Re: [PATCH 0/2] xen/common: llc-coloring parser fixes

[Jan Beulich <jbeulich@xxxxxxxx>]

On 18.05.2026 09:19, Mykola Kvach wrote:
> On Mon, May 18, 2026 at 10:06 AM Jan Beulich <jbeulich@xxxxxxxx> wrote:
>> On 18.05.2026 09:01, Mykola Kvach wrote:
>>> On Mon, May 18, 2026 at 9:52 AM Jan Beulich <jbeulich@xxxxxxxx> wrote:
>>>> On 18.05.2026 08:42, Mykola Kvach wrote:
>>>>> On Mon, May 18, 2026 at 9:20 AM Jan Beulich <jbeulich@xxxxxxxx> wrote:
>>>>>> On 16.05.2026 17:03, Mykola Kvach wrote:
>>>>>>> This small series fixes two issues in parse_color_config().
>>>>>>>
>>>>>>> The first patch makes parse failures leave the caller-visible color 
>>>>>>> count
>>>>>>> at zero.  This prevents a rejected command-line value from leaving a
>>>>>>> partially parsed configuration behind for later init paths to consume.
>>>>>>>
>>>>>>> The second patch rejects empty color tokens.  Previously, delimiters in
>>>>>>> places where a color value was expected could be interpreted as color 0,
>>>>>>> because simple_strtoul() returns zero without advancing the input 
>>>>>>> pointer.
>>>>>>> The patch checks that each parsed color value consumed input.  It also
>>>>>>> adds the missing newline to the DT color parsing error message.
>>>>>>>
>>>>>>> Mykola Kvach (2):
>>>>>>>   xen/common: llc-coloring: clear color count on parse failure
>>>>>>>   xen/common: llc-coloring: reject empty color tokens
>>>>>>
>>>>>> For both of these, a question which isn't even considered in the 
>>>>>> reasoning
>>>>>> is whether the present behavior may be intentional. Especially for the 
>>>>>> 2nd
>>>>>> ISTR Stefano(?) not so long ago indicating that the behavior is indeed
>>>>>> intended to be this way. That may have been somewhere on Matrix rather 
>>>>>> than
>>>>>> on the list, though.
>>>>>
>>>>> Thank you for pointing this out.
>>>>>
>>>>> For the first patch, my reasoning was that returning an error while
>>>>> leaving a partially parsed caller-visible color count behind looks
>>>>> surprising. If the value is rejected, I think later init paths should
>>>>> not be able to consume the partially parsed state.
>>>>>
>>>>> For the second patch, my reasoning was that the current behavior looks
>>>>> accidental rather than an intentional extension of the syntax.
>>>>
>>>> That was my impression as well, hence why I had raised the question back
>>>> then.
>>>>
>>>>> The parser comment says:
>>>>>
>>>>> COLOR_CONFIGURATION ::= COLOR | RANGE,...,COLOR | RANGE
>>>>> RANGE ::= COLOR-COLOR
>>>>>
>>>>> The user guide also describes this as a comma-separated list of colors
>>>>> or ranges, with ranges written as hyphen-separated inclusive intervals.
>>>>> I don't see an empty-token production there.
>>>>
>>>> What you quote is insufficient to determine: COLOR may be allowed to be
>>>> <nothing>. Iirc the reasoning went in particular towards a range with
>>>> merely the upper end specified being something (halfway) meaningful.
>>>
>>> Right, I see your point.
>>>
>>> The grammar does not define COLOR explicitly, so it does not by itself
>>> prove that an empty token is invalid. I was implicitly reading COLOR as
>>> a numeric color value, partly because all examples seem to use numeric
>>> values, but I agree that this is not stated there.
>>>
>>> If this behavior is intentional, then I agree the second patch should
>>> not go in as-is.
>>
>> Just to mention: Something like "-5" won't be interpreted as "0-5" even
>> right now. Instead it's taken as a single color with value -5U, afaict.
> 
> I don't think that is what happens with the current parser.
> 
> I tested this without the patches from this series:
> 
> (XEN) Command line: dom0_mem=2048M console=dtuart dtuart=serial0
> (XEN) loglvl=all console_timestamps=boot llc-coloring=on
> (XEN) xen-llc-colors=-5
> ...
> (XEN) LLC coloring info:
> (XEN) Number of LLC colors supported: 32
> (XEN) Xen LLC colors (6): { 0-5 }
> 
> So "-5" is currently interpreted as "0-5", not as a single color with
> value -5U.
> 
> That seems to happen because simple_strtoul() does not consume the
> leading '-', so start remains 0 and the parser then takes the range
> path.

Oh, I didn't recall this incompatibility with strtoul(). We also don't
permit a leading + there. IOW the behavior of the color parsing would
change if we made the functions (more) compatible with the standard.

Jan