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Message-ID: <20180121104019.GS17719@n2100.armlinux.org.uk>
Date: Sun, 21 Jan 2018 10:40:19 +0000
From: Russell King - ARM Linux <linux@...linux.org.uk>
To: Dan Williams <dan.j.williams@...el.com>
Cc: tglx@...utronix.de, linux-arch@...r.kernel.org,
kernel-hardening@...ts.openwall.com,
Peter Zijlstra <peterz@...radead.org>,
Catalin Marinas <catalin.marinas@....com>, x86@...nel.org,
Will Deacon <will.deacon@....com>, Ingo Molnar <mingo@...hat.com>,
gregkh@...uxfoundation.org, "H. Peter Anvin" <hpa@...or.com>,
torvalds@...ux-foundation.org, alan@...ux.intel.com
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.
or:
* 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.
--
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