On Fri, 2018-06-15 at 11:04 -0700, Nick Desaulniers wrote: > 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) 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@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
