[PATCH] fix comparison of unsigned expression < 0

[Andrew Morton]

On Tue, 28 Nov 2023 15:55:32 +0800 Haibo Li <haibo.li at mediatek.com> wrote:

> Kernel test robot reported:
> 
> '''
> mm/kasan/report.c:637 kasan_non_canonical_hook() warn:
> unsigned 'addr' is never less than zero.
> '''
> The KASAN_SHADOW_OFFSET is 0 on loongarch64.
> 
> To fix it,check the KASAN_SHADOW_OFFSET before do comparison.
> 
> --- a/mm/kasan/report.c
> +++ b/mm/kasan/report.c
> @@ -634,10 +634,10 @@ void kasan_non_canonical_hook(unsigned long addr)
>  {
>  	unsigned long orig_addr;
>  	const char *bug_type;
> -
> +#if KASAN_SHADOW_OFFSET > 0
>  	if (addr < KASAN_SHADOW_OFFSET)
>  		return;
> -
> +#endif

We'd rather not add ugly ifdefs for a simple test like this.  If we
replace "<" with "<=", does it fix?  I suspect that's wrong.

But really, some hardwired comparison with an absolute address seems
lazy.  If KASAN_SHADOW_OFFSET is variable on a per-architecture basis
then the expression which checks the validity of an arbitrary address
should also be per-architecture.