linux-kernel - Re: [PATCH v3 30/60] arm64: idreg-override: Create a pseudo feature for rodata=off

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Message-ID: <CAMj1kXFK3uAMXgrtFofb0e2Z=eW+bMQBkbU-3MAazV3PgX_SpA@mail.gmail.com>
Date:   Mon, 17 Apr 2023 16:30:32 +0200
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Ryan Roberts <ryan.roberts@....com>
Cc:     linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>, Marc Zyngier <maz@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH v3 30/60] arm64: idreg-override: Create a pseudo feature
 for rodata=off

On Mon, 17 Apr 2023 at 16:28, Ryan Roberts <ryan.roberts@....com> wrote:
>
> On 07/03/2023 14:04, Ard Biesheuvel wrote:
> > Add rodata=off to the set of kernel command line options that is parsed
> > early using the CPU feature override detection code, so we can easily
> > refer to it when creating the kernel mapping.
> >
> > Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
> > ---
> >  arch/arm64/include/asm/cpufeature.h   | 1 +
> >  arch/arm64/kernel/pi/idreg-override.c | 2 ++
> >  2 files changed, 3 insertions(+)
> >
> > diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
> > index bc10098901808c00..edc7733aa49846b2 100644
> > --- a/arch/arm64/include/asm/cpufeature.h
> > +++ b/arch/arm64/include/asm/cpufeature.h
> > @@ -16,6 +16,7 @@
> >  #define cpu_feature(x)               KERNEL_HWCAP_ ## x
> >
> >  #define ARM64_SW_FEATURE_OVERRIDE_NOKASLR    0
> > +#define ARM64_SW_FEATURE_OVERRIDE_RODATA_OFF 4
>
> I assume these are bit numbers? Why not just use the next available bit (bit 1)
> for this new flag?
>

This (ab)uses the CPU feature framework, which is based on 4-bit
quantities. I don't remember if it matters or not, but IIRC the
default macros use 4-bit wide values.