Re: [PATCH 05/21] powerpc: Avoid comparison of unsigned long >= 0 in pfn_valid
From: christophe leroy <hidden>
Date: 2018-03-03 07:46:02
Also in:
lkml
Le 02/03/2018 à 20:54, Mathieu Malaterre a écrit :
On Mon, Feb 26, 2018 at 9:46 AM, Segher Boessenkool [off-list ref] wrote:quoted
On Mon, Feb 26, 2018 at 07:32:03AM +0100, Christophe LEROY wrote:quoted
Le 25/02/2018 à 18:22, Mathieu Malaterre a écrit :quoted
-#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 ?
No it is fine for me. Reviewed-by: Christophe Leroy <redacted> --- L'absence de virus dans ce courrier électronique a été vérifiée par le logiciel antivirus Avast. https://www.avast.com/antivirus