1 /* $NetBSD: nospec.h,v 1.3 2021/12/19 01:45:38 riastradh Exp $ */
2
3 /*-
4 * Copyright (c) 2018 The NetBSD Foundation, Inc.
5 * All rights reserved.
6 *
7 * This code is derived from software contributed to The NetBSD Foundation
8 * by Taylor R. Campbell.
9 *
10 * Redistribution and use in source and binary forms, with or without
11 * modification, are permitted provided that the following conditions
12 * are met:
13 * 1. Redistributions of source code must retain the above copyright
14 * notice, this list of conditions and the following disclaimer.
15 * 2. Redistributions in binary form must reproduce the above copyright
16 * notice, this list of conditions and the following disclaimer in the
17 * documentation and/or other materials provided with the distribution.
18 *
19 * THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS
20 * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
21 * TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
22 * PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS
23 * BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
24 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
25 * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
26 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
27 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
28 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
29 * POSSIBILITY OF SUCH DAMAGE.
30 */
31
32 #ifndef _LINUX_NOSPEC_H_
33 #define _LINUX_NOSPEC_H_
34
35 #include <sys/param.h>
36 #include <sys/types.h>
37
38 /*
39 * array_index_nospec(i, n)
40 *
41 * If i < n, return i; otherwise return 0. Guarantees that
42 * speculative execution will behave the same even if the compiler
43 * can prove in the caller that i < n.
44 */
45 static inline size_t
array_index_nospec(size_t i,size_t n)46 array_index_nospec(size_t i, size_t n)
47 {
48 size_t j, diff, mask;
49
50 /*
51 * Let j = i, but force the compiler to assume nothing about
52 * the value of j by issuing an asm block that is tagged as
53 * reading from and writing to j but actually does nothing.
54 */
55 j = i;
56 asm volatile ("" : "=r"(j) : "0"(j));
57
58 /* If i >= n, then diff has the high bit set; otherwise clear. */
59 diff = n - 1 - j;
60
61 /*
62 * If i >= n so that diff's high bit is set, mask = 0;
63 * otherwise mask = -1 = ~0.
64 */
65 mask = (diff >> (NBBY*sizeof(diff) - 1)) - 1;
66
67 return i & mask;
68 }
69
70 #endif /* _LINUX_NOSPEC_H_ */
71