KCOV(4) | Device Drivers Manual | 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>

May 28, 2019 | NetBSD 11.99