xref: /linux/kernel/kcsan/encoding.h (revision 8be98d2f2a0a262f8bf8a0bc1fdf522b3c7aab17)
1  /* SPDX-License-Identifier: GPL-2.0 */
2  /*
3   * KCSAN watchpoint encoding.
4   *
5   * Copyright (C) 2019, Google LLC.
6   */
7  
8  #ifndef _KERNEL_KCSAN_ENCODING_H
9  #define _KERNEL_KCSAN_ENCODING_H
10  
11  #include <linux/bits.h>
12  #include <linux/log2.h>
13  #include <linux/mm.h>
14  
15  #include "kcsan.h"
16  
17  #define SLOT_RANGE PAGE_SIZE
18  
19  #define INVALID_WATCHPOINT  0
20  #define CONSUMED_WATCHPOINT 1
21  
22  /*
23   * The maximum useful size of accesses for which we set up watchpoints is the
24   * max range of slots we check on an access.
25   */
26  #define MAX_ENCODABLE_SIZE (SLOT_RANGE * (1 + KCSAN_CHECK_ADJACENT))
27  
28  /*
29   * Number of bits we use to store size info.
30   */
31  #define WATCHPOINT_SIZE_BITS bits_per(MAX_ENCODABLE_SIZE)
32  /*
33   * This encoding for addresses discards the upper (1 for is-write + SIZE_BITS);
34   * however, most 64-bit architectures do not use the full 64-bit address space.
35   * Also, in order for a false positive to be observable 2 things need to happen:
36   *
37   *	1. different addresses but with the same encoded address race;
38   *	2. and both map onto the same watchpoint slots;
39   *
40   * Both these are assumed to be very unlikely. However, in case it still
41   * happens, the report logic will filter out the false positive (see report.c).
42   */
43  #define WATCHPOINT_ADDR_BITS (BITS_PER_LONG-1 - WATCHPOINT_SIZE_BITS)
44  
45  /* Bitmasks for the encoded watchpoint access information. */
46  #define WATCHPOINT_WRITE_MASK	BIT(BITS_PER_LONG-1)
47  #define WATCHPOINT_SIZE_MASK	GENMASK(BITS_PER_LONG-2, WATCHPOINT_ADDR_BITS)
48  #define WATCHPOINT_ADDR_MASK	GENMASK(WATCHPOINT_ADDR_BITS-1, 0)
49  static_assert(WATCHPOINT_ADDR_MASK == (1UL << WATCHPOINT_ADDR_BITS) - 1);
50  static_assert((WATCHPOINT_WRITE_MASK ^ WATCHPOINT_SIZE_MASK ^ WATCHPOINT_ADDR_MASK) == ~0UL);
51  
check_encodable(unsigned long addr,size_t size)52  static inline bool check_encodable(unsigned long addr, size_t size)
53  {
54  	/*
55  	 * While we can encode addrs<PAGE_SIZE, avoid crashing with a NULL
56  	 * pointer deref inside KCSAN.
57  	 */
58  	return addr >= PAGE_SIZE && size <= MAX_ENCODABLE_SIZE;
59  }
60  
61  static inline long
encode_watchpoint(unsigned long addr,size_t size,bool is_write)62  encode_watchpoint(unsigned long addr, size_t size, bool is_write)
63  {
64  	return (long)((is_write ? WATCHPOINT_WRITE_MASK : 0) |
65  		      (size << WATCHPOINT_ADDR_BITS) |
66  		      (addr & WATCHPOINT_ADDR_MASK));
67  }
68  
decode_watchpoint(long watchpoint,unsigned long * addr_masked,size_t * size,bool * is_write)69  static __always_inline bool decode_watchpoint(long watchpoint,
70  					      unsigned long *addr_masked,
71  					      size_t *size,
72  					      bool *is_write)
73  {
74  	if (watchpoint == INVALID_WATCHPOINT ||
75  	    watchpoint == CONSUMED_WATCHPOINT)
76  		return false;
77  
78  	*addr_masked =    (unsigned long)watchpoint & WATCHPOINT_ADDR_MASK;
79  	*size	     =   ((unsigned long)watchpoint & WATCHPOINT_SIZE_MASK) >> WATCHPOINT_ADDR_BITS;
80  	*is_write    = !!((unsigned long)watchpoint & WATCHPOINT_WRITE_MASK);
81  
82  	return true;
83  }
84  
85  /*
86   * Return watchpoint slot for an address.
87   */
watchpoint_slot(unsigned long addr)88  static __always_inline int watchpoint_slot(unsigned long addr)
89  {
90  	return (addr / PAGE_SIZE) % CONFIG_KCSAN_NUM_WATCHPOINTS;
91  }
92  
matching_access(unsigned long addr1,size_t size1,unsigned long addr2,size_t size2)93  static __always_inline bool matching_access(unsigned long addr1, size_t size1,
94  					    unsigned long addr2, size_t size2)
95  {
96  	unsigned long end_range1 = addr1 + size1 - 1;
97  	unsigned long end_range2 = addr2 + size2 - 1;
98  
99  	return addr1 <= end_range2 && addr2 <= end_range1;
100  }
101  
102  #endif /* _KERNEL_KCSAN_ENCODING_H */
103