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

[Mykola Kvach <xakep.amatop@xxxxxxxxx>]

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.

Best regards,
Mykola