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+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Special rules for ignoring entire classes of data-racy memory accesses. None
+ * of the rules here imply that such data races are generally safe!
+ *
+ * All rules in this file can be configured via CONFIG_KCSAN_PERMISSIVE. Keep
+ * them separate from core code to make it easier to audit.
+ *
+ * Copyright (C) 2019, Google LLC.
+ */
+
+#ifndef _KERNEL_KCSAN_PERMISSIVE_H
+#define _KERNEL_KCSAN_PERMISSIVE_H
+
+#include <linux/bitops.h>
+#include <linux/sched.h>
+#include <linux/types.h>
+
+/*
+ * Access ignore rules based on address.
+ */
+static __always_inline bool kcsan_ignore_address(const volatile void *ptr)
+{
+ if (!IS_ENABLED(CONFIG_KCSAN_PERMISSIVE))
+ return false;
+
+ /*
+ * Data-racy bitops on current->flags are too common, ignore completely
+ * for now.
+ */
+ return ptr == &current->flags;
+}
+
+/*
+ * Data race ignore rules based on access type and value change patterns.
+ */
+static bool
+kcsan_ignore_data_race(size_t size, int type, u64 old, u64 new, u64 diff)
+{
+ if (!IS_ENABLED(CONFIG_KCSAN_PERMISSIVE))
+ return false;
+
+ /*
+ * Rules here are only for plain read accesses, so that we still report
+ * data races between plain read-write accesses.
+ */
+ if (type || size > sizeof(long))
+ return false;
+
+ /*
+ * A common pattern is checking/setting just 1 bit in a variable; for
+ * example:
+ *
+ * if (flags & SOME_FLAG) { ... }
+ *
+ * and elsewhere flags is updated concurrently:
+ *
+ * flags |= SOME_OTHER_FLAG; // just 1 bit
+ *
+ * While it is still recommended that such accesses be marked
+ * appropriately, in many cases these types of data races are so common
+ * that marking them all is often unrealistic and left to maintainer
+ * preference.
+ *
+ * The assumption in all cases is that with all known compiler
+ * optimizations (including those that tear accesses), because no more
+ * than 1 bit changed, the plain accesses are safe despite the presence
+ * of data races.
+ *
+ * The rules here will ignore the data races if we observe no more than
+ * 1 bit changed.
+ *
+ * Of course many operations can effecively change just 1 bit, but the
+ * general assuption that data races involving 1-bit changes can be
+ * tolerated still applies.
+ *
+ * And in case a true bug is missed, the bug likely manifests as a
+ * reportable data race elsewhere.
+ */
+ if (hweight64(diff) == 1) {
+ /*
+ * Exception: Report data races where the values look like
+ * ordinary booleans (one of them was 0 and the 0th bit was
+ * changed) More often than not, they come with interesting
+ * memory ordering requirements, so let's report them.
+ */
+ if (!((!old || !new) && diff == 1))
+ return true;
+ }
+
+ return false;
+}
+
+#endif /* _KERNEL_KCSAN_PERMISSIVE_H */