nospec.h revision 1.2
1 1.2 riastrad /* $NetBSD: nospec.h,v 1.2 2021/12/19 01:44:02 riastradh Exp $ */ 2 1.2 riastrad 3 1.2 riastrad /*- 4 1.2 riastrad * Copyright (c) 2018 The NetBSD Foundation, Inc. 5 1.2 riastrad * All rights reserved. 6 1.2 riastrad * 7 1.2 riastrad * This code is derived from software contributed to The NetBSD Foundation 8 1.2 riastrad * by Taylor R. Campbell. 9 1.2 riastrad * 10 1.2 riastrad * Redistribution and use in source and binary forms, with or without 11 1.2 riastrad * modification, are permitted provided that the following conditions 12 1.2 riastrad * are met: 13 1.2 riastrad * 1. Redistributions of source code must retain the above copyright 14 1.2 riastrad * notice, this list of conditions and the following disclaimer. 15 1.2 riastrad * 2. Redistributions in binary form must reproduce the above copyright 16 1.2 riastrad * notice, this list of conditions and the following disclaimer in the 17 1.2 riastrad * documentation and/or other materials provided with the distribution. 18 1.2 riastrad * 19 1.2 riastrad * THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS 20 1.2 riastrad * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED 21 1.2 riastrad * TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR 22 1.2 riastrad * PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS 23 1.2 riastrad * BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR 24 1.2 riastrad * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF 25 1.2 riastrad * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS 26 1.2 riastrad * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN 27 1.2 riastrad * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) 28 1.2 riastrad * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE 29 1.2 riastrad * POSSIBILITY OF SUCH DAMAGE. 30 1.2 riastrad */ 31 1.2 riastrad 32 1.2 riastrad #ifndef _LINUX_NOSPEC_H_ 33 1.2 riastrad #define _LINUX_NOSPEC_H_ 34 1.2 riastrad 35 1.2 riastrad #include <sys/param.h> 36 1.2 riastrad #include <sys/types.h> 37 1.2 riastrad 38 1.2 riastrad /* 39 1.2 riastrad * array_index_nospec(i, n) 40 1.2 riastrad * 41 1.2 riastrad * If i < n, return i; otherwise return 0. Guarantees that 42 1.2 riastrad * speculative execution will behave the same even if the compiler 43 1.2 riastrad * can prove in the caller that i < n. 44 1.2 riastrad */ 45 1.2 riastrad static inline size_t 46 1.2 riastrad array_index_nospec(size_t i, size_t n) 47 1.2 riastrad { 48 1.2 riastrad size_t j, diff, mask; 49 1.2 riastrad 50 1.2 riastrad /* 51 1.2 riastrad * Let j = i, but force the compiler to assume nothing about 52 1.2 riastrad * the value of j by issuing an asm block that is tagged as 53 1.2 riastrad * reading from and writing to j but actually does nothing. 54 1.2 riastrad */ 55 1.2 riastrad j = i; 56 1.2 riastrad asm volatile ("" : "=r"(j) : "r"(j)); 57 1.2 riastrad 58 1.2 riastrad /* If i >= n, then diff has the high bit set; otherwise clear. */ 59 1.2 riastrad diff = n - 1 - j; 60 1.2 riastrad 61 1.2 riastrad /* 62 1.2 riastrad * If i >= n so that diff's high bit is set, mask = 0; 63 1.2 riastrad * otherwise mask = -1 = ~0. 64 1.2 riastrad */ 65 1.2 riastrad mask = (diff >> (NBBY*sizeof(diff) - 1)) - 1; 66 1.2 riastrad 67 1.2 riastrad return i & mask; 68 1.2 riastrad } 69 1.2 riastrad 70 1.2 riastrad #endif /* _LINUX_NOSPEC_H_ */ 71
Indexes created Mon Sep 22 13:09:51 GMT 2025