[PATCH -rcu 2/2] kcsan: Rate-limit reporting per data races

[Marco Elver]

Adds support for rate limiting reports. This uses a time based rate
limit, that limits any given data race report to no more than one in a
fixed time window (default is 3 sec). This should prevent the console
from being spammed with data race reports, that would render the system
unusable.

The implementation assumes that unique data races and the rate at which
they occur is bounded, since we cannot store arbitrarily many past data
race report information: we use a fixed-size array to store the required
information. We cannot use kmalloc/krealloc and resize the list when
needed, as reporting is triggered by the instrumentation calls; to
permit using KCSAN on the allocators, we cannot (re-)allocate any memory
during report generation (data races in the allocators lead to
deadlock).

Reported-by: Qian Cai <cai@xxxxxx>
Suggested-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
Signed-off-by: Marco Elver <elver@xxxxxxxxxx>
---
 kernel/kcsan/report.c | 112 ++++++++++++++++++++++++++++++++++++++----
 lib/Kconfig.kcsan     |  10 ++++
 2 files changed, 112 insertions(+), 10 deletions(-)

diff --git a/kernel/kcsan/report.c b/kernel/kcsan/report.c
index 9f503ca2ff7a..e324af7d14c9 100644
--- a/kernel/kcsan/report.c
+++ b/kernel/kcsan/report.c
@@ -1,6 +1,7 @@
 // SPDX-License-Identifier: GPL-2.0
 
 #include <linux/kernel.h>
+#include <linux/ktime.h>
 #include <linux/preempt.h>
 #include <linux/printk.h>
 #include <linux/sched.h>
@@ -31,12 +32,101 @@ static struct {
 	int			num_stack_entries;
 } other_info = { .ptr = NULL };
 
+/*
+ * Information about reported data races; used to rate limit reporting.
+ */
+struct report_time {
+	/*
+	 * The last time the data race was reported.
+	 */
+	ktime_t time;
+
+	/*
+	 * The frames of the 2 threads; if only 1 thread is known, one frame
+	 * will be 0.
+	 */
+	unsigned long frame1;
+	unsigned long frame2;
+};
+
+/*
+ * Since we also want to be able to debug allocators with KCSAN, to avoid
+ * deadlock, report_times cannot be dynamically resized with krealloc in
+ * rate_limit_report.
+ *
+ * Therefore, we use a fixed-size array, which at most will occupy a page. This
+ * still adequately rate limits reports, assuming that a) number of unique data
+ * races is not excessive, and b) occurrence of unique data races within the
+ * same time window is limited.
+ */
+#define REPORT_TIMES_MAX (PAGE_SIZE / sizeof(struct report_time))
+#define REPORT_TIMES_SIZE                                                      \
+	(CONFIG_KCSAN_REPORT_ONCE_IN_MS > REPORT_TIMES_MAX ?                   \
+		 REPORT_TIMES_MAX :                                            \
+		 CONFIG_KCSAN_REPORT_ONCE_IN_MS)
+static struct report_time report_times[REPORT_TIMES_SIZE];
+
 /*
  * This spinlock protects reporting and other_info, since other_info is usually
  * required when reporting.
  */
 static DEFINE_SPINLOCK(report_lock);
 
+/*
+ * Checks if the data race identified by thread frames frame1 and frame2 has
+ * been reported since (now - KCSAN_REPORT_ONCE_IN_MS).
+ */
+static bool rate_limit_report(unsigned long frame1, unsigned long frame2)
+{
+	struct report_time *use_entry = &report_times[0];
+	ktime_t now;
+	ktime_t invalid_before;
+	int i;
+
+	BUILD_BUG_ON(CONFIG_KCSAN_REPORT_ONCE_IN_MS != 0 && REPORT_TIMES_SIZE == 0);
+
+	if (CONFIG_KCSAN_REPORT_ONCE_IN_MS == 0)
+		return false;
+
+	now = ktime_get();
+	invalid_before = ktime_sub_ms(now, CONFIG_KCSAN_REPORT_ONCE_IN_MS);
+
+	/* Check if a matching data race report exists. */
+	for (i = 0; i < REPORT_TIMES_SIZE; ++i) {
+		struct report_time *rt = &report_times[i];
+
+		/*
+		 * Must always select an entry for use to store info as we
+		 * cannot resize report_times; at the end of the scan, use_entry
+		 * will be the oldest entry, which ideally also happened before
+		 * KCSAN_REPORT_ONCE_IN_MS ago.
+		 */
+		if (ktime_before(rt->time, use_entry->time))
+			use_entry = rt;
+
+		/*
+		 * Initially, no need to check any further as this entry as well
+		 * as following entries have never been used.
+		 */
+		if (rt->time == 0)
+			break;
+
+		/* Check if entry expired. */
+		if (ktime_before(rt->time, invalid_before))
+			continue; /* before KCSAN_REPORT_ONCE_IN_MS ago */
+
+		/* Reported recently, check if data race matches. */
+		if ((rt->frame1 == frame1 && rt->frame2 == frame2) ||
+		    (rt->frame1 == frame2 && rt->frame2 == frame1))
+			return true;
+	}
+
+	use_entry->time = now;
+	use_entry->frame1 = frame1;
+	use_entry->frame2 = frame2;
+	return false;
+}
+
 /*
  * Special rules to skip reporting.
  */
@@ -132,7 +222,9 @@ static bool print_report(const volatile void *ptr, size_t size, int access_type,
 	unsigned long stack_entries[NUM_STACK_ENTRIES] = { 0 };
 	int num_stack_entries = stack_trace_save(stack_entries, NUM_STACK_ENTRIES, 1);
 	int skipnr = get_stack_skipnr(stack_entries, num_stack_entries);
-	int other_skipnr;
+	unsigned long this_frame = stack_entries[skipnr];
+	unsigned long other_frame = 0;
+	int other_skipnr = 0; /* silence uninit warnings */
 
 	/*
 	 * Must check report filter rules before starting to print.
@@ -143,34 +235,34 @@ static bool print_report(const volatile void *ptr, size_t size, int access_type,
 	if (type == KCSAN_REPORT_RACE_SIGNAL) {
 		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(other_info.access_type, value_change,
-				other_info.stack_entries[other_skipnr]))
+		if (skip_report(other_info.access_type, value_change, other_frame))
 			return false;
 	}
 
+	if (rate_limit_report(this_frame, other_frame))
+		return false;
+
 	/* Print report header. */
 	pr_err("==================================================================\n");
 	switch (type) {
 	case KCSAN_REPORT_RACE_SIGNAL: {
-		void *this_fn = (void *)stack_entries[skipnr];
-		void *other_fn = (void *)other_info.stack_entries[other_skipnr];
 		int cmp;
 
 		/*
 		 * Order functions lexographically for consistent bug titles.
 		 * Do not print offset of functions to keep title short.
 		 */
-		cmp = sym_strcmp(other_fn, this_fn);
+		cmp = sym_strcmp((void *)other_frame, (void *)this_frame);
 		pr_err("BUG: KCSAN: data-race in %ps / %ps\n",
-		       cmp < 0 ? other_fn : this_fn,
-		       cmp < 0 ? this_fn : other_fn);
+		       (void *)(cmp < 0 ? other_frame : this_frame),
+		       (void *)(cmp < 0 ? this_frame : other_frame));
 	} break;
 
 	case KCSAN_REPORT_RACE_UNKNOWN_ORIGIN:
-		pr_err("BUG: KCSAN: data-race in %pS\n",
-		       (void *)stack_entries[skipnr]);
+		pr_err("BUG: KCSAN: data-race in %pS\n", (void *)this_frame);
 		break;
 
 	default:
diff --git a/lib/Kconfig.kcsan b/lib/Kconfig.kcsan
index 3f78b1434375..3552990abcfe 100644
--- a/lib/Kconfig.kcsan
+++ b/lib/Kconfig.kcsan
@@ -81,6 +81,16 @@ config KCSAN_SKIP_WATCH_RANDOMIZE
 	  KCSAN_WATCH_SKIP. If false, the chosen value is always
 	  KCSAN_WATCH_SKIP.
 
+config KCSAN_REPORT_ONCE_IN_MS
+	int "Duration in milliseconds, in which any given data race is only reported once"
+	default 3000
+	help
+	  Any given data race is only reported once in the defined time window.
+	  Different data races may still generate reports within a duration
+	  that is smaller than the duration defined here. This allows rate
+	  limiting reporting to avoid flooding the console with reports.
+	  Setting this to 0 disables rate limiting.
+
 # Note that, while some of the below options could be turned into boot
 # parameters, to optimize for the common use-case, we avoid this because: (a)
 # it would impact performance (and we want to avoid static branch for all
-- 
2.25.0.rc1.283.g88dfdc4193-goog