Re: [PATCH] powerpc/mm: fix always true/false warning in slice.c

[Christophe Leroy]

On 06/29/2018 02:55 AM, Michael Ellerman wrote:
> Christophe Leroy <christophe.leroy@xxxxxx> writes:
>
> > This patch fixes the following warnings (obtained with make W=1).
> >
> > arch/powerpc/mm/slice.c: In function 'slice_range_to_mask':
> > arch/powerpc/mm/slice.c:73:12: error: comparison is always true due to limited range of data type [-Werror=type-limits]
> >    if (start < SLICE_LOW_TOP) {
>
> Presumably only on 32-bit ?

Sure

> > diff --git a/arch/powerpc/mm/slice.c b/arch/powerpc/mm/slice.c
> > index 9530c6db406a..17c57760e06c 100644
> > --- a/arch/powerpc/mm/slice.c
> > +++ b/arch/powerpc/mm/slice.c
> > @@ -79,7 +86,7 @@ static void slice_range_to_mask(unsigned long start, unsigned long len,
> >   			- (1u << GET_LOW_SLICE_INDEX(start));
> >   	}
> >   
> > -	if ((start + len) > SLICE_LOW_TOP) {
> > +	if (!slice_addr_is_low(end)) {
> >   		unsigned long start_index = GET_HIGH_SLICE_INDEX(start);
> >   		unsigned long align_end = ALIGN(end, (1UL << SLICE_HIGH_SHIFT));
> >   		unsigned long count = GET_HIGH_SLICE_INDEX(align_end) - start_index;
>
> This worries me.

I'll change it in v2 to:
 	if (SLICE_NUM_HIGH && !slice_addr_is_low(end)) {


> By casting before the comparison in the helper you squash the compiler
> warning, but the code is still broken if (start + len) overflows.
>
> Presumably that "never happens", but it just seems fishy.
>
> The other similar check in that file does:
>
>    if (SLICE_NUM_HIGH && ((start + len) > SLICE_LOW_TOP)) {
>
> Where SLICE_NUM_HIGH == 0 on 32-bit.
>
>
> Could we fix the less than comparisons with SLICE_LOW_TOP with something
> similar, eg:
>
> 	if (!SLICE_NUM_HIGH || start < SLICE_LOW_TOP) {
>
> ie. limit them to the 64-bit code?

That's not enough to make GCC happy:

arch/powerpc/mm/slice.c: In function âslice_range_to_maskâ:
arch/powerpc/mm/slice.c:74:31: error: comparison is always true due to 
limited range of data type [-Werror=type-limits]
  if (!SLICE_NUM_HIGH || start < SLICE_LOW_TOP) {

Christophe


> cheers