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 */ 45static inline size_t 46array_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