Merge branch 'kcsan.2021.05.18a' of git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu

Pull KCSAN updates from Paul McKenney.

* 'kcsan.2021.05.18a' of git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu:
  kcsan: Use URL link for pointing access-marking.txt
  kcsan: Document "value changed" line
  kcsan: Report observed value changes
  kcsan: Remove kcsan_report_type
  kcsan: Remove reporting indirection
  kcsan: Refactor access_info initialization
  kcsan: Fold panic() call into print_report()
  kcsan: Refactor passing watchpoint/other_info
  kcsan: Distinguish kcsan_report() calls
  kcsan: Simplify value change detection
  kcsan: Add pointer to access-marking.txt to data_race() bullet

3 files changed, 127 insertions, 134 deletions

diff --git a/kernel/kcsan/core.c b/kernel/kcsan/core.c
index 45c821d4e8bd..26709ea65c71 100644
--- a/kernel/kcsan/core.c
+++ b/kernel/kcsan/core.c
@@ -380,9 +380,7 @@ static noinline void kcsan_found_watchpoint(const volatile void *ptr,
 
 	if (consumed) {
 		kcsan_save_irqtrace(current);
-		kcsan_report(ptr, size, type, KCSAN_VALUE_CHANGE_MAYBE,
-			     KCSAN_REPORT_CONSUMED_WATCHPOINT,
-			     watchpoint - watchpoints);
+		kcsan_report_set_info(ptr, size, type, watchpoint - watchpoints);
 		kcsan_restore_irqtrace(current);
 	} else {
 		/*
@@ -407,12 +405,7 @@ kcsan_setup_watchpoint(const volatile void *ptr, size_t size, int type)
 	const bool is_write = (type & KCSAN_ACCESS_WRITE) != 0;
 	const bool is_assert = (type & KCSAN_ACCESS_ASSERT) != 0;
 	atomic_long_t *watchpoint;
-	union {
-		u8 _1;
-		u16 _2;
-		u32 _4;
-		u64 _8;
-	} expect_value;
+	u64 old, new, diff;
 	unsigned long access_mask;
 	enum kcsan_value_change value_change = KCSAN_VALUE_CHANGE_MAYBE;
 	unsigned long ua_flags = user_access_save();
@@ -468,19 +461,19 @@ kcsan_setup_watchpoint(const volatile void *ptr, size_t size, int type)
 	 * Read the current value, to later check and infer a race if the data
 	 * was modified via a non-instrumented access, e.g. from a device.
 	 */
-	expect_value._8 = 0;
+	old = 0;
 	switch (size) {
 	case 1:
-		expect_value._1 = READ_ONCE(*(const u8 *)ptr);
+		old = READ_ONCE(*(const u8 *)ptr);
 		break;
 	case 2:
-		expect_value._2 = READ_ONCE(*(const u16 *)ptr);
+		old = READ_ONCE(*(const u16 *)ptr);
 		break;
 	case 4:
-		expect_value._4 = READ_ONCE(*(const u32 *)ptr);
+		old = READ_ONCE(*(const u32 *)ptr);
 		break;
 	case 8:
-		expect_value._8 = READ_ONCE(*(const u64 *)ptr);
+		old = READ_ONCE(*(const u64 *)ptr);
 		break;
 	default:
 		break; /* ignore; we do not diff the values */
@@ -506,33 +499,30 @@ kcsan_setup_watchpoint(const volatile void *ptr, size_t size, int type)
 	 * racy access.
 	 */
 	access_mask = get_ctx()->access_mask;
+	new = 0;
 	switch (size) {
 	case 1:
-		expect_value._1 ^= READ_ONCE(*(const u8 *)ptr);
-		if (access_mask)
-			expect_value._1 &= (u8)access_mask;
+		new = READ_ONCE(*(const u8 *)ptr);
 		break;
 	case 2:
-		expect_value._2 ^= READ_ONCE(*(const u16 *)ptr);
-		if (access_mask)
-			expect_value._2 &= (u16)access_mask;
+		new = READ_ONCE(*(const u16 *)ptr);
 		break;
 	case 4:
-		expect_value._4 ^= READ_ONCE(*(const u32 *)ptr);
-		if (access_mask)
-			expect_value._4 &= (u32)access_mask;
+		new = READ_ONCE(*(const u32 *)ptr);
 		break;
 	case 8:
-		expect_value._8 ^= READ_ONCE(*(const u64 *)ptr);
-		if (access_mask)
-			expect_value._8 &= (u64)access_mask;
+		new = READ_ONCE(*(const u64 *)ptr);
 		break;
 	default:
 		break; /* ignore; we do not diff the values */
 	}
 
+	diff = old ^ new;
+	if (access_mask)
+		diff &= access_mask;
+
 	/* Were we able to observe a value-change? */
-	if (expect_value._8 != 0)
+	if (diff != 0)
 		value_change = KCSAN_VALUE_CHANGE_TRUE;
 
 	/* Check if this access raced with another. */
@@ -566,8 +556,9 @@ kcsan_setup_watchpoint(const volatile void *ptr, size_t size, int type)
 		if (is_assert && value_change == KCSAN_VALUE_CHANGE_TRUE)
 			atomic_long_inc(&kcsan_counters[KCSAN_COUNTER_ASSERT_FAILURES]);
 
-		kcsan_report(ptr, size, type, value_change, KCSAN_REPORT_RACE_SIGNAL,
-			     watchpoint - watchpoints);
+		kcsan_report_known_origin(ptr, size, type, value_change,
+					  watchpoint - watchpoints,
+					  old, new, access_mask);
 	} else if (value_change == KCSAN_VALUE_CHANGE_TRUE) {
 		/* Inferring a race, since the value should not have changed. */
 
@@ -576,9 +567,7 @@ kcsan_setup_watchpoint(const volatile void *ptr, size_t size, int type)
 			atomic_long_inc(&kcsan_counters[KCSAN_COUNTER_ASSERT_FAILURES]);
 
 		if (IS_ENABLED(CONFIG_KCSAN_REPORT_RACE_UNKNOWN_ORIGIN) || is_assert)
-			kcsan_report(ptr, size, type, KCSAN_VALUE_CHANGE_TRUE,
-				     KCSAN_REPORT_RACE_UNKNOWN_ORIGIN,
-				     watchpoint - watchpoints);
+			kcsan_report_unknown_origin(ptr, size, type, old, new, access_mask);
 	}
 
 	/*
diff --git a/kernel/kcsan/kcsan.h b/kernel/kcsan/kcsan.h
index 9881099d4179..f36e25c497ed 100644
--- a/kernel/kcsan/kcsan.h
+++ b/kernel/kcsan/kcsan.h
@@ -116,30 +116,27 @@ enum kcsan_value_change {
 	KCSAN_VALUE_CHANGE_TRUE,
 };
 
-enum kcsan_report_type {
-	/*
-	 * The thread that set up the watchpoint and briefly stalled was
-	 * signalled that another thread triggered the watchpoint.
-	 */
-	KCSAN_REPORT_RACE_SIGNAL,
-
-	/*
-	 * A thread found and consumed a matching watchpoint.
-	 */
-	KCSAN_REPORT_CONSUMED_WATCHPOINT,
+/*
+ * The calling thread hit and consumed a watchpoint: set the access information
+ * to be consumed by the reporting thread. No report is printed yet.
+ */
+void kcsan_report_set_info(const volatile void *ptr, size_t size, int access_type,
+			   int watchpoint_idx);
 
-	/*
-	 * No other thread was observed to race with the access, but the data
-	 * value before and after the stall differs.
-	 */
-	KCSAN_REPORT_RACE_UNKNOWN_ORIGIN,
-};
+/*
+ * The calling thread observed that the watchpoint it set up was hit and
+ * consumed: print the full report based on information set by the racing
+ * thread.
+ */
+void kcsan_report_known_origin(const volatile void *ptr, size_t size, int access_type,
+			       enum kcsan_value_change value_change, int watchpoint_idx,
+			       u64 old, u64 new, u64 mask);
 
 /*
- * Print a race report from thread that encountered the race.
+ * No other thread was observed to race with the access, but the data value
+ * before and after the stall differs. Reports a race of "unknown origin".
  */
-extern void kcsan_report(const volatile void *ptr, size_t size, int access_type,
-			 enum kcsan_value_change value_change,
-			 enum kcsan_report_type type, int watchpoint_idx);
+void kcsan_report_unknown_origin(const volatile void *ptr, size_t size, int access_type,
+				 u64 old, u64 new, u64 mask);
 
 #endif /* _KERNEL_KCSAN_KCSAN_H */
diff --git a/kernel/kcsan/report.c b/kernel/kcsan/report.c
index 56016e8e7461..21137929d428 100644
--- a/kernel/kcsan/report.c
+++ b/kernel/kcsan/report.c
@@ -325,13 +325,10 @@ static void print_verbose_info(struct task_struct *task)
 	print_irqtrace_events(task);
 }
 
-/*
- * Returns true if a report was generated, false otherwise.
- */
-static bool print_report(enum kcsan_value_change value_change,
-			 enum kcsan_report_type type,
+static void print_report(enum kcsan_value_change value_change,
 			 const struct access_info *ai,
-			 const struct other_info *other_info)
+			 const struct other_info *other_info,
+			 u64 old, u64 new, u64 mask)
 {
 	unsigned long stack_entries[NUM_STACK_ENTRIES] = { 0 };
 	int num_stack_entries = stack_trace_save(stack_entries, NUM_STACK_ENTRIES, 1);
@@ -344,25 +341,24 @@ static bool print_report(enum kcsan_value_change value_change,
 	 * Must check report filter rules before starting to print.
 	 */
 	if (skip_report(KCSAN_VALUE_CHANGE_TRUE, stack_entries[skipnr]))
-		return false;
+		return;
 
-	if (type == KCSAN_REPORT_RACE_SIGNAL) {
+	if (other_info) {
 		other_skipnr = get_stack_skipnr(other_info->stack_entries,
 						other_info->num_stack_entries);
 		other_frame = other_info->stack_entries[other_skipnr];
 
 		/* @value_change is only known for the other thread */
 		if (skip_report(value_change, other_frame))
-			return false;
+			return;
 	}
 
 	if (rate_limit_report(this_frame, other_frame))
-		return false;
+		return;
 
 	/* Print report header. */
 	pr_err("==================================================================\n");
-	switch (type) {
-	case KCSAN_REPORT_RACE_SIGNAL: {
+	if (other_info) {
 		int cmp;
 
 		/*
@@ -374,22 +370,15 @@ static bool print_report(enum kcsan_value_change value_change,
 		       get_bug_type(ai->access_type | other_info->ai.access_type),
 		       (void *)(cmp < 0 ? other_frame : this_frame),
 		       (void *)(cmp < 0 ? this_frame : other_frame));
-	} break;
-
-	case KCSAN_REPORT_RACE_UNKNOWN_ORIGIN:
+	} else {
 		pr_err("BUG: KCSAN: %s in %pS\n", get_bug_type(ai->access_type),
 		       (void *)this_frame);
-		break;
-
-	default:
-		BUG();
 	}
 
 	pr_err("\n");
 
 	/* Print information about the racing accesses. */
-	switch (type) {
-	case KCSAN_REPORT_RACE_SIGNAL:
+	if (other_info) {
 		pr_err("%s to 0x%px of %zu bytes by %s on cpu %i:\n",
 		       get_access_type(other_info->ai.access_type), other_info->ai.ptr,
 		       other_info->ai.size, get_thread_desc(other_info->ai.task_pid),
@@ -407,16 +396,10 @@ static bool print_report(enum kcsan_value_change value_change,
 		pr_err("%s to 0x%px of %zu bytes by %s on cpu %i:\n",
 		       get_access_type(ai->access_type), ai->ptr, ai->size,
 		       get_thread_desc(ai->task_pid), ai->cpu_id);
-		break;
-
-	case KCSAN_REPORT_RACE_UNKNOWN_ORIGIN:
+	} else {
 		pr_err("race at unknown origin, with %s to 0x%px of %zu bytes by %s on cpu %i:\n",
 		       get_access_type(ai->access_type), ai->ptr, ai->size,
 		       get_thread_desc(ai->task_pid), ai->cpu_id);
-		break;
-
-	default:
-		BUG();
 	}
 	/* Print stack trace of this thread. */
 	stack_trace_print(stack_entries + skipnr, num_stack_entries - skipnr,
@@ -425,24 +408,41 @@ static bool print_report(enum kcsan_value_change value_change,
 	if (IS_ENABLED(CONFIG_KCSAN_VERBOSE))
 		print_verbose_info(current);
 
+	/* Print observed value change. */
+	if (ai->size <= 8) {
+		int hex_len = ai->size * 2;
+		u64 diff = old ^ new;
+
+		if (mask)
+			diff &= mask;
+		if (diff) {
+			pr_err("\n");
+			pr_err("value changed: 0x%0*llx -> 0x%0*llx\n",
+			       hex_len, old, hex_len, new);
+			if (mask) {
+				pr_err(" bits changed: 0x%0*llx with mask 0x%0*llx\n",
+				       hex_len, diff, hex_len, mask);
+			}
+		}
+	}
+
 	/* Print report footer. */
 	pr_err("\n");
 	pr_err("Reported by Kernel Concurrency Sanitizer on:\n");
 	dump_stack_print_info(KERN_DEFAULT);
 	pr_err("==================================================================\n");
 
-	return true;
+	if (panic_on_warn)
+		panic("panic_on_warn set ...\n");
 }
 
 static void release_report(unsigned long *flags, struct other_info *other_info)
 {
-	if (other_info)
-		/*
-		 * Use size to denote valid/invalid, since KCSAN entirely
-		 * ignores 0-sized accesses.
-		 */
-		other_info->ai.size = 0;
-
+	/*
+	 * Use size to denote valid/invalid, since KCSAN entirely ignores
+	 * 0-sized accesses.
+	 */
+	other_info->ai.size = 0;
 	raw_spin_unlock_irqrestore(&report_lock, *flags);
 }
 
@@ -575,48 +575,42 @@ discard:
 	return false;
 }
 
-/*
- * Depending on the report type either sets @other_info and returns false, or
- * awaits @other_info and returns true. If @other_info is not required for the
- * report type, simply acquires @report_lock and returns true.
- */
-static noinline bool prepare_report(unsigned long *flags,
-				    enum kcsan_report_type type,
-				    const struct access_info *ai,
-				    struct other_info *other_info)
+static struct access_info prepare_access_info(const volatile void *ptr, size_t size,
+					      int access_type)
 {
-	switch (type) {
-	case KCSAN_REPORT_CONSUMED_WATCHPOINT:
-		prepare_report_producer(flags, ai, other_info);
-		return false;
-	case KCSAN_REPORT_RACE_SIGNAL:
-		return prepare_report_consumer(flags, ai, other_info);
-	default:
-		/* @other_info not required; just acquire @report_lock. */
-		raw_spin_lock_irqsave(&report_lock, *flags);
-		return true;
-	}
-}
-
-void kcsan_report(const volatile void *ptr, size_t size, int access_type,
-		  enum kcsan_value_change value_change,
-		  enum kcsan_report_type type, int watchpoint_idx)
-{
-	unsigned long flags = 0;
-	const struct access_info ai = {
+	return (struct access_info) {
 		.ptr		= ptr,
 		.size		= size,
 		.access_type	= access_type,
 		.task_pid	= in_task() ? task_pid_nr(current) : -1,
 		.cpu_id		= raw_smp_processor_id()
 	};
-	struct other_info *other_info = type == KCSAN_REPORT_RACE_UNKNOWN_ORIGIN
-					? NULL : &other_infos[watchpoint_idx];
+}
+
+void kcsan_report_set_info(const volatile void *ptr, size_t size, int access_type,
+			   int watchpoint_idx)
+{
+	const struct access_info ai = prepare_access_info(ptr, size, access_type);
+	unsigned long flags;
 
 	kcsan_disable_current();
-	if (WARN_ON(watchpoint_idx < 0 || watchpoint_idx >= ARRAY_SIZE(other_infos)))
-		goto out;
+	lockdep_off(); /* See kcsan_report_known_origin(). */
+
+	prepare_report_producer(&flags, &ai, &other_infos[watchpoint_idx]);
+
+	lockdep_on();
+	kcsan_enable_current();
+}
+
+void kcsan_report_known_origin(const volatile void *ptr, size_t size, int access_type,
+			       enum kcsan_value_change value_change, int watchpoint_idx,
+			       u64 old, u64 new, u64 mask)
+{
+	const struct access_info ai = prepare_access_info(ptr, size, access_type);
+	struct other_info *other_info = &other_infos[watchpoint_idx];
+	unsigned long flags = 0;
 
+	kcsan_disable_current();
 	/*
 	 * Because we may generate reports when we're in scheduler code, the use
 	 * of printk() could deadlock. Until such time that all printing code
@@ -626,22 +620,35 @@ void kcsan_report(const volatile void *ptr, size_t size, int access_type,
 	 */
 	lockdep_off();
 
-	if (prepare_report(&flags, type, &ai, other_info)) {
-		/*
-		 * Never report if value_change is FALSE, only if we it is
-		 * either TRUE or MAYBE. In case of MAYBE, further filtering may
-		 * be done once we know the full stack trace in print_report().
-		 */
-		bool reported = value_change != KCSAN_VALUE_CHANGE_FALSE &&
-				print_report(value_change, type, &ai, other_info);
+	if (!prepare_report_consumer(&flags, &ai, other_info))
+		goto out;
+	/*
+	 * Never report if value_change is FALSE, only when it is
+	 * either TRUE or MAYBE. In case of MAYBE, further filtering may
+	 * be done once we know the full stack trace in print_report().
+	 */
+	if (value_change != KCSAN_VALUE_CHANGE_FALSE)
+		print_report(value_change, &ai, other_info, old, new, mask);
 
-		if (reported && panic_on_warn)
-			panic("panic_on_warn set ...\n");
+	release_report(&flags, other_info);
+out:
+	lockdep_on();
+	kcsan_enable_current();
+}
 
-		release_report(&flags, other_info);
-	}
+void kcsan_report_unknown_origin(const volatile void *ptr, size_t size, int access_type,
+				 u64 old, u64 new, u64 mask)
+{
+	const struct access_info ai = prepare_access_info(ptr, size, access_type);
+	unsigned long flags;
+
+	kcsan_disable_current();
+	lockdep_off(); /* See kcsan_report_known_origin(). */
+
+	raw_spin_lock_irqsave(&report_lock, flags);
+	print_report(KCSAN_VALUE_CHANGE_TRUE, &ai, NULL, old, new, mask);
+	raw_spin_unlock_irqrestore(&report_lock, flags);
 
 	lockdep_on();
-out:
 	kcsan_enable_current();
 }