Re: [PATCH v2 06/19] asm-generic/barrier: mask speculative execution flows

[Eric W. Biederman]

Dan Williams <dan.j.williams@xxxxxxxxx> writes:

> diff --git a/include/linux/nospec.h b/include/linux/nospec.h
> new file mode 100644
> index 000000000000..5c66fc30f919
> --- /dev/null
> +++ b/include/linux/nospec.h
> @@ -0,0 +1,71 @@
> +// SPDX-License-Identifier: GPL-2.0
> +// Copyright(c) 2018 Intel Corporation. All rights reserved.
> +
> +#ifndef __NOSPEC_H__
> +#define __NOSPEC_H__
> +
> +#include <linux/jump_label.h>
> +#include <asm/barrier.h>
> +
> +#ifndef array_ptr_mask
> +#define array_ptr_mask(idx, sz)						\
> +({									\
> +	unsigned long mask;						\
> +	unsigned long _i = (idx);					\
> +	unsigned long _s = (sz);					\
> +									\
> +	mask = ~(long)(_i | (_s - 1 - _i)) >> (BITS_PER_LONG - 1);	\
> +	mask;								\
> +})
> +#endif

This could really use a comment that explains it generates 0
for out of bound accesses and -1L aka 0xffffffffffffffff for
all other accesses.

The code is clever enough which values it generates is not obvious.

Eric