcov - kernel code coverage tracing

SYNOPSIS
options KCOV

#include <sys/kcov.h>

DESCRIPTION
The kcov driver implements collection of code coverage inside the kernel. It can be enabled on a per thread basis from userland, allowing the kernel program counter to be collected during syscalls triggered by the same thread.

The kcov descriptors (KD) are allocated during open(2), and are associated with a file descriptor. A thread can enable the kcov device. When this happens, this thread becomes the owner of the kcov descriptors (KD), and no thread can disable this KD except the owner.

A kcov descriptor (KD) is freed when its file descriptor is closed iff the KD is not active on a thread. If it is, we ask the thread to free it when it exits.

The collected coverage can be accessed by mapping the device using mmap(2). The buffers are mapped without risk that the kernel frees a buffer still mapped in a process.

By default, kcov is not enabled but requires the compile-time configuration makeoptions KCOV options KCOV to be present, see options(4).

The following ioctl(2) calls are provided:

KCOV_IOC_SETBUFSIZE uint64_t *nentries
    Allocate a coverage buffer with a capacity of nentries. The buffer can be accessed using mmap(2) whereas the returned pointer must be interpreted as an array of kcov_int_t entries. Note that kcov_int_t is volatile. The first entry contains the number of entries in the array, excluding the first entry.

KCOV_IOC_ENABLE int *mode
    Enable code coverage tracing for the current thread. The mode must be one of the following:

    KCOV_MODE_NONE
    No trace selected. This option is useful for testing the kcov device.

    KCOV_MODE_TRACE_PC
    Trace the kernel program counter.

    KCOV_MODE_TRACE_CMP
    Trace comparison instructions and switch statements. For switch statements, the number of traced comparison instructions is equal to the number of switch cases. Each traced comparison instruction is represented by 4 entries in the coverage buffer:

    1.  A mask where the least significant bit is set if one of the comparison operands is a compile-time constant, which is always true for switch statements. The remaining bits represents the log2 size of the operands, ranging from 0 to 3.

    2.  First comparison operand. For switch statements, this operand corresponds to the case value.

    3.  Second comparison operand. For switch statements, this operand corresponds to the value passed to switch.

    4.  Kernel program counter where the comparison instruction took place.

    In this mode, the first entry in the coverage buffer reflects the number of traced comparison instructions. Thus, the effective number of entries in the coverage buffer is given by multiplying the first entry by 4.

KCOV_IOC_DISABLE void
    Disable code coverage tracing for the current thread.

FILES
/dev/kcov
    Default device node.

EXAMPLES
In the following example, the read(2) syscall is traced and the coverage displayed, which in turn can be passed to addr2line(1) in order to translate the kernel program counter into the file name and line number it corresponds to.

    #include <err.h>
    #include <fcntl.h>
    #include <stdio.h>
    #include <stdlib.h>
    #include <unistd.h>
    #include <sys/ioccom.h>
    #include <sys/ioctl.h>
    #include <sys/mman.h>
    #include <sys/kcov.h>
    int
    main(void)
    {
    	kcov_int_t *cover, i, n;
    	uint64_t size = 1024 * 100;
    	int fd;
    	int mode;
    	fd = open("/dev/kcov", O_RDWR);
    	if (fd == -1)
    		err(1, "open");
    	if (ioctl(fd, KCOV_IOC_SETBUFSIZE, &size) == -1)
    		err(1, "ioctl: KCOV_IOC_SETBUFSIZE");
    	cover = mmap(NULL, size * KCOV_ENTRY_SIZE,
    	    PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
    	if (cover == MAP_FAILED)
    		err(1, "mmap");
    	mode = KCOV_MODE_TRACE_PC;
    	if (ioctl(fd, KCOV_IOC_ENABLE, &mode) == -1)
    		err(1, "ioctl: KCOV_IOC_ENABLE");
    	cover[0] = 0;
    	read(-1, NULL, 0); /* syscall paths to be traced */
    	n = cover[0];
    	if (ioctl(fd, KCOV_IOC_DISABLE) == -1)
    		err(1, "ioctl: KCOV_IOC_DISABLE");
    	for (i = 0; i < n; i++)
    		printf("%p\n", (void *)cover[i + 1]);
    	if (munmap(cover, size * KCOV_ENTRY_SIZE) == -1)
    		err(1, "munmap");
    	close(fd);
    	return 0;
    }

SEE ALSO
options(4)

HISTORY
The kcov driver was initially developed in Linux. A driver based on the same concept was then implemented in NetBSD 9.

AUTHORS
Siddharth Muralee <siddharth.muralee@gmail.com>