1 files changed, 774 insertions, 96 deletions

diff --git a/kernel/kcov.c b/kernel/kcov.c
index c2277dbdbfb1..6563141f5de9 100644
--- a/kernel/kcov.c
+++ b/kernel/kcov.c
@@ -9,7 +9,10 @@
 #include <linux/types.h>
 #include <linux/file.h>
 #include <linux/fs.h>
+#include <linux/hashtable.h>
 #include <linux/init.h>
+#include <linux/jiffies.h>
+#include <linux/kmsan-checks.h>
 #include <linux/mm.h>
 #include <linux/preempt.h>
 #include <linux/printk.h>
@@ -20,8 +23,12 @@
 #include <linux/debugfs.h>
 #include <linux/uaccess.h>
 #include <linux/kcov.h>
+#include <linux/refcount.h>
+#include <linux/log2.h>
 #include <asm/setup.h>
 
+#define kcov_debug(fmt, ...) pr_debug("%s: " fmt, __func__, ##__VA_ARGS__)
+
 /* Number of 64-bit words written per one comparison: */
 #define KCOV_WORDS_PER_CMP 4
 
@@ -43,28 +50,137 @@ struct kcov {
 	 * Reference counter. We keep one for:
 	 *  - opened file descriptor
 	 *  - task with enabled coverage (we can't unwire it from another task)
+	 *  - each code section for remote coverage collection
 	 */
-	atomic_t		refcount;
+	refcount_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;
+	/* Size of arena (in long's). */
+	unsigned int		size;
 	/* Coverage buffer shared with user space. */
 	void			*area;
 	/* Task for which we collect coverage, or NULL. */
 	struct task_struct	*t;
+	/* Collecting coverage from remote (background) threads. */
+	bool			remote;
+	/* Size of remote area (in long's). */
+	unsigned int		remote_size;
+	/*
+	 * Sequence is incremented each time kcov is reenabled, used by
+	 * kcov_remote_stop(), see the comment there.
+	 */
+	int			sequence;
+};
+
+struct kcov_remote_area {
+	struct list_head	list;
+	unsigned int		size;
+};
+
+struct kcov_remote {
+	u64			handle;
+	struct kcov		*kcov;
+	struct hlist_node	hnode;
+};
+
+static DEFINE_SPINLOCK(kcov_remote_lock);
+static DEFINE_HASHTABLE(kcov_remote_map, 4);
+static struct list_head kcov_remote_areas = LIST_HEAD_INIT(kcov_remote_areas);
+
+struct kcov_percpu_data {
+	void			*irq_area;
+	local_lock_t		lock;
+
+	unsigned int		saved_mode;
+	unsigned int		saved_size;
+	void			*saved_area;
+	struct kcov		*saved_kcov;
+	int			saved_sequence;
+};
+
+static DEFINE_PER_CPU(struct kcov_percpu_data, kcov_percpu_data) = {
+	.lock = INIT_LOCAL_LOCK(lock),
 };
 
+/* Must be called with kcov_remote_lock locked. */
+static struct kcov_remote *kcov_remote_find(u64 handle)
+{
+	struct kcov_remote *remote;
+
+	hash_for_each_possible(kcov_remote_map, remote, hnode, handle) {
+		if (remote->handle == handle)
+			return remote;
+	}
+	return NULL;
+}
+
+/* Must be called with kcov_remote_lock locked. */
+static struct kcov_remote *kcov_remote_add(struct kcov *kcov, u64 handle)
+{
+	struct kcov_remote *remote;
+
+	if (kcov_remote_find(handle))
+		return ERR_PTR(-EEXIST);
+	remote = kmalloc(sizeof(*remote), GFP_ATOMIC);
+	if (!remote)
+		return ERR_PTR(-ENOMEM);
+	remote->handle = handle;
+	remote->kcov = kcov;
+	hash_add(kcov_remote_map, &remote->hnode, handle);
+	return remote;
+}
+
+/* Must be called with kcov_remote_lock locked. */
+static struct kcov_remote_area *kcov_remote_area_get(unsigned int size)
+{
+	struct kcov_remote_area *area;
+	struct list_head *pos;
+
+	list_for_each(pos, &kcov_remote_areas) {
+		area = list_entry(pos, struct kcov_remote_area, list);
+		if (area->size == size) {
+			list_del(&area->list);
+			return area;
+		}
+	}
+	return NULL;
+}
+
+/* Must be called with kcov_remote_lock locked. */
+static void kcov_remote_area_put(struct kcov_remote_area *area,
+					unsigned int size)
+{
+	INIT_LIST_HEAD(&area->list);
+	area->size = size;
+	list_add(&area->list, &kcov_remote_areas);
+	/*
+	 * KMSAN doesn't instrument this file, so it may not know area->list
+	 * is initialized. Unpoison it explicitly to avoid reports in
+	 * kcov_remote_area_get().
+	 */
+	kmsan_unpoison_memory(&area->list, sizeof(area->list));
+}
+
+/*
+ * Unlike in_serving_softirq(), this function returns false when called during
+ * a hardirq or an NMI that happened in the softirq context.
+ */
+static __always_inline bool in_softirq_really(void)
+{
+	return in_serving_softirq() && !in_hardirq() && !in_nmi();
+}
+
 static notrace bool check_kcov_mode(enum kcov_mode needed_mode, struct task_struct *t)
 {
 	unsigned int mode;
 
 	/*
 	 * We are interested in code coverage as a function of a syscall inputs,
-	 * so we ignore code executed in interrupts.
+	 * so we ignore code executed in interrupts, unless we are in a remote
+	 * coverage collection section in a softirq.
 	 */
-	if (!in_task())
+	if (!in_task() && !(in_softirq_really() && t->kcov_softirq))
 		return false;
 	mode = READ_ONCE(t->kcov_mode);
 	/*
@@ -72,7 +188,7 @@ static notrace bool check_kcov_mode(enum kcov_mode needed_mode, struct task_stru
 	 * 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().
+	 * kcov_start().
 	 */
 	barrier();
 	return mode == needed_mode;
@@ -105,8 +221,16 @@ void notrace __sanitizer_cov_trace_pc(void)
 	/* The first 64-bit word is the number of subsequent PCs. */
 	pos = READ_ONCE(area[0]) + 1;
 	if (likely(pos < t->kcov_size)) {
-		area[pos] = ip;
+		/* Previously we write pc before updating pos. However, some
+		 * early interrupt code could bypass check_kcov_mode() check
+		 * and invoke __sanitizer_cov_trace_pc(). If such interrupt is
+		 * raised between writing pc and updating pos, the pc could be
+		 * overitten by the recursive __sanitizer_cov_trace_pc().
+		 * Update pos before writing pc to avoid such interleaving.
+		 */
 		WRITE_ONCE(area[0], pos);
+		barrier();
+		area[pos] = ip;
 	}
 }
 EXPORT_SYMBOL(__sanitizer_cov_trace_pc);
@@ -137,11 +261,13 @@ static void notrace write_comp_data(u64 type, u64 arg1, u64 arg2, u64 ip)
 	start_index = 1 + count * KCOV_WORDS_PER_CMP;
 	end_pos = (start_index + KCOV_WORDS_PER_CMP) * sizeof(u64);
 	if (likely(end_pos <= max_pos)) {
+		/* See comment in __sanitizer_cov_trace_pc(). */
+		WRITE_ONCE(area[0], count + 1);
+		barrier();
 		area[start_index] = type;
 		area[start_index + 1] = arg1;
 		area[start_index + 2] = arg2;
 		area[start_index + 3] = ip;
-		WRITE_ONCE(area[0], count + 1);
 	}
 }
 
@@ -163,7 +289,7 @@ void notrace __sanitizer_cov_trace_cmp4(u32 arg1, u32 arg2)
 }
 EXPORT_SYMBOL(__sanitizer_cov_trace_cmp4);
 
-void notrace __sanitizer_cov_trace_cmp8(u64 arg1, u64 arg2)
+void notrace __sanitizer_cov_trace_cmp8(kcov_u64 arg1, kcov_u64 arg2)
 {
 	write_comp_data(KCOV_CMP_SIZE(3), arg1, arg2, _RET_IP_);
 }
@@ -190,16 +316,17 @@ void notrace __sanitizer_cov_trace_const_cmp4(u32 arg1, u32 arg2)
 }
 EXPORT_SYMBOL(__sanitizer_cov_trace_const_cmp4);
 
-void notrace __sanitizer_cov_trace_const_cmp8(u64 arg1, u64 arg2)
+void notrace __sanitizer_cov_trace_const_cmp8(kcov_u64 arg1, kcov_u64 arg2)
 {
 	write_comp_data(KCOV_CMP_SIZE(3) | KCOV_CMP_CONST, arg1, arg2,
 			_RET_IP_);
 }
 EXPORT_SYMBOL(__sanitizer_cov_trace_const_cmp8);
 
-void notrace __sanitizer_cov_trace_switch(u64 val, u64 *cases)
+void notrace __sanitizer_cov_trace_switch(kcov_u64 val, void *arg)
 {
 	u64 i;
+	u64 *cases = arg;
 	u64 count = cases[0];
 	u64 size = cases[1];
 	u64 type = KCOV_CMP_CONST;
@@ -226,81 +353,165 @@ void notrace __sanitizer_cov_trace_switch(u64 val, u64 *cases)
 EXPORT_SYMBOL(__sanitizer_cov_trace_switch);
 #endif /* ifdef CONFIG_KCOV_ENABLE_COMPARISONS */
 
+static void kcov_start(struct task_struct *t, struct kcov *kcov,
+			unsigned int size, void *area, enum kcov_mode mode,
+			int sequence)
+{
+	kcov_debug("t = %px, size = %u, area = %px\n", t, size, area);
+	t->kcov = kcov;
+	/* Cache in task struct for performance. */
+	t->kcov_size = size;
+	t->kcov_area = area;
+	t->kcov_sequence = sequence;
+	/* See comment in check_kcov_mode(). */
+	barrier();
+	WRITE_ONCE(t->kcov_mode, mode);
+}
+
+static void kcov_stop(struct task_struct *t)
+{
+	WRITE_ONCE(t->kcov_mode, KCOV_MODE_DISABLED);
+	barrier();
+	t->kcov = NULL;
+	t->kcov_size = 0;
+	t->kcov_area = NULL;
+}
+
+static void kcov_task_reset(struct task_struct *t)
+{
+	kcov_stop(t);
+	t->kcov_sequence = 0;
+	t->kcov_handle = 0;
+}
+
+void kcov_task_init(struct task_struct *t)
+{
+	kcov_task_reset(t);
+	t->kcov_handle = current->kcov_handle;
+}
+
+static void kcov_reset(struct kcov *kcov)
+{
+	kcov->t = NULL;
+	kcov->mode = KCOV_MODE_INIT;
+	kcov->remote = false;
+	kcov->remote_size = 0;
+	kcov->sequence++;
+}
+
+static void kcov_remote_reset(struct kcov *kcov)
+{
+	int bkt;
+	struct kcov_remote *remote;
+	struct hlist_node *tmp;
+	unsigned long flags;
+
+	spin_lock_irqsave(&kcov_remote_lock, flags);
+	hash_for_each_safe(kcov_remote_map, bkt, tmp, remote, hnode) {
+		if (remote->kcov != kcov)
+			continue;
+		hash_del(&remote->hnode);
+		kfree(remote);
+	}
+	/* Do reset before unlock to prevent races with kcov_remote_start(). */
+	kcov_reset(kcov);
+	spin_unlock_irqrestore(&kcov_remote_lock, flags);
+}
+
+static void kcov_disable(struct task_struct *t, struct kcov *kcov)
+{
+	kcov_task_reset(t);
+	if (kcov->remote)
+		kcov_remote_reset(kcov);
+	else
+		kcov_reset(kcov);
+}
+
 static void kcov_get(struct kcov *kcov)
 {
-	atomic_inc(&kcov->refcount);
+	refcount_inc(&kcov->refcount);
 }
 
 static void kcov_put(struct kcov *kcov)
 {
-	if (atomic_dec_and_test(&kcov->refcount)) {
+	if (refcount_dec_and_test(&kcov->refcount)) {
+		kcov_remote_reset(kcov);
 		vfree(kcov->area);
 		kfree(kcov);
 	}
 }
 
-void kcov_task_init(struct task_struct *t)
-{
-	WRITE_ONCE(t->kcov_mode, KCOV_MODE_DISABLED);
-	barrier();
-	t->kcov_size = 0;
-	t->kcov_area = NULL;
-	t->kcov = NULL;
-}
-
 void kcov_task_exit(struct task_struct *t)
 {
 	struct kcov *kcov;
+	unsigned long flags;
 
 	kcov = t->kcov;
 	if (kcov == NULL)
 		return;
-	spin_lock(&kcov->lock);
+
+	spin_lock_irqsave(&kcov->lock, flags);
+	kcov_debug("t = %px, kcov->t = %px\n", t, kcov->t);
+	/*
+	 * For KCOV_ENABLE devices we want to make sure that t->kcov->t == t,
+	 * which comes down to:
+	 *        WARN_ON(!kcov->remote && kcov->t != t);
+	 *
+	 * For KCOV_REMOTE_ENABLE devices, the exiting task is either:
+	 *
+	 * 1. A remote task between kcov_remote_start() and kcov_remote_stop().
+	 *    In this case we should print a warning right away, since a task
+	 *    shouldn't be exiting when it's in a kcov coverage collection
+	 *    section. Here t points to the task that is collecting remote
+	 *    coverage, and t->kcov->t points to the thread that created the
+	 *    kcov device. Which means that to detect this case we need to
+	 *    check that t != t->kcov->t, and this gives us the following:
+	 *        WARN_ON(kcov->remote && kcov->t != t);
+	 *
+	 * 2. The task that created kcov exiting without calling KCOV_DISABLE,
+	 *    and then again we make sure that t->kcov->t == t:
+	 *        WARN_ON(kcov->remote && kcov->t != t);
+	 *
+	 * By combining all three checks into one we get:
+	 */
 	if (WARN_ON(kcov->t != t)) {
-		spin_unlock(&kcov->lock);
+		spin_unlock_irqrestore(&kcov->lock, flags);
 		return;
 	}
 	/* Just to not leave dangling references behind. */
-	kcov_task_init(t);
-	kcov->t = NULL;
-	kcov->mode = KCOV_MODE_INIT;
-	spin_unlock(&kcov->lock);
+	kcov_disable(t, kcov);
+	spin_unlock_irqrestore(&kcov->lock, flags);
 	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;
+	unsigned long flags;
 
-	area = vmalloc_user(vma->vm_end - vma->vm_start);
-	if (!area)
-		return -ENOMEM;
-
-	spin_lock(&kcov->lock);
+	spin_lock_irqsave(&kcov->lock, flags);
 	size = kcov->size * sizeof(unsigned long);
-	if (kcov->mode != KCOV_MODE_INIT || vma->vm_pgoff != 0 ||
+	if (kcov->area == NULL || 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");
+	spin_unlock_irqrestore(&kcov->lock, flags);
+	vm_flags_set(vma, VM_DONTEXPAND);
+	for (off = 0; off < size; off += PAGE_SIZE) {
+		page = vmalloc_to_page(kcov->area + off);
+		res = vm_insert_page(vma, vma->vm_start + off, page);
+		if (res) {
+			pr_warn_once("kcov: vm_insert_page() failed\n");
+			return res;
 		}
-		return 0;
 	}
+	return 0;
 exit:
-	spin_unlock(&kcov->lock);
-	vfree(area);
+	spin_unlock_irqrestore(&kcov->lock, flags);
 	return res;
 }
 
@@ -312,7 +523,8 @@ static int kcov_open(struct inode *inode, struct file *filep)
 	if (!kcov)
 		return -ENOMEM;
 	kcov->mode = KCOV_MODE_DISABLED;
-	atomic_set(&kcov->refcount, 1);
+	kcov->sequence = 1;
+	refcount_set(&kcov->refcount, 1);
 	spin_lock_init(&kcov->lock);
 	filep->private_data = kcov;
 	return nonseekable_open(inode, filep);
@@ -324,9 +536,23 @@ static int kcov_close(struct inode *inode, struct file *filep)
 	return 0;
 }
 
+static int kcov_get_mode(unsigned long arg)
+{
+	if (arg == KCOV_TRACE_PC)
+		return KCOV_MODE_TRACE_PC;
+	else if (arg == KCOV_TRACE_CMP)
+#ifdef CONFIG_KCOV_ENABLE_COMPARISONS
+		return KCOV_MODE_TRACE_CMP;
+#else
+		return -ENOTSUPP;
+#endif
+	else
+		return -EINVAL;
+}
+
 /*
  * Fault in a lazily-faulted vmalloc area before it can be used by
- * __santizer_cov_trace_pc(), to avoid recursion issues if any code on the
+ * __sanitizer_cov_trace_pc(), to avoid recursion issues if any code on the
  * vmalloc fault handling path is instrumented.
  */
 static void kcov_fault_in_area(struct kcov *kcov)
@@ -339,31 +565,33 @@ static void kcov_fault_in_area(struct kcov *kcov)
 		READ_ONCE(area[offset]);
 }
 
+static inline bool kcov_check_handle(u64 handle, bool common_valid,
+				bool uncommon_valid, bool zero_valid)
+{
+	if (handle & ~(KCOV_SUBSYSTEM_MASK | KCOV_INSTANCE_MASK))
+		return false;
+	switch (handle & KCOV_SUBSYSTEM_MASK) {
+	case KCOV_SUBSYSTEM_COMMON:
+		return (handle & KCOV_INSTANCE_MASK) ?
+			common_valid : zero_valid;
+	case KCOV_SUBSYSTEM_USB:
+		return uncommon_valid;
+	default:
+		return false;
+	}
+	return false;
+}
+
 static int kcov_ioctl_locked(struct kcov *kcov, unsigned int cmd,
 			     unsigned long arg)
 {
 	struct task_struct *t;
-	unsigned long size, unused;
+	unsigned long flags, unused;
+	int mode, i;
+	struct kcov_remote_arg *remote_arg;
+	struct kcov_remote *remote;
 
 	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_INIT;
-		return 0;
 	case KCOV_ENABLE:
 		/*
 		 * Enable coverage for the current task.
@@ -377,26 +605,15 @@ static int kcov_ioctl_locked(struct kcov *kcov, unsigned int cmd,
 		t = current;
 		if (kcov->t != NULL || t->kcov != NULL)
 			return -EBUSY;
-		if (arg == KCOV_TRACE_PC)
-			kcov->mode = KCOV_MODE_TRACE_PC;
-		else if (arg == KCOV_TRACE_CMP)
-#ifdef CONFIG_KCOV_ENABLE_COMPARISONS
-			kcov->mode = KCOV_MODE_TRACE_CMP;
-#else
-		return -ENOTSUPP;
-#endif
-		else
-			return -EINVAL;
+		mode = kcov_get_mode(arg);
+		if (mode < 0)
+			return mode;
 		kcov_fault_in_area(kcov);
-		/* Cache in task struct for performance. */
-		t->kcov_size = kcov->size;
-		t->kcov_area = kcov->area;
-		/* See comment in check_kcov_mode(). */
-		barrier();
-		WRITE_ONCE(t->kcov_mode, kcov->mode);
-		t->kcov = kcov;
+		kcov->mode = mode;
+		kcov_start(t, kcov, kcov->size, kcov->area, kcov->mode,
+				kcov->sequence);
 		kcov->t = t;
-		/* This is put either in kcov_task_exit() or in KCOV_DISABLE. */
+		/* Put either in kcov_task_exit() or in KCOV_DISABLE. */
 		kcov_get(kcov);
 		return 0;
 	case KCOV_DISABLE:
@@ -407,11 +624,67 @@ static int kcov_ioctl_locked(struct kcov *kcov, unsigned int cmd,
 		t = current;
 		if (WARN_ON(kcov->t != t))
 			return -EINVAL;
-		kcov_task_init(t);
-		kcov->t = NULL;
-		kcov->mode = KCOV_MODE_INIT;
+		kcov_disable(t, kcov);
 		kcov_put(kcov);
 		return 0;
+	case KCOV_REMOTE_ENABLE:
+		if (kcov->mode != KCOV_MODE_INIT || !kcov->area)
+			return -EINVAL;
+		t = current;
+		if (kcov->t != NULL || t->kcov != NULL)
+			return -EBUSY;
+		remote_arg = (struct kcov_remote_arg *)arg;
+		mode = kcov_get_mode(remote_arg->trace_mode);
+		if (mode < 0)
+			return mode;
+		if ((unsigned long)remote_arg->area_size >
+		    LONG_MAX / sizeof(unsigned long))
+			return -EINVAL;
+		kcov->mode = mode;
+		t->kcov = kcov;
+	        t->kcov_mode = KCOV_MODE_REMOTE;
+		kcov->t = t;
+		kcov->remote = true;
+		kcov->remote_size = remote_arg->area_size;
+		spin_lock_irqsave(&kcov_remote_lock, flags);
+		for (i = 0; i < remote_arg->num_handles; i++) {
+			if (!kcov_check_handle(remote_arg->handles[i],
+						false, true, false)) {
+				spin_unlock_irqrestore(&kcov_remote_lock,
+							flags);
+				kcov_disable(t, kcov);
+				return -EINVAL;
+			}
+			remote = kcov_remote_add(kcov, remote_arg->handles[i]);
+			if (IS_ERR(remote)) {
+				spin_unlock_irqrestore(&kcov_remote_lock,
+							flags);
+				kcov_disable(t, kcov);
+				return PTR_ERR(remote);
+			}
+		}
+		if (remote_arg->common_handle) {
+			if (!kcov_check_handle(remote_arg->common_handle,
+						true, false, false)) {
+				spin_unlock_irqrestore(&kcov_remote_lock,
+							flags);
+				kcov_disable(t, kcov);
+				return -EINVAL;
+			}
+			remote = kcov_remote_add(kcov,
+					remote_arg->common_handle);
+			if (IS_ERR(remote)) {
+				spin_unlock_irqrestore(&kcov_remote_lock,
+							flags);
+				kcov_disable(t, kcov);
+				return PTR_ERR(remote);
+			}
+			t->kcov_handle = remote_arg->common_handle;
+		}
+		spin_unlock_irqrestore(&kcov_remote_lock, flags);
+		/* Put either in kcov_task_exit() or in KCOV_DISABLE. */
+		kcov_get(kcov);
+		return 0;
 	default:
 		return -ENOTTY;
 	}
@@ -421,12 +694,67 @@ static long kcov_ioctl(struct file *filep, unsigned int cmd, unsigned long arg)
 {
 	struct kcov *kcov;
 	int res;
+	struct kcov_remote_arg *remote_arg = NULL;
+	unsigned int remote_num_handles;
+	unsigned long remote_arg_size;
+	unsigned long size, flags;
+	void *area;
 
 	kcov = filep->private_data;
-	spin_lock(&kcov->lock);
-	res = kcov_ioctl_locked(kcov, cmd, arg);
-	spin_unlock(&kcov->lock);
-	return res;
+	switch (cmd) {
+	case KCOV_INIT_TRACE:
+		/*
+		 * Enable kcov in trace mode and setup buffer size.
+		 * Must happen before anything else.
+		 *
+		 * First check the size argument - it must be at least 2
+		 * to hold the current position and one PC.
+		 */
+		size = arg;
+		if (size < 2 || size > INT_MAX / sizeof(unsigned long))
+			return -EINVAL;
+		area = vmalloc_user(size * sizeof(unsigned long));
+		if (area == NULL)
+			return -ENOMEM;
+		spin_lock_irqsave(&kcov->lock, flags);
+		if (kcov->mode != KCOV_MODE_DISABLED) {
+			spin_unlock_irqrestore(&kcov->lock, flags);
+			vfree(area);
+			return -EBUSY;
+		}
+		kcov->area = area;
+		kcov->size = size;
+		kcov->mode = KCOV_MODE_INIT;
+		spin_unlock_irqrestore(&kcov->lock, flags);
+		return 0;
+	case KCOV_REMOTE_ENABLE:
+		if (get_user(remote_num_handles, (unsigned __user *)(arg +
+				offsetof(struct kcov_remote_arg, num_handles))))
+			return -EFAULT;
+		if (remote_num_handles > KCOV_REMOTE_MAX_HANDLES)
+			return -EINVAL;
+		remote_arg_size = struct_size(remote_arg, handles,
+					remote_num_handles);
+		remote_arg = memdup_user((void __user *)arg, remote_arg_size);
+		if (IS_ERR(remote_arg))
+			return PTR_ERR(remote_arg);
+		if (remote_arg->num_handles != remote_num_handles) {
+			kfree(remote_arg);
+			return -EINVAL;
+		}
+		arg = (unsigned long)remote_arg;
+		fallthrough;
+	default:
+		/*
+		 * All other commands can be normally executed under a spin lock, so we
+		 * obtain and release it here in order to simplify kcov_ioctl_locked().
+		 */
+		spin_lock_irqsave(&kcov->lock, flags);
+		res = kcov_ioctl_locked(kcov, cmd, arg);
+		spin_unlock_irqrestore(&kcov->lock, flags);
+		kfree(remote_arg);
+		return res;
+	}
 }
 
 static const struct file_operations kcov_fops = {
@@ -437,17 +765,367 @@ static const struct file_operations kcov_fops = {
 	.release        = kcov_close,
 };
 
+/*
+ * kcov_remote_start() and kcov_remote_stop() can be used to annotate a section
+ * of code in a kernel background thread or in a softirq to allow kcov to be
+ * used to collect coverage from that part of code.
+ *
+ * The handle argument of kcov_remote_start() identifies a code section that is
+ * used for coverage collection. A userspace process passes this handle to
+ * KCOV_REMOTE_ENABLE ioctl to make the used kcov device start collecting
+ * coverage for the code section identified by this handle.
+ *
+ * The usage of these annotations in the kernel code is different depending on
+ * the type of the kernel thread whose code is being annotated.
+ *
+ * For global kernel threads that are spawned in a limited number of instances
+ * (e.g. one USB hub_event() worker thread is spawned per USB HCD) and for
+ * softirqs, each instance must be assigned a unique 4-byte instance id. The
+ * instance id is then combined with a 1-byte subsystem id to get a handle via
+ * kcov_remote_handle(subsystem_id, instance_id).
+ *
+ * For local kernel threads that are spawned from system calls handler when a
+ * user interacts with some kernel interface (e.g. vhost workers), a handle is
+ * passed from a userspace process as the common_handle field of the
+ * kcov_remote_arg struct (note, that the user must generate a handle by using
+ * kcov_remote_handle() with KCOV_SUBSYSTEM_COMMON as the subsystem id and an
+ * arbitrary 4-byte non-zero number as the instance id). This common handle
+ * then gets saved into the task_struct of the process that issued the
+ * KCOV_REMOTE_ENABLE ioctl. When this process issues system calls that spawn
+ * kernel threads, the common handle must be retrieved via kcov_common_handle()
+ * and passed to the spawned threads via custom annotations. Those kernel
+ * threads must in turn be annotated with kcov_remote_start(common_handle) and
+ * kcov_remote_stop(). All of the threads that are spawned by the same process
+ * obtain the same handle, hence the name "common".
+ *
+ * See Documentation/dev-tools/kcov.rst for more details.
+ *
+ * Internally, kcov_remote_start() looks up the kcov device associated with the
+ * provided handle, allocates an area for coverage collection, and saves the
+ * pointers to kcov and area into the current task_struct to allow coverage to
+ * be collected via __sanitizer_cov_trace_pc().
+ * In turns kcov_remote_stop() clears those pointers from task_struct to stop
+ * collecting coverage and copies all collected coverage into the kcov area.
+ */
+
+static inline bool kcov_mode_enabled(unsigned int mode)
+{
+	return (mode & ~KCOV_IN_CTXSW) != KCOV_MODE_DISABLED;
+}
+
+static void kcov_remote_softirq_start(struct task_struct *t)
+{
+	struct kcov_percpu_data *data = this_cpu_ptr(&kcov_percpu_data);
+	unsigned int mode;
+
+	mode = READ_ONCE(t->kcov_mode);
+	barrier();
+	if (kcov_mode_enabled(mode)) {
+		data->saved_mode = mode;
+		data->saved_size = t->kcov_size;
+		data->saved_area = t->kcov_area;
+		data->saved_sequence = t->kcov_sequence;
+		data->saved_kcov = t->kcov;
+		kcov_stop(t);
+	}
+}
+
+static void kcov_remote_softirq_stop(struct task_struct *t)
+{
+	struct kcov_percpu_data *data = this_cpu_ptr(&kcov_percpu_data);
+
+	if (data->saved_kcov) {
+		kcov_start(t, data->saved_kcov, data->saved_size,
+				data->saved_area, data->saved_mode,
+				data->saved_sequence);
+		data->saved_mode = 0;
+		data->saved_size = 0;
+		data->saved_area = NULL;
+		data->saved_sequence = 0;
+		data->saved_kcov = NULL;
+	}
+}
+
+void kcov_remote_start(u64 handle)
+{
+	struct task_struct *t = current;
+	struct kcov_remote *remote;
+	struct kcov *kcov;
+	unsigned int mode;
+	void *area;
+	unsigned int size;
+	int sequence;
+	unsigned long flags;
+
+	if (WARN_ON(!kcov_check_handle(handle, true, true, true)))
+		return;
+	if (!in_task() && !in_softirq_really())
+		return;
+
+	local_lock_irqsave(&kcov_percpu_data.lock, flags);
+
+	/*
+	 * Check that kcov_remote_start() is not called twice in background
+	 * threads nor called by user tasks (with enabled kcov).
+	 */
+	mode = READ_ONCE(t->kcov_mode);
+	if (WARN_ON(in_task() && kcov_mode_enabled(mode))) {
+		local_unlock_irqrestore(&kcov_percpu_data.lock, flags);
+		return;
+	}
+	/*
+	 * Check that kcov_remote_start() is not called twice in softirqs.
+	 * Note, that kcov_remote_start() can be called from a softirq that
+	 * happened while collecting coverage from a background thread.
+	 */
+	if (WARN_ON(in_serving_softirq() && t->kcov_softirq)) {
+		local_unlock_irqrestore(&kcov_percpu_data.lock, flags);
+		return;
+	}
+
+	spin_lock(&kcov_remote_lock);
+	remote = kcov_remote_find(handle);
+	if (!remote) {
+		spin_unlock(&kcov_remote_lock);
+		local_unlock_irqrestore(&kcov_percpu_data.lock, flags);
+		return;
+	}
+	kcov_debug("handle = %llx, context: %s\n", handle,
+			in_task() ? "task" : "softirq");
+	kcov = remote->kcov;
+	/* Put in kcov_remote_stop(). */
+	kcov_get(kcov);
+	/*
+	 * Read kcov fields before unlock to prevent races with
+	 * KCOV_DISABLE / kcov_remote_reset().
+	 */
+	mode = kcov->mode;
+	sequence = kcov->sequence;
+	if (in_task()) {
+		size = kcov->remote_size;
+		area = kcov_remote_area_get(size);
+	} else {
+		size = CONFIG_KCOV_IRQ_AREA_SIZE;
+		area = this_cpu_ptr(&kcov_percpu_data)->irq_area;
+	}
+	spin_unlock(&kcov_remote_lock);
+
+	/* Can only happen when in_task(). */
+	if (!area) {
+		local_unlock_irqrestore(&kcov_percpu_data.lock, flags);
+		area = vmalloc(size * sizeof(unsigned long));
+		if (!area) {
+			kcov_put(kcov);
+			return;
+		}
+		local_lock_irqsave(&kcov_percpu_data.lock, flags);
+	}
+
+	/* Reset coverage size. */
+	*(u64 *)area = 0;
+
+	if (in_serving_softirq()) {
+		kcov_remote_softirq_start(t);
+		t->kcov_softirq = 1;
+	}
+	kcov_start(t, kcov, size, area, mode, sequence);
+
+	local_unlock_irqrestore(&kcov_percpu_data.lock, flags);
+
+}
+EXPORT_SYMBOL(kcov_remote_start);
+
+static void kcov_move_area(enum kcov_mode mode, void *dst_area,
+				unsigned int dst_area_size, void *src_area)
+{
+	u64 word_size = sizeof(unsigned long);
+	u64 count_size, entry_size_log;
+	u64 dst_len, src_len;
+	void *dst_entries, *src_entries;
+	u64 dst_occupied, dst_free, bytes_to_move, entries_moved;
+
+	kcov_debug("%px %u <= %px %lu\n",
+		dst_area, dst_area_size, src_area, *(unsigned long *)src_area);
+
+	switch (mode) {
+	case KCOV_MODE_TRACE_PC:
+		dst_len = READ_ONCE(*(unsigned long *)dst_area);
+		src_len = *(unsigned long *)src_area;
+		count_size = sizeof(unsigned long);
+		entry_size_log = __ilog2_u64(sizeof(unsigned long));
+		break;
+	case KCOV_MODE_TRACE_CMP:
+		dst_len = READ_ONCE(*(u64 *)dst_area);
+		src_len = *(u64 *)src_area;
+		count_size = sizeof(u64);
+		BUILD_BUG_ON(!is_power_of_2(KCOV_WORDS_PER_CMP));
+		entry_size_log = __ilog2_u64(sizeof(u64) * KCOV_WORDS_PER_CMP);
+		break;
+	default:
+		WARN_ON(1);
+		return;
+	}
+
+	/* As arm can't divide u64 integers use log of entry size. */
+	if (dst_len > ((dst_area_size * word_size - count_size) >>
+				entry_size_log))
+		return;
+	dst_occupied = count_size + (dst_len << entry_size_log);
+	dst_free = dst_area_size * word_size - dst_occupied;
+	bytes_to_move = min(dst_free, src_len << entry_size_log);
+	dst_entries = dst_area + dst_occupied;
+	src_entries = src_area + count_size;
+	memcpy(dst_entries, src_entries, bytes_to_move);
+	entries_moved = bytes_to_move >> entry_size_log;
+
+	/*
+	 * A write memory barrier is required here, to ensure
+	 * that the writes from the memcpy() are visible before
+	 * the count is updated. Without this, it is possible for
+	 * a user to observe a new count value but stale
+	 * coverage data.
+	 */
+	smp_wmb();
+
+	switch (mode) {
+	case KCOV_MODE_TRACE_PC:
+		WRITE_ONCE(*(unsigned long *)dst_area, dst_len + entries_moved);
+		break;
+	case KCOV_MODE_TRACE_CMP:
+		WRITE_ONCE(*(u64 *)dst_area, dst_len + entries_moved);
+		break;
+	default:
+		break;
+	}
+}
+
+/* See the comment before kcov_remote_start() for usage details. */
+void kcov_remote_stop(void)
+{
+	struct task_struct *t = current;
+	struct kcov *kcov;
+	unsigned int mode;
+	void *area;
+	unsigned int size;
+	int sequence;
+	unsigned long flags;
+
+	if (!in_task() && !in_softirq_really())
+		return;
+
+	local_lock_irqsave(&kcov_percpu_data.lock, flags);
+
+	mode = READ_ONCE(t->kcov_mode);
+	barrier();
+	if (!kcov_mode_enabled(mode)) {
+		local_unlock_irqrestore(&kcov_percpu_data.lock, flags);
+		return;
+	}
+	/*
+	 * When in softirq, check if the corresponding kcov_remote_start()
+	 * actually found the remote handle and started collecting coverage.
+	 */
+	if (in_serving_softirq() && !t->kcov_softirq) {
+		local_unlock_irqrestore(&kcov_percpu_data.lock, flags);
+		return;
+	}
+	/* Make sure that kcov_softirq is only set when in softirq. */
+	if (WARN_ON(!in_serving_softirq() && t->kcov_softirq)) {
+		local_unlock_irqrestore(&kcov_percpu_data.lock, flags);
+		return;
+	}
+
+	kcov = t->kcov;
+	area = t->kcov_area;
+	size = t->kcov_size;
+	sequence = t->kcov_sequence;
+
+	kcov_stop(t);
+	if (in_serving_softirq()) {
+		t->kcov_softirq = 0;
+		kcov_remote_softirq_stop(t);
+	}
+
+	spin_lock(&kcov->lock);
+	/*
+	 * KCOV_DISABLE could have been called between kcov_remote_start()
+	 * and kcov_remote_stop(), hence the sequence check.
+	 */
+	if (sequence == kcov->sequence && kcov->remote)
+		kcov_move_area(kcov->mode, kcov->area, kcov->size, area);
+	spin_unlock(&kcov->lock);
+
+	if (in_task()) {
+		spin_lock(&kcov_remote_lock);
+		kcov_remote_area_put(area, size);
+		spin_unlock(&kcov_remote_lock);
+	}
+
+	local_unlock_irqrestore(&kcov_percpu_data.lock, flags);
+
+	/* Get in kcov_remote_start(). */
+	kcov_put(kcov);
+}
+EXPORT_SYMBOL(kcov_remote_stop);
+
+/* See the comment before kcov_remote_start() for usage details. */
+u64 kcov_common_handle(void)
+{
+	if (!in_task())
+		return 0;
+	return current->kcov_handle;
+}
+EXPORT_SYMBOL(kcov_common_handle);
+
+#ifdef CONFIG_KCOV_SELFTEST
+static void __init selftest(void)
+{
+	unsigned long start;
+
+	pr_err("running self test\n");
+	/*
+	 * Test that interrupts don't produce spurious coverage.
+	 * The coverage callback filters out interrupt code, but only
+	 * after the handler updates preempt count. Some code periodically
+	 * leaks out of that section and leads to spurious coverage.
+	 * It's hard to call the actual interrupt handler directly,
+	 * so we just loop here for a bit waiting for a timer interrupt.
+	 * We set kcov_mode to enable tracing, but don't setup the area,
+	 * so any attempt to trace will crash. Note: we must not call any
+	 * potentially traced functions in this region.
+	 */
+	start = jiffies;
+	current->kcov_mode = KCOV_MODE_TRACE_PC;
+	while ((jiffies - start) * MSEC_PER_SEC / HZ < 300)
+		;
+	current->kcov_mode = 0;
+	pr_err("done running self test\n");
+}
+#endif
+
 static int __init kcov_init(void)
 {
+	int cpu;
+
+	for_each_possible_cpu(cpu) {
+		void *area = vmalloc_node(CONFIG_KCOV_IRQ_AREA_SIZE *
+				sizeof(unsigned long), cpu_to_node(cpu));
+		if (!area)
+			return -ENOMEM;
+		per_cpu_ptr(&kcov_percpu_data, cpu)->irq_area = area;
+	}
+
 	/*
 	 * The kcov debugfs file won't ever get removed and thus,
 	 * there is no need to protect it against removal races. The
 	 * use of debugfs_create_file_unsafe() is actually safe here.
 	 */
-	if (!debugfs_create_file_unsafe("kcov", 0600, NULL, NULL, &kcov_fops)) {
-		pr_err("failed to create kcov in debugfs\n");
-		return -ENOMEM;
-	}
+	debugfs_create_file_unsafe("kcov", 0600, NULL, NULL, &kcov_fops);
+
+#ifdef CONFIG_KCOV_SELFTEST
+	selftest();
+#endif
+
 	return 0;
 }