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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 == ¤t->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 */
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