diff --git a/kernel/kcov.c b/kernel/kcov.c
new file mode 100644
index 0000000..3efbee0
--- /dev/null
+++ b/kernel/kcov.c
@@ -0,0 +1,273 @@
+#define pr_fmt(fmt) "kcov: " fmt
+
+#include <linux/compiler.h>
+#include <linux/types.h>
+#include <linux/file.h>
+#include <linux/fs.h>
+#include <linux/mm.h>
+#include <linux/printk.h>
+#include <linux/slab.h>
+#include <linux/spinlock.h>
+#include <linux/vmalloc.h>
+#include <linux/debugfs.h>
+#include <linux/uaccess.h>
+#include <linux/kcov.h>
+
+/*
+ * kcov descriptor (one per opened debugfs file).
+ * State transitions of the descriptor:
+ *  - initial state after open()
+ *  - then there must be a single ioctl(KCOV_INIT_TRACE) call
+ *  - then, mmap() call (several calls are allowed but not useful)
+ *  - then, repeated enable/disable for a task (only one task a time allowed)
+ */
+struct kcov {
+	/*
+	 * Reference counter. We keep one for:
+	 *  - opened file descriptor
+	 *  - task with enabled coverage (we can't unwire it from another task)
+	 */
+	atomic_t		refcount;
+	/* The lock protects mode, size, area and t. */
+	spinlock_t		lock;
+	enum kcov_mode		mode;
+	/* Size of arena (in long's for KCOV_MODE_TRACE). */
+	unsigned		size;
+	/* Coverage buffer shared with user space. */
+	void			*area;
+	/* Task for which we collect coverage, or NULL. */
+	struct task_struct	*t;
+};
+
+/*
+ * Entry point from instrumented code.
+ * This is called once per basic-block/edge.
+ */
+void __sanitizer_cov_trace_pc(void)
+{
+	struct task_struct *t;
+	enum kcov_mode mode;
+
+	t = current;
+	/*
+	 * We are interested in code coverage as a function of a syscall inputs,
+	 * so we ignore code executed in interrupts.
+	 */
+	if (!t || in_interrupt())
+		return;
+	mode = READ_ONCE(t->kcov_mode);
+	if (mode == KCOV_MODE_TRACE) {
+		unsigned long *area;
+		unsigned long pos;
+
+		/*
+		 * There is some code that runs in interrupts but for which
+		 * in_interrupt() returns false (e.g. preempt_schedule_irq()).
+		 * READ_ONCE()/barrier() effectively provides load-acquire wrt
+		 * interrupts, there are paired barrier()/WRITE_ONCE() in
+		 * kcov_ioctl_locked().
+		 */
+		barrier();
+		area = t->kcov_area;
+		/* The first word is number of subsequent PCs. */
+		pos = READ_ONCE(area[0]) + 1;
+		if (likely(pos < t->kcov_size)) {
+			area[pos] = _RET_IP_;
+			WRITE_ONCE(area[0], pos);
+		}
+	}
+}
+EXPORT_SYMBOL(__sanitizer_cov_trace_pc);
+
+static void kcov_get(struct kcov *kcov)
+{
+	atomic_inc(&kcov->refcount);
+}
+
+static void kcov_put(struct kcov *kcov)
+{
+	if (atomic_dec_and_test(&kcov->refcount)) {
+		vfree(kcov->area);
+		kfree(kcov);
+	}
+}
+
+void kcov_task_init(struct task_struct *t)
+{
+	t->kcov_mode = KCOV_MODE_DISABLED;
+	t->kcov_size = 0;
+	t->kcov_area = NULL;
+	t->kcov = NULL;
+}
+
+void kcov_task_exit(struct task_struct *t)
+{
+	struct kcov *kcov;
+
+	kcov = t->kcov;
+	if (kcov == NULL)
+		return;
+	spin_lock(&kcov->lock);
+	if (WARN_ON(kcov->t != t)) {
+		spin_unlock(&kcov->lock);
+		return;
+	}
+	/* Just to not leave dangling references behind. */
+	kcov_task_init(t);
+	kcov->t = NULL;
+	spin_unlock(&kcov->lock);
+	kcov_put(kcov);
+}
+
+static int kcov_mmap(struct file *filep, struct vm_area_struct *vma)
+{
+	int res = 0;
+	void *area;
+	struct kcov *kcov = vma->vm_file->private_data;
+	unsigned long size, off;
+	struct page *page;
+
+	area = vmalloc_user(vma->vm_end - vma->vm_start);
+	if (!area)
+		return -ENOMEM;
+
+	spin_lock(&kcov->lock);
+	size = kcov->size * sizeof(unsigned long);
+	if (kcov->mode == KCOV_MODE_DISABLED || vma->vm_pgoff != 0 ||
+	    vma->vm_end - vma->vm_start != size) {
+		res = -EINVAL;
+		goto exit;
+	}
+	if (!kcov->area) {
+		kcov->area = area;
+		vma->vm_flags |= VM_DONTEXPAND;
+		spin_unlock(&kcov->lock);
+		for (off = 0; off < size; off += PAGE_SIZE) {
+			page = vmalloc_to_page(kcov->area + off);
+			if (vm_insert_page(vma, vma->vm_start + off, page))
+				WARN_ONCE(1, "vm_insert_page() failed");
+		}
+		return 0;
+	}
+exit:
+	spin_unlock(&kcov->lock);
+	vfree(area);
+	return res;
+}
+
+static int kcov_open(struct inode *inode, struct file *filep)
+{
+	struct kcov *kcov;
+
+	kcov = kzalloc(sizeof(*kcov), GFP_KERNEL);
+	if (!kcov)
+		return -ENOMEM;
+	atomic_set(&kcov->refcount, 1);
+	spin_lock_init(&kcov->lock);
+	filep->private_data = kcov;
+	return nonseekable_open(inode, filep);
+}
+
+static int kcov_close(struct inode *inode, struct file *filep)
+{
+	kcov_put(filep->private_data);
+	return 0;
+}
+
+static int kcov_ioctl_locked(struct kcov *kcov, unsigned int cmd,
+			     unsigned long arg)
+{
+	struct task_struct *t;
+	unsigned long size, unused;
+
+	switch (cmd) {
+	case KCOV_INIT_TRACE:
+		/*
+		 * Enable kcov in trace mode and setup buffer size.
+		 * Must happen before anything else.
+		 */
+		if (kcov->mode != KCOV_MODE_DISABLED)
+			return -EBUSY;
+		/*
+		 * Size must be at least 2 to hold current position and one PC.
+		 * Later we allocate size * sizeof(unsigned long) memory,
+		 * that must not overflow.
+		 */
+		size = arg;
+		if (size < 2 || size > INT_MAX / sizeof(unsigned long))
+			return -EINVAL;
+		kcov->size = size;
+		kcov->mode = KCOV_MODE_TRACE;
+		return 0;
+	case KCOV_ENABLE:
+		/*
+		 * Enable coverage for the current task.
+		 * At this point user must have been enabled trace mode,
+		 * and mmapped the file. Coverage collection is disabled only
+		 * at task exit or voluntary by KCOV_DISABLE. After that it can
+		 * be enabled for another task.
+		 */
+		unused = arg;
+		if (unused != 0 || kcov->mode == KCOV_MODE_DISABLED ||
+		    kcov->area == NULL)
+			return -EINVAL;
+		if (kcov->t != NULL)
+			return -EBUSY;
+		t = current;
+		/* Cache in task struct for performance. */
+		t->kcov_size = kcov->size;
+		t->kcov_area = kcov->area;
+		/* See comment in __sanitizer_cov_trace_pc(). */
+		barrier();
+		WRITE_ONCE(t->kcov_mode, kcov->mode);
+		t->kcov = kcov;
+		kcov->t = t;
+		/* This is put either in kcov_task_exit() or in KCOV_DISABLE. */
+		kcov_get(kcov);
+		return 0;
+	case KCOV_DISABLE:
+		/* Disable coverage for the current task. */
+		unused = arg;
+		if (unused != 0 || current->kcov != kcov)
+			return -EINVAL;
+		t = current;
+		if (WARN_ON(kcov->t != t))
+			return -EINVAL;
+		kcov_task_init(t);
+		kcov->t = NULL;
+		kcov_put(kcov);
+		return 0;
+	default:
+		return -ENOTTY;
+	}
+}
+
+static long kcov_ioctl(struct file *filep, unsigned int cmd, unsigned long arg)
+{
+	struct kcov *kcov;
+	int res;
+
+	kcov = filep->private_data;
+	spin_lock(&kcov->lock);
+	res = kcov_ioctl_locked(kcov, cmd, arg);
+	spin_unlock(&kcov->lock);
+	return res;
+}
+
+static const struct file_operations kcov_fops = {
+	.open		= kcov_open,
+	.unlocked_ioctl	= kcov_ioctl,
+	.mmap		= kcov_mmap,
+	.release        = kcov_close,
+};
+
+static int __init kcov_init(void)
+{
+	if (!debugfs_create_file("kcov", 0600, NULL, NULL, &kcov_fops)) {
+		pr_err("failed to create kcov in debugfs\n");
+		return -ENOMEM;
+	}
+	return 0;
+}
+
+device_initcall(kcov_init);