include/linux/nospec.h

// 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.

#ifndef _LINUX_NOSPEC_H
#define _LINUX_NOSPEC_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)
{
	/*
	 * Warn developers about inappropriate array_index_nospec() usage.
	 *
	 * Even if the CPU speculates past the WARN_ONCE branch, the
	 * sign bit of @index is taken into account when generating the
	 * mask.
	 *
	 * This warning is compiled out when the compiler can infer that
	 * @index and @size are less than LONG_MAX.
	 */
	if (WARN_ONCE(index > LONG_MAX || size > LONG_MAX,
			"array_index_nospec() limited to range of [0, LONG_MAX]\n"))
		return 0;

	/*
	 * 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));			\
									\
	_i &= _mask;							\
	_i;								\
})
#endif /* _LINUX_NOSPEC_H */