linux-kernel - Re: [PATCH 05/21] powerpc: Avoid comparison of unsigned long >= 0 in pfn

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Date:   Fri, 2 Mar 2018 20:54:27 +0100
From:   Mathieu Malaterre <malat@...ian.org>
To:     Segher Boessenkool <segher@...nel.crashing.org>
Cc:     Christophe LEROY <christophe.leroy@....fr>,
        Michael Ellerman <mpe@...erman.id.au>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Paul Mackerras <paulus@...ba.org>,
        LKML <linux-kernel@...r.kernel.org>, Jiri Slaby <jslaby@...e.com>
Subject: Re: [PATCH 05/21] powerpc: Avoid comparison of unsigned long >= 0 in pfn_valid

On Mon, Feb 26, 2018 at 9:46 AM, Segher Boessenkool
<segher@...nel.crashing.org> wrote:
> On Mon, Feb 26, 2018 at 07:32:03AM +0100, Christophe LEROY wrote:
>> Le 25/02/2018 à 18:22, Mathieu Malaterre a écrit :
>> >-#define pfn_valid(pfn)              ((pfn) >= ARCH_PFN_OFFSET && (pfn) <
>> >max_mapnr)
>> >+#define pfn_valid(pfn) \
>> >+            (((pfn) - ARCH_PFN_OFFSET) < (max_mapnr - ARCH_PFN_OFFSET))
>>
>> What will happen when ARCH_PFN_OFFSET is not nul and pfn is lower than
>> ARCH_PFN_OFFSET ?
>
> It will work fine.
>
> Say you are asking for  a <= x < b  so (in actual integers, no overflow)
> that is  0 <= x-a < b-a  and you also assume x-a overflows, so that we
> are actually comparing  x-a+M < b-a  with M = 2**32 or such (the maximum
> value in the unsigned integer type plus one).  This comparison is
> obviously always false.
>
> (It also works if b < a btw).
>
>
Thanks Segher !

Christophe does that clarify things or do you want me to update the
commit message ?