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Re: [PATCH 2/3] xen/common: llc-coloring: Fix off-by-one in parse_color_config()
- To: Jan Beulich <jbeulich@xxxxxxxx>
- From: Luca Fancellu <Luca.Fancellu@xxxxxxx>
- Date: Thu, 9 Apr 2026 13:34:10 +0000
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- Cc: "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Anthony PERARD <anthony.perard@xxxxxxxxxx>, Julien Grall <julien@xxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Michal Orzel <michal.orzel@xxxxxxx>
- Delivery-date: Thu, 09 Apr 2026 13:35:25 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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- Thread-index: AQHcyBWdW2bjkWPifku6XWocvaXOWrXWpuWAgAAHZ4CAAADggIAAC5wA
- Thread-topic: [PATCH 2/3] xen/common: llc-coloring: Fix off-by-one in parse_color_config()
Hi Jan,
> On 9 Apr 2026, at 13:52, Luca Fancellu <Luca.Fancellu@xxxxxxx> wrote:
>
>
>
>> On 9 Apr 2026, at 13:48, Jan Beulich <jbeulich@xxxxxxxx> wrote:
>>
>> On 09.04.2026 14:22, Luca Fancellu wrote:
>>>> On 9 Apr 2026, at 12:39, Michal Orzel <michal.orzel@xxxxxxx> wrote:
>>>>
>>>> The check uses >= to compare the total number of colors against
>>>> max_num_colors (which is ARRAY_SIZE of the colors array). This
>>>> incorrectly rejects input that would exactly fill the array.
>>>>
>>>> For example, with NR_LLC_COLORS=16, specifying 1 color for Xen and 15
>>>> for dom0 would fail.
>>>>
>>>> Change >= to > so that exactly filling the array is permitted.
>>>>
>>>> Fixes: 95ef5ddf8a ("xen/arm: add Dom0 cache coloring support")
>>>> Signed-off-by: Michal Orzel <michal.orzel@xxxxxxx>
>>>> ---
>>>
>>> Reviewed-by: Luca Fancellu <luca.fancellu@xxxxxxx>
>>
>> Did you see Andrew's reply? If that earlier (recent) commit was wrong, I
>> think a 2nd Fixes: tag may be needed here. For now I can't help the
>> impression though that there might have been a re-basing mistake, where
>> that re-base may have wanted to result in this patch dissolving into
>> nothing. Yet of course I'm all ears to learn otherwise.
>>
>> Jan
>
> Oh, no I didn’t see that! Thanks for pointing that out, I will have a closer
> look.
I had a closer look, I feel that the patch is ok and commit
cba8a584de171c8c4510709c2edc9f1cf86b21ab
was missing this corner case.
Let’s say max_num_colors = 8 (array capacity), *num_colors = 4 so we stored
already 4 entries and the
next parsed range gives start = 4, end = 7:
(*num_colors + (end - start + 1)) >= max_num_colors will compute as
(4 + (7 - 4 + 1)) >= 8 which will be
8 >= 8 that will be true and the input will be rejected, instead of being a
valid entry.
Did I miss anything?
Cheers,
Luca
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