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

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Date:   Fri, 15 Jun 2018 11:18:12 -0700
From:   Joe Perches <joe@...ches.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Matthias Kaehlcke <mka@...omium.org>
Cc:     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, 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;


> > Signed-off-by: Matthias Kaehlcke <mka@...omium.org>
> > ---
> >  arch/x86/kvm/mmu.c | 6 +++---
> >  1 file changed, 3 insertions(+), 3 deletions(-)
> > 
> > diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> > index d634f0332c0f..a83817fdbd87 100644
> > --- a/arch/x86/kvm/mmu.c
> > +++ b/arch/x86/kvm/mmu.c
> > @@ -4275,11 +4275,11 @@ static void update_permission_bitmask(struct kvm_vcpu *vcpu,
> >                  */
> > 
> >                 /* Faults from writes to non-writable pages */
> > -               u8 wf = (pfec & PFERR_WRITE_MASK) ? ~w : 0;
> > +               u8 wf = (pfec & PFERR_WRITE_MASK) ? (u8)~w : 0;
> >                 /* Faults from user mode accesses to supervisor pages */
> > -               u8 uf = (pfec & PFERR_USER_MASK) ? ~u : 0;
> > +               u8 uf = (pfec & PFERR_USER_MASK) ? (u8)~u : 0;
> >                 /* Faults from fetches of non-executable pages*/
> > -               u8 ff = (pfec & PFERR_FETCH_MASK) ? ~x : 0;
> > +               u8 ff = (pfec & PFERR_FETCH_MASK) ? (u8)~x : 0;
> >                 /* Faults from kernel mode fetches of user pages */
> >                 u8 smepf = 0;
> >                 /* Faults from kernel mode accesses of user pages */
> > --
> > 2.18.0.rc1.244.gcf134e6275-goog
> > 
> 
> This fixes -Woverflow warnings in gcc and -Wconstant-conversion
> warnings in clang, without changing the disassembly.  Further
> modification to mka's test case (to show no change in codegen):
> https://godbolt.org/g/ynAoeJ
> 
> See also: https://wiki.sei.cmu.edu/confluence/display/c/EXP14-C.+Beware+of+integer+promotion+when+performing+bitwise+operations+on+integer+types+smaller+than+int
> 
> Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>
> --
> Thanks,
> ~Nick Desaulniers