linux-kernel - Re: [PATCH v2 1/1] arm64: mm: correct the inside linear map boundaries during hotplug check

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Date:   Mon, 15 Feb 2021 20:34:26 +0100
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Pavel Tatashin <pasha.tatashin@...een.com>
Cc:     Tyler Hicks <tyhicks@...ux.microsoft.com>,
        James Morris <jmorris@...ei.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Mike Rapoport <rppt@...nel.org>,
        Logan Gunthorpe <logang@...tatee.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/1] arm64: mm: correct the inside linear map
 boundaries during hotplug check

On Mon, 15 Feb 2021 at 20:30, Pavel Tatashin <pasha.tatashin@...een.com> wrote:
>
> > Can't we simply use signed arithmetic here? This expression works fine
> > if the quantities are all interpreted as s64 instead of u64
>
> I was thinking about that, but I do not like the idea of using sign
> arithmetics for physical addresses. Also, I am worried that someone in
> the future will unknowingly change it to unsigns or to phys_addr_t. It
> is safer to have start explicitly set to 0 in case of wrap.

memstart_addr is already a s64 for this exact reason.

Btw, the KASLR check is incorrect: memstart_addr could also be
negative when running the 52-bit VA kernel on hardware that is only
48-bit VA capable.