Re: [PATCH RFC 09/15] arm_mpam: Fix MSC MMIO window size to use resource_size() instead of end - start

[Ben Horgan]

Hi Yin,

On 09/09/2026 10:25, Yin Li wrote:
> 
> 
> On 9/4/2026 11:05 AM, Yin Li wrote:
>>
>>
>> On 9/3/2026 9:23 PM, Ben Horgan wrote:
>>> Hi Yin,
>>>
>>> On 03/09/2026 11:20, Ben Horgan wrote:
>>>> Hi Yin,
>>>>
>>>> On 11/08/2026 14:30, Yin Li wrote:
>>>>> struct resource uses an inclusive end address, so the correct size is
>>>>> end - start + 1. The previous calculation of end - start was off by one,
>>>>> resulting in a mapped window one byte smaller than the actual resource.
>>>>> Use resource_size() which correctly computes end - start + 1.
>>>>>
>>>>> Signed-off-by: Yin Li <yin.li@xxxxxxxxxxxxxxxx>
>>>>
>>>> I just got a kernel ci report for this one which asks for tags:
>>>>
>>>> Reported-by: kernel test robot <lkp@xxxxxxxxx>
>>>> Closes: https://lore.kernel.org/oe-kbuild-all/202609030809.ObirDhR3- lkp@xxxxxxxxx/
>>>>
>>>> It doesn't look to give a useful fixes tag though. I'd go with this as that's where the error was
>>>> introduced.
>>>>
>>>> Fixes: f04046f2577a ("arm_mpam: Add probe/remove for mpam msc driver and kbuild boiler plate")
>>>>
>>>> Looks good to me.
>>>>
>>>> Reviewed-by: Ben Horgan <ben.horgan@xxxxxxx>
>>>
>>> Scratch that. As Ilpo points out,[1], there is no functional bug but just some misleading naming
>>> which never the less would be good to fix. This does require > in the warnings becoming >= though
>>> and there would be no need for fixes tag. Do you agree with this analysis?
>>>
>>> Thanks,
>>>
>>> Ben
>>>
>>> [1]
>>> https://lore.kernel.org/ lkml/03055fbc-281f-4ed9-9282-4853d560e17f@xxxxxxx/T/
>>> #mdb57d40c888ff4ce656a9d9a00ecf5d99466530c
>>>
>>>
>>
>> Hi Ben,
>>
>> Thanks, and thanks to Ilpo for the detailed analysis.
>>
>> Agreed — the current code is functionally correct because the off-by-one
>> in "end - start" is cancelled out by the ">" checks, since
>> mapped_hwpage_sz effectively holds the last mapped byte rather than the
>> size. So there's no functional bug and no Fixes tag is needed.
>>
>> I'll update the patch to switch to resource_size() and change the
>> corresponding ">" checks to ">=" together, so the naming becomes
>> accurate while keeping the behaviour unchanged. I'll also reword the
>> commit message to describe this as a naming/readability cleanup rather
>> than a bugfix.
>>
>>
> 
> Hi Ben,
> 
> I looked into this more closely, and I think the situation is a bit
> different from previous analysis — could you and Ilpo double-check?
> 
> All three bounds checks have the access width included on the left-hand
> side, e.g.:
> 
>       WARN_ON_ONCE(reg + sizeof(u32) > msc->mapped_hwpage_sz);
> 
> Here "reg + sizeof(u32)" is the one-past-the-end offset of the write, so
> a legal access needs "reg + 4 <= size", i.e. the out-of-bounds condition
> is correctly "> size".
> 
> Take a 0x1000-sized window (valid offsets 0x000..0xFFF):
> 
>     - With the old value, mapped_hwpage_sz = end - start = 0xFFF (size - 1).
>     A write at reg = 0xFFC touches bytes 0xFFC..0xFFF — exactly the last
>     4 bytes, which is legal. But the check computes 0xFFC + 4 = 0x1000 >
>     0xFFF → true, so it falsely warns on a valid access. That's a real
>     off-by-one.
> 
>     - With resource_size() = 0x1000 (true size), the same access gives
>     0x1000 > 0x1000 → false, so it correctly passes, while an access at
>     reg = 0xFFD (0x1001 > 0x1000 → true) is correctly rejected.
> 
> So resource_size() fixes a genuine off-by-one here, and the ">" checks
> should stay as ">". Changing them to ">=" would break the reg = 0xFFC
> case (0x1000 >= 0x1000 → true) and wrongly reject a legal access.
> 
> So for the next version I plan to keep the resource_size() change, keep
> the ">" checks unchanged, and describe it as an off-by-one fix rather
> than a naming cleanup. Does this match what you and Ilpo see?

Yes. Thank you very much for taking the time and looking into this more closely.
It looks like I introduced the functional bug whilst upstreaming James' patches.

Ben

> 
> Best regards,
> Yin Li
> 
> 
> 
> 
>>>>
>>>> Thanks,
>>>>
>>>> Ben
>>>>
>>>>> ---
>>>>>   drivers/resctrl/mpam_devices.c | 2 +-
>>>>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>>>>
>>>>> diff --git a/drivers/resctrl/mpam_devices.c b/drivers/resctrl/ mpam_devices.c
>>>>> index 1e082fb60e30..5d1854d97371 100644
>>>>> --- a/drivers/resctrl/mpam_devices.c
>>>>> +++ b/drivers/resctrl/mpam_devices.c
>>>>> @@ -2296,7 +2296,7 @@ static struct mpam_msc *do_mpam_msc_drv_probe(struct platform_device *pdev)
>>>>>               dev_err_once(dev, "Failed to map MSC base address\n");
>>>>>               return ERR_CAST(io);
>>>>>           }
>>>>> -        msc->mapped_hwpage_sz = msc_res->end - msc_res->start;
>>>>> +        msc->mapped_hwpage_sz = resource_size(msc_res);
>>>>>           msc->mapped_hwpage = io;
>>>>>       } else {
>>>>>           return ERR_PTR(-EINVAL);
>>>>>
>>>>
>>>
>>
>