NAME

SYNOPSIS

The following header file defines the application interface provided by KCOV:

#include <sys/kcov.h>

DESCRIPTION

In typical usage, a user-mode thread first allocates a chunk of memory to be shared with kcov. The thread then enables coverage tracing, with coverage data being written by the kernel to the shared memory region. When tracing is disabled, the kernel relinquishes its access to the shared memory region, and the written coverage data may be consumed.

The shared memory buffer can be treated as a 64-bit unsigned integer followed by an array of records. The integer records the number of valid records and is updated by the kernel as coverage information is recorded. The state of the tracing buffer can be reset by writing the value 0 to this field. The record layout depends on the tracing mode set using the KIOENABLE ioctl.

Two tracing modes are implemented, KCOV_MODE_TRACE_PC and KCOV_MODE_TRACE_CMP. PC-tracing records a program counter value for each basic block executed while tracing is enabled. In this mode, each record is a single 64-bit unsigned integer containing the program counter value. Comparison tracing provides information about data flow; information about dynamic variable comparisons is recorded. Such records provide information about the results of c(7) ‘if’ or ‘switch’ statements, for example. In this mode each record consists of four 64-bit unsigned integers. The first integer is a bitmask defining attributes of the variables involved in the comparison. KCOV_CMP_CONST is set if one of the variables has a constant value at compile-time, and KCOV_CMP_SIZE(n) specifies the width of the variables:

KCOV_CMP_SIZE(0):

a comparison of 8-bit integers

KCOV_CMP_SIZE(1):

a comparison of 16-bit integers

KCOV_CMP_SIZE(2):

a comparison of 32-bit integers

KCOV_CMP_SIZE(3):

a comparison of 64-bit integers

The second and third fields record the values of the two variables, and the fourth and final field stores the program counter value of the comparison.

IOCTL INTERFACE

KIOSETBUFSIZE size_t entries

Set the size of the tracing buffer in units of KCOV_ENTRY_SIZE. The buffer may then be mapped into the calling thread's address space by calling mmap(2) on the kcov device file.

KIOENABLE int mode

Enable coverage tracing for the calling thread. Valid modes are KCOV_MODE_TRACE_PC and KCOV_MODE_TRACE_CMP.

KIODISABLE

Disable coverage tracing for the calling thread.

FILES

EXAMPLES

size_t sz;
uint64_t *buf;
int fd;

fd = open("/dev/kcov", O_RDWR);
if (fd == -1)
	err(1, "open(/dev/kcov)");
sz = 1ul << 20; /* 1MB */
if (ioctl(fd, KIOSETBUFSIZE, sz / KCOV_ENTRY_SIZE) != 0)
	err(1, "ioctl(KIOSETBUFSIZE)");
buf = mmap(NULL, sz, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
if (buf == MAP_FAILED)
	err(1, "mmap");

/* Enable PC tracing. */
if (ioctl(fd, KIOENABLE, KCOV_MODE_TRACE_PC) != 0)
	err(1, "ioctl(KIOENABLE)");

/* Clear trace records from the preceding ioctl() call. */
buf[0] = 0;

printf("Hello, world!\n");

/* Disable PC tracing. */
if (ioctl(fd, KIODISABLE, 0) != 0)
	err(1, "ioctl(KIODISABLE)");

for (uint64_t i = 1; i < buf[0]; i++)
	printf("%#jx\n", (uintmax_t)buf[i]);

# ./kcov-test | sed 1d | addr2line -e /usr/lib/debug/boot/kernel/kernel.debug

SEE ALSO

HISTORY

BUGS