kcov: Enable context analysis

Enable context analysis for the KCOV subsystem.

Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20251219154418.3592607-30-elver@google.com

kernel/Makefile

@@ -43,6 +43,8 @@ KASAN_SANITIZE_kcov.o := n
 KCSAN_SANITIZE_kcov.o := n
 UBSAN_SANITIZE_kcov.o := n
 KMSAN_SANITIZE_kcov.o := n
+
+CONTEXT_ANALYSIS_kcov.o := y
 CFLAGS_kcov.o := $(call cc-option, -fno-conserve-stack) -fno-stack-protector
 
 obj-y += sched/

kernel/kcov.c

@@ -55,13 +55,13 @@ struct kcov {
 	refcount_t		refcount;
 	/* The lock protects mode, size, area and t. */
 	spinlock_t		lock;
-	enum kcov_mode		mode;
+	enum kcov_mode		mode __guarded_by(&lock);
 	/* Size of arena (in long's). */
-	unsigned int		size;
+	unsigned int		size __guarded_by(&lock);
 	/* Coverage buffer shared with user space. */
-	void			*area;
+	void			*area __guarded_by(&lock);
 	/* Task for which we collect coverage, or NULL. */
-	struct task_struct	*t;
+	struct task_struct	*t __guarded_by(&lock);
 	/* Collecting coverage from remote (background) threads. */
 	bool			remote;
 	/* Size of remote area (in long's). */
@@ -391,6 +391,7 @@ void kcov_task_init(struct task_struct *t)
 }
 
 static void kcov_reset(struct kcov *kcov)
+	__must_hold(&kcov->lock)
 {
 	kcov->t = NULL;
 	kcov->mode = KCOV_MODE_INIT;
@@ -400,6 +401,7 @@ static void kcov_reset(struct kcov *kcov)
 }
 
 static void kcov_remote_reset(struct kcov *kcov)
+	__must_hold(&kcov->lock)
 {
 	int bkt;
 	struct kcov_remote *remote;
@@ -419,6 +421,7 @@ static void kcov_remote_reset(struct kcov *kcov)
 }
 
 static void kcov_disable(struct task_struct *t, struct kcov *kcov)
+	__must_hold(&kcov->lock)
 {
 	kcov_task_reset(t);
 	if (kcov->remote)
@@ -435,8 +438,11 @@ static void kcov_get(struct kcov *kcov)
 static void kcov_put(struct kcov *kcov)
 {
 	if (refcount_dec_and_test(&kcov->refcount)) {
-		kcov_remote_reset(kcov);
+		/* Context-safety: no references left, object being destroyed. */
-		vfree(kcov->area);
+		context_unsafe(
+			kcov_remote_reset(kcov);
+			vfree(kcov->area);
+		);
 		kfree(kcov);
 	}
 }
@@ -491,6 +497,7 @@ static int kcov_mmap(struct file *filep, struct vm_area_struct *vma)
 	unsigned long size, off;
 	struct page *page;
 	unsigned long flags;
+	void *area;
 
 	spin_lock_irqsave(&kcov->lock, flags);
 	size = kcov->size * sizeof(unsigned long);
@@ -499,10 +506,11 @@ static int kcov_mmap(struct file *filep, struct vm_area_struct *vma)
 		res = -EINVAL;
 		goto exit;
 	}
+	area = kcov->area;
 	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);
+		page = vmalloc_to_page(area + off);
 		res = vm_insert_page(vma, vma->vm_start + off, page);
 		if (res) {
 			pr_warn_once("kcov: vm_insert_page() failed\n");
@@ -522,10 +530,10 @@ static int kcov_open(struct inode *inode, struct file *filep)
 	kcov = kzalloc(sizeof(*kcov), GFP_KERNEL);
 	if (!kcov)
 		return -ENOMEM;
+	spin_lock_init(&kcov->lock);
 	kcov->mode = KCOV_MODE_DISABLED;
 	kcov->sequence = 1;
 	refcount_set(&kcov->refcount, 1);
-	spin_lock_init(&kcov->lock);
 	filep->private_data = kcov;
 	return nonseekable_open(inode, filep);
 }
@@ -556,6 +564,7 @@ static int kcov_get_mode(unsigned long arg)
  * vmalloc fault handling path is instrumented.
  */
 static void kcov_fault_in_area(struct kcov *kcov)
+	__must_hold(&kcov->lock)
 {
 	unsigned long stride = PAGE_SIZE / sizeof(unsigned long);
 	unsigned long *area = kcov->area;
@@ -584,6 +593,7 @@ static inline bool kcov_check_handle(u64 handle, bool common_valid,
 
 static int kcov_ioctl_locked(struct kcov *kcov, unsigned int cmd,
 			     unsigned long arg)
+	__must_hold(&kcov->lock)
 {
 	struct task_struct *t;
 	unsigned long flags, unused;
@@ -814,6 +824,7 @@ static inline bool kcov_mode_enabled(unsigned int mode)
 }
 
 static void kcov_remote_softirq_start(struct task_struct *t)
+	__must_hold(&kcov_percpu_data.lock)
 {
 	struct kcov_percpu_data *data = this_cpu_ptr(&kcov_percpu_data);
 	unsigned int mode;
@@ -831,6 +842,7 @@ static void kcov_remote_softirq_start(struct task_struct *t)
 }
 
 static void kcov_remote_softirq_stop(struct task_struct *t)
+	__must_hold(&kcov_percpu_data.lock)
 {
 	struct kcov_percpu_data *data = this_cpu_ptr(&kcov_percpu_data);
 
@@ -896,10 +908,12 @@ void kcov_remote_start(u64 handle)
 	/* Put in kcov_remote_stop(). */
 	kcov_get(kcov);
 	/*
-	 * Read kcov fields before unlock to prevent races with
+	 * Read kcov fields before unlocking kcov_remote_lock to prevent races
-	 * KCOV_DISABLE / kcov_remote_reset().
+	 * with KCOV_DISABLE and kcov_remote_reset(); cannot acquire kcov->lock
+	 * here, because it might lead to deadlock given kcov_remote_lock is
+	 * acquired _after_ kcov->lock elsewhere.
 	 */
-	mode = kcov->mode;
+	mode = context_unsafe(kcov->mode);
 	sequence = kcov->sequence;
 	if (in_task()) {
 		size = kcov->remote_size;