linux-kernel - Re: [PATCH v2] kvm: x86: mmu: Add cast to negated bitmasks in update_permission

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Date:   Mon, 25 Jun 2018 12:47:07 -0400
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     pbonzini@...hat.com
Cc:     Matthias Kaehlcke <mka@...omium.org>, rkrcmar@...hat.com,
        Thomas Gleixner <tglx@...utronix.de>, hpa@...or.com,
        x86@...nel.org, kvm@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>, joe@...ches.com
Subject: Re: [PATCH v2] kvm: x86: mmu: Add cast to negated bitmasks in update_permission_bitmask()

On Mon, Jun 25, 2018 at 12:05 PM Paolo Bonzini <pbonzini@...hat.com> wrote:
>
> On 19/06/2018 21:25, Matthias Kaehlcke wrote:
> > update_permission_bitmask() negates u8 bitmask values and assigns them
> > to variables of type u8. Since the MSB is set in the bitmask values the
> > compiler expands the negated values to int, which then is assigned to
> > an u8 variable. Cast the negated value back to u8.
> >
> > This fixes several warnings like this when building with clang:
> >
> > arch/x86/kvm/mmu.c:4266:39: error: implicit conversion from 'int' to 'u8'
> >   (aka 'unsigned char') changes value from -205 to 51 [-Werror,
> >   -Wconstant-conversion]
> >     u8 wf = (pfec & PFERR_WRITE_MASK) ? ~w : 0;
> >        ~~                               ^~
> >
> > (gcc also raises a warning (see https://godbolt.org/g/6JWfWk), however it
> > doesn't seem to be universally enabled)
> >
> > Suggested-by: Nick Desaulniers <ndesaulniers@...gle.com>
> > Signed-off-by: Matthias Kaehlcke <mka@...omium.org>
> > ---
> > Changes in v2:
> > - negate the bitmask at initialization and rename variables to not_X
>
> The patch is not that bad, but I'd like to get confirmation that other
> maintainers are applying fixes like this.  Honestly I'm not really
> impressed by most new clang warnings, these days.

Here's an actual bug this warning caught applied to drivers/input/:

dae1a432ab1f ("Input: mousedev - fix implicit conversion warning"):
https://patchwork.kernel.org/patch/9753771/