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

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Date:   Fri, 15 Jun 2018 11:04:43 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     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, 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)
>
> 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