Add support for the Clang Coverage Sanitizer in the kernel (KCOV).

When building with KCOV enabled the compiler will insert function calls
to probes allowing us to trace the execution of the kernel from userspace.
These probes are on function entry (trace-pc) and on comparison operations
(trace-cmp).

Userspace can enable the use of these probes on a single kernel thread with
an ioctl interface. It can allocate space for the probe with KIOSETBUFSIZE,
then mmap the allocated buffer and enable tracing with KIOENABLE, with the
trace mode being passed in as the int argument. When complete KIODISABLE
is used to disable tracing.

The first item in the buffer is the number of trace event that have
happened. Userspace can write 0 to this to reset the tracing, and is
expected to do so on first use.

The format of the buffer depends on the trace mode. When in PC tracing just
the return address of the probe is stored. Under comparison tracing the
comparison type, the two arguments, and the return address are traced. The
former method uses on entry per trace event, while the later uses 4. As
such they are incompatible so only a single mode may be enabled.

KCOV is expected to help fuzzing the kernel, and while in development has
already found a number of issues. It is required for the syzkaller system
call fuzzer [1]. Other kernel fuzzers could also make use of it, either
with the current interface, or by extending it with new modes.

A man page is currently being worked on and is expected to be committed
soon, however having the code in the kernel now is useful for other
developers to use.

[1] https://github.com/google/syzkaller

Submitted by:	Mitchell Horne <mhorne063@gmail.com> (Earlier version)
Reviewed by:	kib
Testing by:	tuexen
Sponsored by:	DARPA, AFRL
Sponsored by:	The FreeBSD Foundation (Mitchell Horne)
Differential Revision:	https://reviews.freebsd.org/D14599

2 files changed, 678 insertions, 2 deletions

diff --git a/sys/kern/kern_kcov.c b/sys/kern/kern_kcov.c
new file mode 100644
index 000000000000..c90fdc36d6a1
--- /dev/null
+++ b/sys/kern/kern_kcov.c
@@ -0,0 +1,676 @@
+/*-
+ * SPDX-License-Identifier: BSD-2-Clause-FreeBSD
+ *
+ * Copyright (C) 2018 The FreeBSD Foundation. All rights reserved.
+ * Copyright (C) 2018, 2019 Andrew Turner
+ *
+ * This software was developed by Mitchell Horne under sponsorship of
+ * the FreeBSD Foundation.
+ *
+ * This software was developed by SRI International and the University of
+ * Cambridge Computer Laboratory under DARPA/AFRL contract FA8750-10-C-0237
+ * ("CTSRD"), as part of the DARPA CRASH research programme.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
+ * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+ * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+ * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+ * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ *
+ * $FreeBSD$
+ */
+
+#include <sys/cdefs.h>
+__FBSDID("$FreeBSD$");
+
+#include <sys/param.h>
+#include <sys/conf.h>
+#include <sys/file.h>
+#include <sys/kcov.h>
+#include <sys/kernel.h>
+#include <sys/lock.h>
+#include <sys/malloc.h>
+#include <sys/mman.h>
+#include <sys/mutex.h>
+#include <sys/proc.h>
+#include <sys/rwlock.h>
+#include <sys/stat.h>
+#include <sys/sysctl.h>
+#include <sys/systm.h>
+#include <sys/types.h>
+
+#include <vm/vm.h>
+#include <vm/vm_extern.h>
+#include <vm/vm_object.h>
+#include <vm/vm_page.h>
+#include <vm/vm_pager.h>
+
+#include <vm/pmap.h>
+
+MALLOC_DEFINE(M_KCOV_INFO, "kcovinfo", "KCOV info type");
+
+#define	KCOV_ELEMENT_SIZE	sizeof(uint64_t)
+
+/*
+ * To know what the code can safely perform at any point in time we use a
+ * state machine. In the normal case the state transitions are:
+ *
+ * OPEN -> READY -> RUNNING -> DYING
+ *  |       | ^        |        ^ ^
+ *  |       | +--------+        | |
+ *  |       +-------------------+ |
+ *  +-----------------------------+
+ *
+ * The states are:
+ *  OPEN:   The kcov fd has been opened, but no buffer is available to store
+ *          coverage data.
+ *  READY:  The buffer to store coverage data has been allocated. Userspace
+ *          can set this by using ioctl(fd, KIOSETBUFSIZE, entries);. When
+ *          this has been set the buffer can be written to by the kernel,
+ *          and mmaped by userspace.
+ * RUNNING: The coverage probes are able to store coverage data in the buffer.
+ *          This is entered with ioctl(fd, KIOENABLE, mode);. The READY state
+ *          can be exited by ioctl(fd, KIODISABLE); or exiting the thread to
+ *          return to the READY state to allow tracing to be reused, or by
+ *          closing the kcov fd to enter the DYING state.
+ * DYING:   The fd has been closed. All states can enter into this state when
+ *          userspace closes the kcov fd.
+ *
+ * We need to be careful when moving into and out of the RUNNING state. As
+ * an interrupt may happen while this is happening the ordering of memory
+ * operations is important so struct kcov_info is valid for the tracing
+ * functions.
+ *
+ * When moving into the RUNNING state prior stores to struct kcov_info need
+ * to be observed before the state is set. This allows for interrupts that
+ * may call into one of the coverage functions to fire at any point while
+ * being enabled and see a consistent struct kcov_info.
+ *
+ * When moving out of the RUNNING state any later stores to struct kcov_info
+ * need to be observed after the state is set. As with entering this is to
+ * present a consistent struct kcov_info to interrupts.
+ */
+typedef enum {
+	KCOV_STATE_INVALID,
+	KCOV_STATE_OPEN,	/* The device is open, but with no buffer */
+	KCOV_STATE_READY,	/* The buffer has been allocated */
+	KCOV_STATE_RUNNING,	/* Recording trace data */
+	KCOV_STATE_DYING,	/* The fd was closed */
+} kcov_state_t;
+
+/*
+ * (l) Set while holding the kcov_lock mutex and not in the RUNNING state.
+ * (o) Only set once while in the OPEN state. Cleaned up while in the DYING
+ *     state, and with no thread associated with the struct kcov_info.
+ * (s) Set atomically to enter or exit the RUNNING state, non-atomically
+ *     otherwise. See above for a description of the other constraints while
+ *     moving into or out of the RUNNING state.
+ */
+struct kcov_info {
+	struct thread	*thread;	/* (l) */
+	vm_object_t	bufobj;		/* (o) */
+	vm_offset_t	kvaddr;		/* (o) */
+	size_t		entries;	/* (o) */
+	size_t		bufsize;	/* (o) */
+	kcov_state_t	state;		/* (s) */
+	int		mode;		/* (l) */
+	bool		mmap;
+};
+
+/* Prototypes */
+static d_open_t		kcov_open;
+static d_close_t	kcov_close;
+static d_mmap_single_t	kcov_mmap_single;
+static d_ioctl_t	kcov_ioctl;
+
+void __sanitizer_cov_trace_pc(void);
+void __sanitizer_cov_trace_cmp1(uint8_t, uint8_t);
+void __sanitizer_cov_trace_cmp2(uint16_t, uint16_t);
+void __sanitizer_cov_trace_cmp4(uint32_t, uint32_t);
+void __sanitizer_cov_trace_cmp8(uint64_t, uint64_t);
+void __sanitizer_cov_trace_const_cmp1(uint8_t, uint8_t);
+void __sanitizer_cov_trace_const_cmp2(uint16_t, uint16_t);
+void __sanitizer_cov_trace_const_cmp4(uint32_t, uint32_t);
+void __sanitizer_cov_trace_const_cmp8(uint64_t, uint64_t);
+void __sanitizer_cov_trace_switch(uint64_t, uint64_t *);
+
+static int  kcov_alloc(struct kcov_info *info, size_t entries);
+static void kcov_init(const void *unused);
+
+static struct cdevsw kcov_cdevsw = {
+	.d_version =	D_VERSION,
+	.d_open =	kcov_open,
+	.d_close =	kcov_close,
+	.d_mmap_single = kcov_mmap_single,
+	.d_ioctl =	kcov_ioctl,
+	.d_name =	"kcov",
+};
+
+SYSCTL_NODE(_kern, OID_AUTO, kcov, CTLFLAG_RW, 0, "Kernel coverage");
+
+static u_int kcov_max_entries = KCOV_MAXENTRIES;
+SYSCTL_UINT(_kern_kcov, OID_AUTO, max_entries, CTLFLAG_RW,
+    &kcov_max_entries, 0,
+    "Maximum number of entries in the kcov buffer");
+
+static struct mtx kcov_lock;
+
+static struct kcov_info *
+get_kinfo(struct thread *td)
+{
+	struct kcov_info *info;
+
+	/* We might have a NULL thread when releasing the secondary CPUs */
+	if (td == NULL)
+		return (NULL);
+
+	/*
+	 * We are in an interrupt, stop tracing as it is not explicitly
+	 * part of a syscall.
+	 */
+	if (td->td_intr_nesting_level > 0 || td->td_intr_frame != NULL)
+		return (NULL);
+
+	/*
+	 * If info is NULL or the state is not running we are not tracing.
+	 */
+	info = td->td_kcov_info;
+	if (info == NULL ||
+	    atomic_load_acq_int(&info->state) != KCOV_STATE_RUNNING)
+		return (NULL);
+
+	return (info);
+}
+
+/*
+ * Main entry point. A call to this function will be inserted
+ * at every edge, and if coverage is enabled for the thread
+ * this function will add the PC to the buffer.
+ */
+void
+__sanitizer_cov_trace_pc(void)
+{
+	struct thread *td;
+	struct kcov_info *info;
+	uint64_t *buf, index;
+
+	/*
+	 * To guarantee curthread is properly set, we exit early
+	 * until the driver has been initialized
+	 */
+	if (cold)
+		return;
+
+	td = curthread;
+	info = get_kinfo(td);
+	if (info == NULL)
+		return;
+
+	/*
+	 * Check we are in the PC-trace mode.
+	 */
+	if (info->mode != KCOV_MODE_TRACE_PC)
+		return;
+
+	KASSERT(info->kvaddr != 0,
+	    ("__sanitizer_cov_trace_pc: NULL buf while running"));
+
+	buf = (uint64_t *)info->kvaddr;
+
+	/* The first entry of the buffer holds the index */
+	index = buf[0];
+	if (index + 2 > info->entries)
+		return;
+
+	buf[index + 1] = (uint64_t)__builtin_return_address(0);
+	buf[0] = index + 1;
+}
+
+static bool
+trace_cmp(uint64_t type, uint64_t arg1, uint64_t arg2, uint64_t ret)
+{
+	struct thread *td;
+	struct kcov_info *info;
+	uint64_t *buf, index;
+
+	/*
+	 * To guarantee curthread is properly set, we exit early
+	 * until the driver has been initialized
+	 */
+	if (cold)
+		return (false);
+
+	td = curthread;
+	info = get_kinfo(td);
+	if (info == NULL)
+		return (false);
+
+	/*
+	 * Check we are in the comparison-trace mode.
+	 */
+	if (info->mode != KCOV_MODE_TRACE_CMP)
+		return (false);
+
+	KASSERT(info->kvaddr != 0,
+	    ("__sanitizer_cov_trace_pc: NULL buf while running"));
+
+	buf = (uint64_t *)info->kvaddr;
+
+	/* The first entry of the buffer holds the index */
+	index = buf[0];
+
+	/* Check we have space to store all elements */
+	if (index * 4 + 4 + 1 > info->entries)
+		return (false);
+
+	buf[index * 4 + 1] = type;
+	buf[index * 4 + 2] = arg1;
+	buf[index * 4 + 3] = arg2;
+	buf[index * 4 + 4] = ret;
+	buf[0] = index + 1;
+
+	return (true);
+}
+
+void
+__sanitizer_cov_trace_cmp1(uint8_t arg1, uint8_t arg2)
+{
+
+	trace_cmp(KCOV_CMP_SIZE(0), arg1, arg2,
+	    (uint64_t)__builtin_return_address(0));
+}
+
+void
+__sanitizer_cov_trace_cmp2(uint16_t arg1, uint16_t arg2)
+{
+
+	trace_cmp(KCOV_CMP_SIZE(1), arg1, arg2,
+	    (uint64_t)__builtin_return_address(0));
+}
+
+void
+__sanitizer_cov_trace_cmp4(uint32_t arg1, uint32_t arg2)
+{
+
+	trace_cmp(KCOV_CMP_SIZE(2), arg1, arg2,
+	    (uint64_t)__builtin_return_address(0));
+}
+
+void
+__sanitizer_cov_trace_cmp8(uint64_t arg1, uint64_t arg2)
+{
+
+	trace_cmp(KCOV_CMP_SIZE(3), arg1, arg2,
+	    (uint64_t)__builtin_return_address(0));
+}
+
+void
+__sanitizer_cov_trace_const_cmp1(uint8_t arg1, uint8_t arg2)
+{
+
+	trace_cmp(KCOV_CMP_SIZE(0) | KCOV_CMP_CONST, arg1, arg2,
+	    (uint64_t)__builtin_return_address(0));
+}
+
+void
+__sanitizer_cov_trace_const_cmp2(uint16_t arg1, uint16_t arg2)
+{
+
+	trace_cmp(KCOV_CMP_SIZE(1) | KCOV_CMP_CONST, arg1, arg2,
+	    (uint64_t)__builtin_return_address(0));
+}
+
+void
+__sanitizer_cov_trace_const_cmp4(uint32_t arg1, uint32_t arg2)
+{
+
+	trace_cmp(KCOV_CMP_SIZE(2) | KCOV_CMP_CONST, arg1, arg2,
+	    (uint64_t)__builtin_return_address(0));
+}
+
+void
+__sanitizer_cov_trace_const_cmp8(uint64_t arg1, uint64_t arg2)
+{
+
+	trace_cmp(KCOV_CMP_SIZE(3) | KCOV_CMP_CONST, arg1, arg2,
+	    (uint64_t)__builtin_return_address(0));
+}
+
+/*
+ * val is the switch operand
+ * cases[0] is the number of case constants
+ * cases[1] is the size of val in bits
+ * cases[2..n] are the case constants
+ */
+void
+__sanitizer_cov_trace_switch(uint64_t val, uint64_t *cases)
+{
+	uint64_t i, count, ret, type;
+
+	count = cases[0];
+	ret = (uint64_t)__builtin_return_address(0);
+
+	switch (cases[1]) {
+	case 8:
+		type = KCOV_CMP_SIZE(0);
+		break;
+	case 16:
+		type = KCOV_CMP_SIZE(1);
+		break;
+	case 32:
+		type = KCOV_CMP_SIZE(2);
+		break;
+	case 64:
+		type = KCOV_CMP_SIZE(3);
+		break;
+	default:
+		return;
+	}
+
+	val |= KCOV_CMP_CONST;
+
+	for (i = 0; i < count; i++)
+		if (!trace_cmp(type, val, cases[i + 2], ret))
+			return;
+}
+
+/*
+ * The fd is being closed, cleanup everything we can.
+ */
+static void
+kcov_mmap_cleanup(void *arg)
+{
+	struct kcov_info *info = arg;
+	struct thread *thread;
+
+	mtx_lock_spin(&kcov_lock);
+	/*
+	 * Move to KCOV_STATE_DYING to stop adding new entries.
+	 *
+	 * If the thread is running we need to wait until thread exit to
+	 * clean up as it may currently be adding a new entry. If this is
+	 * the case being in KCOV_STATE_DYING will signal that the buffer
+	 * needs to be cleaned up.
+	 */
+	atomic_store_int(&info->state, KCOV_STATE_DYING);
+	atomic_thread_fence_seq_cst();
+	thread = info->thread;
+	mtx_unlock_spin(&kcov_lock);
+
+	if (thread != NULL)
+		return;
+
+	/*
+	 * We can safely clean up the info struct as it is in the
+	 * KCOV_STATE_DYING state with no thread associated.
+	 *
+	 * The KCOV_STATE_DYING stops new threads from using it.
+	 * The lack of a thread means nothing is currently using the buffers.
+	 */
+
+	if (info->kvaddr != 0) {
+		pmap_qremove(info->kvaddr, info->bufsize / PAGE_SIZE);
+		kva_free(info->kvaddr, info->bufsize);
+	}
+	if (info->bufobj != NULL && !info->mmap)
+		vm_object_deallocate(info->bufobj);
+	free(info, M_KCOV_INFO);
+}
+
+static int
+kcov_open(struct cdev *dev, int oflags, int devtype, struct thread *td)
+{
+	struct kcov_info *info;
+	int error;
+
+	info = malloc(sizeof(struct kcov_info), M_KCOV_INFO, M_ZERO | M_WAITOK);
+	info->state = KCOV_STATE_OPEN;
+	info->thread = NULL;
+	info->mode = -1;
+	info->mmap = false;
+
+	if ((error = devfs_set_cdevpriv(info, kcov_mmap_cleanup)) != 0)
+		kcov_mmap_cleanup(info);
+
+	return (error);
+}
+
+static int
+kcov_close(struct cdev *dev, int fflag, int devtype, struct thread *td)
+{
+	struct kcov_info *info;
+	int error;
+
+	if ((error = devfs_get_cdevpriv((void **)&info)) != 0)
+		return (error);
+
+	KASSERT(info != NULL, ("kcov_close with no kcov_info structure"));
+
+	/* Trying to close, but haven't disabled */
+	if (info->state == KCOV_STATE_RUNNING)
+		return (EBUSY);
+
+	return (0);
+}
+
+static int
+kcov_mmap_single(struct cdev *dev, vm_ooffset_t *offset, vm_size_t size,
+    struct vm_object **object, int nprot)
+{
+	struct kcov_info *info;
+	int error;
+
+	if ((nprot & (PROT_EXEC | PROT_READ | PROT_WRITE)) !=
+	    (PROT_READ | PROT_WRITE))
+		return (EINVAL);
+
+	if ((error = devfs_get_cdevpriv((void **)&info)) != 0)
+		return (error);
+
+	if (info->kvaddr == 0 || size / KCOV_ELEMENT_SIZE != info->entries ||
+	    info->mmap != false)
+		return (EINVAL);
+
+	info->mmap = true;
+	*offset = 0;
+	*object = info->bufobj;
+	return (0);
+}
+
+static int
+kcov_alloc(struct kcov_info *info, size_t entries)
+{
+	size_t n, pages;
+	vm_page_t *m;
+
+	KASSERT(info->kvaddr == 0, ("kcov_alloc: Already have a buffer"));
+	KASSERT(info->state == KCOV_STATE_OPEN,
+	    ("kcov_alloc: Not in open state (%x)", info->state));
+
+	if (entries < 2 || entries > kcov_max_entries)
+		return (EINVAL);
+
+	/* Align to page size so mmap can't access other kernel memory */
+	info->bufsize = roundup2(entries * KCOV_ELEMENT_SIZE, PAGE_SIZE);
+	pages = info->bufsize / PAGE_SIZE;
+
+	if ((info->kvaddr = kva_alloc(info->bufsize)) == 0)
+		return (ENOMEM);
+
+	info->bufobj = vm_pager_allocate(OBJT_PHYS, 0, info->bufsize,
+	    PROT_READ | PROT_WRITE, 0, curthread->td_ucred);
+
+	m = malloc(sizeof(*m) * pages, M_TEMP, M_WAITOK);
+	VM_OBJECT_WLOCK(info->bufobj);
+	for (n = 0; n < pages; n++) {
+		m[n] = vm_page_grab(info->bufobj, n,
+		    VM_ALLOC_NOBUSY | VM_ALLOC_ZERO | VM_ALLOC_WIRED);
+		m[n]->valid = VM_PAGE_BITS_ALL;
+	}
+	VM_OBJECT_WUNLOCK(info->bufobj);
+	pmap_qenter(info->kvaddr, m, pages);
+	free(m, M_TEMP);
+
+	info->entries = entries;
+
+	return (0);
+}
+
+static int
+kcov_ioctl(struct cdev *dev, u_long cmd, caddr_t data, int fflag __unused,
+    struct thread *td)
+{
+	struct kcov_info *info;
+	int mode, error;
+
+	if ((error = devfs_get_cdevpriv((void **)&info)) != 0)
+		return (error);
+
+	if (cmd == KIOSETBUFSIZE) {
+		/*
+		 * Set the size of the coverage buffer. Should be called
+		 * before enabling coverage collection for that thread.
+		 */
+		if (info->state != KCOV_STATE_OPEN) {
+			return (EBUSY);
+		}
+		error = kcov_alloc(info, *(u_int *)data);
+		if (error == 0)
+			info->state = KCOV_STATE_READY;
+		return (error);
+	}
+
+	mtx_lock_spin(&kcov_lock);
+	switch (cmd) {
+	case KIOENABLE:
+		if (info->state != KCOV_STATE_READY) {
+			error = EBUSY;
+			break;
+		}
+		if (td->td_kcov_info != NULL) {
+			error = EINVAL;
+			break;
+		}
+		mode = *(int *)data;
+		if (mode != KCOV_MODE_TRACE_PC && mode != KCOV_MODE_TRACE_CMP) {
+			error = EINVAL;
+			break;
+		}
+		KASSERT(info->thread == NULL,
+		    ("Enabling kcov when already enabled"));
+		info->thread = td;
+		info->mode = mode;
+		/*
+		 * Ensure the mode has been set before starting coverage
+		 * tracing.
+		 */
+		atomic_store_rel_int(&info->state, KCOV_STATE_RUNNING);
+		td->td_kcov_info = info;
+		break;
+	case KIODISABLE:
+		/* Only the currently enabled thread may disable itself */
+		if (info->state != KCOV_STATE_RUNNING ||
+		    info != td->td_kcov_info) {
+			error = EINVAL;
+			break;
+		}
+		td->td_kcov_info = NULL;
+		atomic_store_int(&info->state, KCOV_STATE_READY);
+		/*
+		 * Ensure we have exited the READY state before clearing the
+		 * rest of the info struct.
+		 */
+		atomic_thread_fence_rel();
+		info->mode = -1;
+		info->thread = NULL;
+		break;
+	default:
+		error = EINVAL;
+		break;
+	}
+	mtx_unlock_spin(&kcov_lock);
+
+	return (error);
+}
+
+static void
+kcov_thread_dtor(void *arg __unused, struct thread *td)
+{
+	struct kcov_info *info;
+
+	info = td->td_kcov_info;
+	if (info == NULL)
+		return;
+
+	mtx_lock_spin(&kcov_lock);
+	td->td_kcov_info = NULL;
+	if (info->state != KCOV_STATE_DYING) {
+		/*
+		 * The kcov file is still open. Mark it as unused and
+		 * wait for it to be closed before cleaning up.
+		 */
+		atomic_store_int(&info->state, KCOV_STATE_READY);
+		atomic_thread_fence_seq_cst();
+		/* This info struct is unused */
+		info->thread = NULL;
+		mtx_unlock_spin(&kcov_lock);
+		return;
+	}
+	mtx_unlock_spin(&kcov_lock);
+
+	/*
+	 * We can safely clean up the info struct as it is in the
+	 * KCOV_STATE_DYING state where the info struct is associated with
+	 * the current thread that's about to exit.
+	 *
+	 * The KCOV_STATE_DYING stops new threads from using it.
+	 * It also stops the current thread from trying to use the info struct.
+	 */
+
+	if (info->kvaddr != 0) {
+		pmap_qremove(info->kvaddr, info->bufsize / PAGE_SIZE);
+		kva_free(info->kvaddr, info->bufsize);
+	}
+	if (info->bufobj != NULL && !info->mmap)
+		vm_object_deallocate(info->bufobj);
+	free(info, M_KCOV_INFO);
+}
+
+static void
+kcov_init(const void *unused)
+{
+	struct make_dev_args args;
+	struct cdev *dev;
+
+	mtx_init(&kcov_lock, "kcov lock", NULL, MTX_SPIN);
+
+	make_dev_args_init(&args);
+	args.mda_devsw = &kcov_cdevsw;
+	args.mda_uid = UID_ROOT;
+	args.mda_gid = GID_WHEEL;
+	args.mda_mode = 0600;
+	if (make_dev_s(&args, &dev, "kcov") != 0) {
+		printf("%s", "Failed to create kcov device");
+		return;
+	}
+
+	EVENTHANDLER_REGISTER(thread_dtor, kcov_thread_dtor, NULL,
+	    EVENTHANDLER_PRI_ANY);
+}
+
+SYSINIT(kcovdev, SI_SUB_DEVFS, SI_ORDER_ANY, kcov_init, NULL);
diff --git a/sys/kern/kern_thread.c b/sys/kern/kern_thread.c
index 2a8eb6d52bce..dc5ee763551c 100644
--- a/sys/kern/kern_thread.c
+++ b/sys/kern/kern_thread.c
@@ -82,9 +82,9 @@ _Static_assert(offsetof(struct thread, td_flags) == 0xfc,
     "struct thread KBI td_flags");
 _Static_assert(offsetof(struct thread, td_pflags) == 0x104,
     "struct thread KBI td_pflags");
-_Static_assert(offsetof(struct thread, td_frame) == 0x470,
+_Static_assert(offsetof(struct thread, td_frame) == 0x478,
     "struct thread KBI td_frame");
-_Static_assert(offsetof(struct thread, td_emuldata) == 0x528,
+_Static_assert(offsetof(struct thread, td_emuldata) == 0x530,
     "struct thread KBI td_emuldata");
 _Static_assert(offsetof(struct proc, p_flag) == 0xb0,
     "struct proc KBI p_flag");