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 10:05:52 -0700
From:   Joe Perches <joe@...ches.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>, 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>
Subject: Re: [PATCH v2] kvm: x86: mmu: Add cast to negated bitmasks in
 update_permission_bitmask()

On Mon, 2018-06-25 at 12:47 -0400, Nick Desaulniers wrote:
> 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/

What bug is that?

$ cat test.c
#include <stdio.h>
#include <stdlib.h>
#include <memory.h>

int main(int argc, char **argv)
{
	static const signed char a[3] = {0x60, 3, 200};
	static const unsigned char b[3] = {0x60, 3, 200};

	printf("a and b are %s\n",
	       memcmp(a, b, 3) == 0 ? "identical" : "different");
}
$ gcc test.c
$ ./a.out
a and b are identical