Re: [PATCH 2/2] memory: XENMEM_add_to_physmap (almost) wrapping checks

[Jan Beulich <jbeulich@xxxxxxxx>]

On 15.10.2021 11:26, Julien Grall wrote:
> On 14/10/2021 15:10, Jan Beulich wrote:
>> On 14.10.2021 13:29, Julien Grall wrote:
>>> On 13/09/2021 07:42, Jan Beulich wrote:
>>>> Determining that behavior is correct (i.e. results in failure) for a
>>>> passed in GFN equaling INVALID_GFN is non-trivial. Make this quite a
>>>> bit more obvious by checking input in generic code - both for singular
>>>> requests to not match the value and for range ones to not pass / wrap
>>>> through it.
>>>>
>>>> For Arm similarly make more obvious that no wrapping of MFNs passed
>>>> for XENMAPSPACE_dev_mmio and thus to map_dev_mmio_region() can occur:
>>>> Drop the "nr" parameter of the function to avoid future callers
>>>> appearing which might not themselves check for wrapping. Otherwise
>>>> the respective ASSERT() in rangeset_contains_range() could trigger.
>>>>
>>>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>>>> ---
>>>> I find it odd that map_dev_mmio_region() returns success upon
>>>> iomem_access_permitted() indicating failure - is this really intended?
>>>
>>> AFAIR yes. The hypercall is not used as "Map the region" but instead
>>> "Make sure the region is mapped if the IOMEM region is accessible".
>>>
>>> It is necessary to return 0 because dom0 OS cannot distinguished between
>>> emulated and non-emulated. So we may report error when there is none.
>>
>> Odd, but I clearly don't understand all the aspects here.
>>
>>>> As per commit 102984bb1987 introducing it this also was added for ACPI
>>>> only - any reason XENMAPSPACE_dev_mmio isn't restricted to CONFIG_ACPI
>>>> builds?
>>>
>>> There is nothing specific to ACPI in the implementation. So I don't
>>> really see the reason to restrict to CONFIG_ACPI.
>>>
>>> However, it is still possible to boot using DT when Xen is built with
>>> CONFIG_ACPI. So if the restriction was desirable, then I think it should
>>> be using !acpi_disabled.
>>
>> My point was rather about this potentially being dead code in non-ACPI
>> builds (i.e. in particular uniformly on 32-bit).
> 
> The hypercall is already wired and a dom0 OS can use it today even on 
> non-ACPI. Whether a dom0 OS will use it is a different question. I know 
> that Linux will limit it to ACPI. It is likely not used by other OS, but 
> I can't guarantee it.
> 
> In this case, the hypercall is only a few lines and already restricted 
> to dom0 only (see xapt_permission_check()). So to me, the #ifdef here is 
> not worth it.

Well, okay then - I've removed that remark.

>>>> @@ -841,6 +844,15 @@ int xenmem_add_to_physmap(struct domain
>>>>        if ( xatp->size < start )
>>>>            return -EILSEQ;
>>>>    
>>>> +    if ( xatp->gpfn + xatp->size < xatp->gpfn ||
>>>> +         xatp->idx + xatp->size < xatp->idx )
>>>> +    {
>>>> +#define _gfn(x) (x)
>>>
>>> AFAICT, _gfn() will already be defined. So some compiler may complain
>>> because will be defined differently on debug build.
>>
>> No - _gfn() is an inline function as per typesafe.h. (Or else it
>> wouldn't be just "some" compiler, but gcc at least would have
>> complained to me.)
> 
> Ah. somehow I thought it was a macro. But looking at the implementation, 
> it makes sense to be an inline funciton.
> 
> Sorry for the noise.
> 
>>
>>> However...
>>>
>>>> +        BUILD_BUG_ON(INVALID_GFN + 1);
>>>
>>> ... I might be missing something... but why can't use gfn_x(INVALID_GFN)
>>> + 1 here?
>>
>> Because gfn_x() also is an inline function, and that's not suitable
>> for a compile-time constant expression.
> 
> Right. How about introduce INVALID_GFN_RAW in mm-frame.h? This could 
> also be used to replace the open-code value in INVALID_GFN and 
> INVALID_GFN_INITIALIZER?

Can do, but that'll be a prereq patch then also taking care of INVALID_MFN.

>>> In fact, I am not entirely sure what's the purpose of this
>>> BUILD_BUG_ON(). Could you give more details?
>>
>> The expression in the surrounding if() relies on INVALID_GFN being the
>> largest representable value, i.e. this ensures that INVALID_GFN doesn't
>> sit anywhere in [xatp->gpfn, xatp->gpfn + xatp->size).
> 
> Thanks the explanation. Can you add the rationale in a comment on top of 
> BUILD_BUG_ON()?

Sure, done.

Jan