1.\"- 2.\" Copyright (c) 2020 The FreeBSD Foundation 3.\" 4.\" This documentation was written by Mark Johnston under sponsorship from 5.\" the FreeBSD Foundation. 6.\" 7.\" Redistribution and use in source and binary forms, with or without 8.\" modification, are permitted provided that the following conditions 9.\" are met: 10.\" 1. Redistributions of source code must retain the above copyright 11.\" notice, this list of conditions and the following disclaimer. 12.\" 2. Redistributions in binary form must reproduce the above copyright 13.\" notice, this list of conditions and the following disclaimer in the 14.\" documentation and/or other materials provided with the distribution. 15.\" 16.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND 17.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 18.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 19.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE 20.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 21.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 22.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 23.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 24.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 25.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 26.\" SUCH DAMAGE. 27.\" 28.\" $FreeBSD$ 29.\" 30.Dd August 18, 2020 31.Dt KCOV 4 32.Os 33.Sh NAME 34.Nm kcov 35.Nd interface for collecting kernel code coverage information 36.Sh SYNOPSIS 37To compile KCOV into the kernel, place the following lines in your kernel 38configuration file: 39.Bd -ragged -offset indent 40.Cd "options COVERAGE" 41.Cd "options KCOV" 42.Ed 43.Pp 44The following header file defines the application interface provided 45by KCOV: 46.Pp 47.In sys/kcov.h 48.Sh DESCRIPTION 49.Nm 50is a module that enables collection of code coverage information from the 51kernel. 52It relies on code instrumentation enabled by the 53.Dv COVERAGE 54kernel option. 55When 56.Nm 57is enabled by a user-mode thread, it collects coverage information only for 58that thread, excluding hard interrupt handlers. 59As a result, 60.Nm 61is not suited to collect comprehensive coverage data for the entire kernel; 62its main purpose is to provide input for coverage-guided system call fuzzers. 63.Pp 64In typical usage, a user-mode thread first allocates a chunk of memory to be 65shared with 66.Nm . 67The thread then enables coverage tracing, with coverage data being written by 68the kernel to the shared memory region. 69When tracing is disabled, the kernel relinquishes its access to the shared 70memory region, and the written coverage data may be consumed. 71.Pp 72The shared memory buffer can be treated as a 64-bit unsigned integer followed 73by an array of records. 74The integer records the number of valid records and is updated by the kernel as 75coverage information is recorded. 76The state of the tracing buffer can be reset by writing the value 0 to this 77field. 78The record layout depends on the tracing mode set using the 79.Dv KIOENABLE 80ioctl. 81.Pp 82Two tracing modes are implemented, 83.Dv KCOV_MODE_TRACE_PC 84and 85.Dv KCOV_MODE_TRACE_CMP . 86PC-tracing records a program counter value for each basic block executed while 87tracing is enabled. 88In this mode, each record is a single 64-bit unsigned integer containing the 89program counter value. 90Comparison tracing provides information about data flow; information about 91dynamic variable comparisons is recorded. 92Such records provide information about the results of 93.Xr c 7 94.Ql if 95or 96.Ql switch 97statements, for example. 98In this mode each record consists of four 64-bit unsigned integers. 99The first integer is a bitmask defining attributes of the variables involved in 100the comparison. 101.Dv KCOV_CMP_CONST 102is set if one of the variables has a constant value at compile-time, and 103.Dv KCOV_CMP_SIZE(n) 104specifies the width of the variables: 105.Pp 106.Bl -inset -offset indent -compact 107.It Dv KCOV_CMP_SIZE(0) : 108a comparison of 8-bit integers 109.It Dv KCOV_CMP_SIZE(1) : 110a comparison of 16-bit integers 111.It Dv KCOV_CMP_SIZE(2) : 112a comparison of 32-bit integers 113.It Dv KCOV_CMP_SIZE(3) : 114a comparison of 64-bit integers 115.El 116.Pp 117The second and third fields record the values of the two variables, and the 118fourth and final field stores the program counter value of the comparison. 119.Sh IOCTL INTERFACE 120Applications interact with 121.Nm 122using the 123.Xr ioctl 2 124system call. 125Each thread making use of 126.Nm 127must use a separate file descriptor for 128.Fa /dev/kcov . 129The following ioctls are defined: 130.Bl -tag -width indent 131.It Dv KIOSETBUFSIZE Fa size_t entries 132Set the size of the tracing buffer in units of 133.Dv KCOV_ENTRY_SIZE . 134The buffer may then be mapped into the calling thread's address space by 135calling 136.Xr mmap 2 137on the 138.Nm 139device file. 140.It Dv KIOENABLE Fa int mode 141Enable coverage tracing for the calling thread. 142Valid modes are 143.Dv KCOV_MODE_TRACE_PC 144and 145.Dv KCOV_MODE_TRACE_CMP . 146.It Dv KIODISABLE 147Disable coverage tracing for the calling thread. 148.El 149.Sh FILES 150.Nm 151creates the 152.Pa /dev/kcov 153device file. 154.Sh EXAMPLES 155The following code sample collects information about basic block coverage for 156kernel code executed while printing 157.Ql "Hello, world" . 158.Bd -literal 159size_t sz; 160uint64_t *buf; 161int fd; 162 163fd = open("/dev/kcov", O_RDWR); 164if (fd == -1) 165 err(1, "open(/dev/kcov)"); 166sz = 1ul << 20; /* 1MB */ 167if (ioctl(fd, KIOSETBUFSIZE, sz / KCOV_ENTRY_SIZE) != 0) 168 err(1, "ioctl(KIOSETBUFSIZE)"); 169buf = mmap(NULL, sz, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0); 170if (buf == MAP_FAILED) 171 err(1, "mmap"); 172 173/* Enable PC tracing. */ 174if (ioctl(fd, KIOENABLE, KCOV_MODE_TRACE_PC) != 0) 175 err(1, "ioctl(KIOENABLE)"); 176 177/* Clear trace records from the preceding ioctl() call. */ 178buf[0] = 0; 179 180printf("Hello, world!\\n"); 181 182/* Disable PC tracing. */ 183if (ioctl(fd, KIODISABLE, 0) != 0) 184 err(1, "ioctl(KIODISABLE)"); 185 186for (uint64_t i = 1; i < buf[0]; i++) 187 printf("%#jx\\n", (uintmax_t)buf[i]); 188.Ed 189The output of this program can be approximately mapped to line numbers 190in kernel source code: 191.Bd -literal 192# ./kcov-test | sed 1d | addr2line -e /usr/lib/debug/boot/kernel/kernel.debug 193.Ed 194.Sh SEE ALSO 195.Xr ioctl 2 , 196.Xr mmap 2 197.Sh HISTORY 198.Nm 199first appeared in 200.Fx 13.0 . 201.Sh BUGS 202The 203.Fx 204implementation of 205.Nm 206does not yet support remote tracing. 207