On Mon, 2018-06-25 at 12:47 -0400, Nick Desaulniers wrote: > On Mon, Jun 25, 2018 at 12:05 PM Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote: > > > > On 19/06/2018 21:25, Matthias Kaehlcke 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 is assigned to > > > an u8 variable. Cast the negated value 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) > > > > > > Suggested-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx> > > > Signed-off-by: Matthias Kaehlcke <mka@xxxxxxxxxxxx> > > > --- > > > Changes in v2: > > > - negate the bitmask at initialization and rename variables to not_X > > > > The patch is not that bad, but I'd like to get confirmation that other > > maintainers are applying fixes like this. Honestly I'm not really > > impressed by most new clang warnings, these days. > > Here's an actual bug this warning caught applied to drivers/input/: > > dae1a432ab1f ("Input: mousedev - fix implicit conversion warning"): > https://patchwork.kernel.org/patch/9753771/ What bug is that? $ cat test.c #include <stdio.h> #include <stdlib.h> #include <memory.h> int main(int argc, char **argv) { static const signed char a[3] = {0x60, 3, 200}; static const unsigned char b[3] = {0x60, 3, 200}; printf("a and b are %s\n", memcmp(a, b, 3) == 0 ? "identical" : "different"); } $ gcc test.c $ ./a.out a and b are identical
