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

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Message-ID: <20180615182945.GN88063@google.com>
Date:   Fri, 15 Jun 2018 11:29:45 -0700
From:   Matthias Kaehlcke <mka@...omium.org>
To:     Joe Perches <joe@...ches.com>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>, pbonzini@...hat.com,
        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] kvm: x86: mmu: Add cast to negated bitmasks in
 update_permission_bitmask()

On Fri, Jun 15, 2018 at 11:18:12AM -0700, Joe Perches wrote:
> On Fri, 2018-06-15 at 11:04 -0700, Nick Desaulniers wrote:
> > On Fri, Jun 15, 2018 at 10:47 AM Matthias Kaehlcke <mka@...omium.org> 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 are assigned to
> > > u8 variables. Cast the negated values 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)
> 
> Perhaps it's better to turn off the warning.
> There are more of these in the kernel too.
> 
> At least:
> drivers/regulator/max8660.c:	u8 mask = (rdev_get_id(rdev) == MAX8660_V3) ? ~1 : ~4;
> drivers/regulator/max8660.c:	u8 mask = (rdev_get_id(rdev) == MAX8660_V6) ? ~2 : ~4;
> fs/ext4/resize.c:	__u16 uninit_mask = (flexbg_size > 1) ? ~EXT4_BG_BLOCK_UNINIT : ~0;

In my experience neither clang nor gcc should promote these values to
int, since the MSB/sign bit is not set.

In any case I think it it preferable to fix the code over disabling
the warning, unless the warning is bogus or there are just too many
occurrences.