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Date: Sun, 21 Jan 2018 10:40:19 +0000
From: Russell King - ARM Linux <>
To: Dan Williams <>
	Peter Zijlstra <>,
	Catalin Marinas <>,,
	Will Deacon <>, Ingo Molnar <>,, "H. Peter Anvin" <>,,
Subject: Re: [PATCH v4.1 02/10] asm/nospec, array_ptr: sanitize speculative
 array de-references

On Sat, Jan 20, 2018 at 01:06:09PM -0800, Dan Williams wrote:
> +/*
> + * If idx is negative or if idx > size then bit 63 is set in the mask,
> + * and the value of ~(-1L) is zero. When the mask is zero, bounds check
> + * failed, array_ptr will return NULL.

The more times I see this the more times I'm unhappy with this comment:

1. does this really mean "idx > size" or "idx >= size" ?  The code
   implements the latter not the former.

2. is "bit 63" relevant here - what if longs are 32-bit?  "the top bit"
   or "the sign bit" would be better.

3. "and the value of ~(-1L) is zero."  So does this mean that when
   0 <= idx < size, somehow the rules of logic change and ~(-1L)
   magically becomes no longer zero!

I'd suggest changing the description to something like:

  * If 0 <= idx < size, return a mask of ~0UL, otherwise return zero.


  * When idx is out of bounds (iow, is negative or idx >= sz), 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 idx, or ~0UL if within bounds.

depending on how deeply you want to describe what's going on here.

RMK's Patch system:
FTTC broadband for 0.8mile line in suburbia: sync at 8.8Mbps down 630kbps up
According to 8.21Mbps down 510kbps up

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