1/* SPDX-License-Identifier: GPL-2.0 */
2/*
3 * Special rules for ignoring entire classes of data-racy memory accesses. None
4 * of the rules here imply that such data races are generally safe!
5 *
6 * All rules in this file can be configured via CONFIG_KCSAN_PERMISSIVE. Keep
7 * them separate from core code to make it easier to audit.
8 *
9 * Copyright (C) 2019, Google LLC.
10 */
11
12#ifndef _KERNEL_KCSAN_PERMISSIVE_H
13#define _KERNEL_KCSAN_PERMISSIVE_H
14
15#include <linux/bitops.h>
16#include <linux/sched.h>
17#include <linux/types.h>
18
19/*
20 * Access ignore rules based on address.
21 */
22static __always_inline bool kcsan_ignore_address(const volatile void *ptr)
23{
24	if (!IS_ENABLED(CONFIG_KCSAN_PERMISSIVE))
25		return false;
26
27	/*
28	 * Data-racy bitops on current->flags are too common, ignore completely
29	 * for now.
30	 */
31	return ptr == &current->flags;
32}
33
34/*
35 * Data race ignore rules based on access type and value change patterns.
36 */
37static bool
38kcsan_ignore_data_race(size_t size, int type, u64 old, u64 new, u64 diff)
39{
40	if (!IS_ENABLED(CONFIG_KCSAN_PERMISSIVE))
41		return false;
42
43	/*
44	 * Rules here are only for plain read accesses, so that we still report
45	 * data races between plain read-write accesses.
46	 */
47	if (type || size > sizeof(long))
48		return false;
49
50	/*
51	 * A common pattern is checking/setting just 1 bit in a variable; for
52	 * example:
53	 *
54	 *	if (flags & SOME_FLAG) { ... }
55	 *
56	 * and elsewhere flags is updated concurrently:
57	 *
58	 *	flags |= SOME_OTHER_FLAG; // just 1 bit
59	 *
60	 * While it is still recommended that such accesses be marked
61	 * appropriately, in many cases these types of data races are so common
62	 * that marking them all is often unrealistic and left to maintainer
63	 * preference.
64	 *
65	 * The assumption in all cases is that with all known compiler
66	 * optimizations (including those that tear accesses), because no more
67	 * than 1 bit changed, the plain accesses are safe despite the presence
68	 * of data races.
69	 *
70	 * The rules here will ignore the data races if we observe no more than
71	 * 1 bit changed.
72	 *
73	 * Of course many operations can effecively change just 1 bit, but the
74	 * general assuption that data races involving 1-bit changes can be
75	 * tolerated still applies.
76	 *
77	 * And in case a true bug is missed, the bug likely manifests as a
78	 * reportable data race elsewhere.
79	 */
80	if (hweight64(diff) == 1) {
81		/*
82		 * Exception: Report data races where the values look like
83		 * ordinary booleans (one of them was 0 and the 0th bit was
84		 * changed) More often than not, they come with interesting
85		 * memory ordering requirements, so let's report them.
86		 */
87		if (!((!old || !new) && diff == 1))
88			return true;
89	}
90
91	return false;
92}
93
94#endif /* _KERNEL_KCSAN_PERMISSIVE_H */
95