linux-kernel - Re: [PATCH] s390: fix normalization bug in exception table sorting

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Date:	Mon, 4 Jan 2016 10:44:01 +0100
From:	Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:	Heiko Carstens <heiko.carstens@...ibm.com>
Cc:	schwidefsky@...ibm.com, linux-s390@...r.kernel.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] s390: fix normalization bug in exception table sorting

On 4 January 2016 at 10:42, Heiko Carstens <heiko.carstens@...ibm.com> wrote:
> On Fri, Jan 01, 2016 at 01:39:22PM +0100, Ard Biesheuvel wrote:
>> The normalization pass in the sorting routine of the relative exception
>> table serves two purposes:
>> - it ensures that the address fields of the exception table entries are
>>   fully ordered, so that no ambiguities arise between entries with
>>   identical instruction offsets (i.e., when two instructions that are
>>   exactly 8 bytes apart each have an exception table entry associated with
>>   them)
>> - it ensures that the offsets of both the instruction and the fixup fields
>>   of each entry are relative to their final location after sorting.
>>
>> Commit eb608fb366de ("s390/exceptions: switch to relative exception table
>> entries") ported the relative exception table format from x86, but modified
>> the sorting routine to only normalize the instruction offset field and not
>> the fixup offset field. The result is that the fixup offset of each entry
>> will be relative to the original location of the entry before sorting,
>> likely leading to crashes when those entries are dereferenced.
>
> Applied, thanks a lot!
>
> I was wondering why this never was observed on s390 during the last three
> years.
>
> The kernel text extable entries will be sorted during build time and I
> verified that for the majority of modules the extable entries are already
> sorted. And even if they are not sorted there isn't any major shuffling.
> So it looks like we were simply lucky...
>

Indeed. I guess most modules only have a single .text section so the
entries are emitted in order.

-- 
Ard.
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