Re: [PATCH] kvm: x86: mmu: Add cast to negated bitmasks in update_permission_bitmask()

From: Nick Desaulniers
Date: Fri Jun 15 2018 - 14:05:01 EST


On Fri, Jun 15, 2018 at 10:47 AM Matthias Kaehlcke <mka@xxxxxxxxxxxx> 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@xxxxxxxxxxxx>
> ---
> 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@xxxxxxxxxx>
--
Thanks,
~Nick Desaulniers