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