/* SPDX-License-Identifier: GPL-2.0 */
/* Copyright(c) 2018 Linus Torvalds. All rights reserved. */
/* Copyright(c) 2018 Alexei Starovoitov. All rights reserved. */
/* Copyright(c) 2018 Intel Corporation. All rights reserved. */
/* Copyright(c) 2018 Citrix Systems R&D Ltd. All rights reserved. */

#ifndef XEN_NOSPEC_H
#define XEN_NOSPEC_H

#include <asm/system.h>

/**
 * array_index_mask_nospec() - generate a ~0 mask when index < size, 0 otherwise
 * @index: array element index
 * @size: number of elements in array
 *
 * When @index is out of bounds (@index >= @size), the sign bit will be
 * set.  Extend the sign bit to all bits and invert, giving a result of
 * zero for an out of bounds index, or ~0 if within bounds [0, @size).
 */
#ifndef array_index_mask_nospec
static inline unsigned long array_index_mask_nospec(unsigned long index,
                                                    unsigned long size)
{
    /*
     * Always calculate and emit the mask even if the compiler
     * thinks the mask is not needed. The compiler does not take
     * into account the value of @index under speculation.
     */
    OPTIMIZER_HIDE_VAR(index);
    return ~(long)(index | (size - 1UL - index)) >> (BITS_PER_LONG - 1);
}
#endif

/*
 * array_index_nospec - sanitize an array index after a bounds check
 *
 * For a code sequence like:
 *
 *     if (index < size) {
 *         index = array_index_nospec(index, size);
 *         val = array[index];
 *     }
 *
 * ...if the CPU speculates past the bounds check then
 * array_index_nospec() will clamp the index within the range of [0,
 * size).
 */
#define array_index_nospec(index, size)                                 \
({                                                                      \
    typeof(index) _i = (index);                                         \
    typeof(size) _s = (size);                                           \
    unsigned long _mask = array_index_mask_nospec(_i, _s);              \
                                                                        \
    BUILD_BUG_ON(sizeof(_i) > sizeof(long));                            \
    BUILD_BUG_ON(sizeof(_s) > sizeof(long));                            \
                                                                        \
    (typeof(_i)) (_i & _mask);                                          \
})

#endif /* XEN_NOSPEC_H */

/*
 * Local variables:
 * mode: C
 * c-file-style: "BSD"
 * c-basic-offset: 4
 * indent-tabs-mode: nil
 * End:
 */
