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

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Date:   Mon, 26 Feb 2018 02:46:02 -0600
From:   Segher Boessenkool <segher@...nel.crashing.org>
To:     Christophe LEROY <christophe.leroy@....fr>
Cc:     Mathieu Malaterre <malat@...ian.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        linuxppc-dev@...ts.ozlabs.org, Paul Mackerras <paulus@...ba.org>,
        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 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).


Segher