linux-kernel - Re: [PATCH v3 2/3] arm64: implement update_fdt

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Date:   Thu, 16 May 2019 16:39:32 +0200
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     Rob Herring <robh+dt@...nel.org>
Cc:     Hsin-Yi Wang <hsinyi@...omium.org>,
        "moderated list:ARM/FREESCALE IMX / MXC ARM ARCHITECTURE" 
        <linux-arm-kernel@...ts.infradead.org>,
        Devicetree List <devicetree@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Frank Rowand <frowand.list@...il.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Miles Chen <miles.chen@...iatek.com>,
        James Morse <james.morse@....com>,
        Andrew Murray <andrew.murray@....com>,
        Mark Rutland <mark.rutland@....com>,
        Chintan Pandya <cpandya@...eaurora.org>,
        Jun Yao <yaojun8558363@...il.com>, Yu Zhao <yuzhao@...gle.com>,
        Robin Murphy <robin.murphy@....com>,
        Laura Abbott <labbott@...hat.com>,
        Stephen Boyd <swboyd@...omium.org>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH v3 2/3] arm64: implement update_fdt_pgprot()

On Thu, 16 May 2019 at 16:37, Rob Herring <robh+dt@...nel.org> wrote:
>
> On Thu, May 16, 2019 at 5:28 AM Hsin-Yi Wang <hsinyi@...omium.org> wrote:
> >
> > Basically does similar things like __fixmap_remap_fdt(). It's supposed
> > to be called after fixmap_remap_fdt() is called at least once, so region
> > checking can be skipped. Since it needs to know dt physical address, make
> > a copy of the value of __fdt_pointer.
> >
> > Signed-off-by: Hsin-Yi Wang <hsinyi@...omium.org>
> > ---
> >  arch/arm64/kernel/setup.c |  2 ++
> >  arch/arm64/mm/mmu.c       | 17 +++++++++++++++++
> >  2 files changed, 19 insertions(+)
>
> Why not just map the FDT R/W at the start and change it to RO just
> before calling unflatten_device_tree? Then all the FDT scanning
> functions or any future fixups we need can just assume R/W. That is
> essentially what Stephen suggested. However, there's no need for a
> weak function as it can all be done within the arch code.
>
> However, I'm still wondering why the FDT needs to be RO in the first place.
>

It was RO because it could be RO, and we wanted to ensure that it
didn't get modified inadvertently (hence the CRC check we added as
well)

If there is a need for the FDT to be RW, let's make it RW.