Re: [PATCH 2/5] llc-coloring: improve checking while parsing

[Jan Beulich <jbeulich@xxxxxxxx>]

On 07.04.2026 12:30, Andrew Cooper wrote:
> On 24/03/2026 4:37 pm, Jan Beulich wrote:
>> We can easily avoid the risk of wrapping UINT_MAX <-> 0 by applying a
>> check against the compile-time-constant maximum number of colors.
>>
>> Additionally the overflow checks suffered from an off-by-1, as the parsed
>> ranges are inclusive (e.g. end == start being possible, requiring 1 array
>> slot, while availability of 0 slots was checked in that case).
>>
>> Fixes: 6cdea3444eaf ("xen/arm: add Dom0 cache coloring support")
>> Reported-by: Kamil Frankowicz <kamil.frankowicz@xxxxxxx>
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>>
>> --- a/xen/common/llc-coloring.c
>> +++ b/xen/common/llc-coloring.c
>> @@ -76,8 +76,9 @@ static int __init parse_color_config(con
>>          else                /* Single value */
>>              end = start;
>>  
>> -        if ( start > end || (end - start) > (UINT_MAX - *num_colors) ||
>> -             (*num_colors + (end - start)) >= max_num_colors )
>> +        if ( end >= NR_LLC_COLORS || start > end ||
>> +             (end - start) >= (UINT_MAX - *num_colors) ||
>> +             (*num_colors + (end - start + 1)) >= max_num_colors )
>>              return -EINVAL;
>>  
>>          /* Colors are range checked in check_colors() */
>>
> 
> I think this is correct, so Acked-by: Andrew Cooper
> <andrew.cooper3@xxxxxxxxxx>

Thanks.

> However, the parsing logic is also ridiculous.  Most of the complexity
> comes because of parsing a bitmap but storing it longhand in an array of
> unsigned ints.
> 
> Instead, the global variables default_colors, dom0_colors and xen_colors
> should be bitmaps sized by NR_LLC_COLORS, and d->llc_colours should be a
> bitmap sized by xen_num_colors (which itself is bound by NR_LLC_COLORS).
> 
> With the default of 32 colours, this would involve no memory allocation
> at all, even on 32bit builds of Xen.

It's pretty space inefficient, yes, but the parsing wouldn't become simpler
when using bitmaps, would it?

Jan