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
