linux-kernel - Re: [PATCH 09/19] x86, mpx: trace entry to bounds exception paths

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Date:	Mon, 18 May 2015 16:06:47 -0700
From:	Dave Hansen <dave@...1.net>
To:	Thomas Gleixner <tglx@...utronix.de>
CC:	linux-kernel@...r.kernel.org, x86@...nel.org,
	dave.hansen@...ux.intel.com
Subject: Re: [PATCH 09/19] x86, mpx: trace entry to bounds exception paths

On 05/18/2015 01:58 PM, Thomas Gleixner wrote:
> On Fri, 8 May 2015, Dave Hansen wrote:
>> > This patch adds a trace point for the case where we are
>> > passing the exception up to userspace with a signal.
>> > 
>> > We are also explicit that we're printing out the inverse of
>> > the 'upper' that we encounter.  If you want to filter, for
>> > instance, you need to ~ the value first.
> Confused. What is the point of printing the complement of upper?

If a pointer's range is

	0x1000 -> 0x2000

it is stored in the bounds table as (32-bits here for brevity):

	lower: 0x00001000
	upper: 0xffffdfff

That is so that an all 0's entry:

	lower: 0x00000000
	upper: 0x00000000

corresponds to the "init" bounds which store a *range* of:

	0x00000000 -> 0xffffffff

The 'upper' stored in the table is gibberish to print by itself, so we
print ~upper to get the *actual*, logical value.
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