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KCOV(4)		       FreeBSD Kernel Interfaces Manual		       KCOV(4)

     kcov -- interface for collecting kernel code coverage information

     To	compile	KCOV into the kernel, place the	following lines	in your	kernel
     configuration file:

	   options COVERAGE
	   options KCOV

     The following header file defines the application interface provided by

     #include <sys/kcov.h>

     kcov is a module that enables collection of code coverage information
     from the kernel.  It relies on code instrumentation enabled by the
     COVERAGE kernel option.  When kcov	is enabled by a	user-mode thread, it
     collects coverage information only	for that thread, excluding hard	inter-
     rupt handlers.  As	a result, kcov is not suited to	collect	comprehensive
     coverage data for the entire kernel; its main purpose is to provide input
     for coverage-guided system	call fuzzers.

     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 cov-
     erage 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 fol-
     lowed 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 in-
     formation about the results of c(7) `if' or `switch' statements, for ex-
     ample.  In	this mode each record consists of four 64-bit unsigned inte-
     gers.  The	first integer is a bitmask defining attributes of the vari-
     ables 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	com-

     Applications interact with	kcov using the ioctl(2)	system call.  Each
     thread making use of kcov must use	a separate file	descriptor for
     /dev/kcov.	 The following ioctls are defined:

     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

	     Disable coverage tracing for the calling thread.

     kcov creates the /dev/kcov	device file.

     The following code	sample collects	information about basic	block coverage
     for kernel	code executed while printing `Hello, world'.

     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]);
     The output	of this	program	can be approximately mapped to line numbers in
     kernel source code:

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

     ioctl(2), mmap(2)

     kcov first	appeared in FreeBSD 13.0.

     The FreeBSD implementation	of kcov	does not yet support remote tracing.

FreeBSD	13.0			August 18, 2020			  FreeBSD 13.0


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