Re: [PATCH v1 04/10] powerpc/kernel/iommu: Add new iommu_table_in_use() helper

[Alexey Kardashevskiy]

On 28/08/2020 04:34, Leonardo Bras wrote:
> On Sat, 2020-08-22 at 20:34 +1000, Alexey Kardashevskiy wrote:
>>> +
>>> +	/*ignore reserved bit0*/
>>
>> s/ignore reserved bit0/ ignore reserved bit0 /  (add spaces)
> 
> Fixed
> 
>>> +	if (tbl->it_offset == 0)
>>> +		p1_start = 1;
>>> +
>>> +	/* Check if reserved memory is valid*/
>>
>> A missing space here.
> 
> Fixed
> 
>>
>>> +	if (tbl->it_reserved_start >= tbl->it_offset &&
>>> +	    tbl->it_reserved_start <= (tbl->it_offset + tbl->it_size) &&
>>> +	    tbl->it_reserved_end   >= tbl->it_offset &&
>>> +	    tbl->it_reserved_end   <= (tbl->it_offset + tbl->it_size)) {
>>
>> Uff. What if tbl->it_reserved_end is bigger than tbl->it_offset +
>> tbl->it_size?
>>
>> The reserved area is to preserve MMIO32 so it is for it_offset==0 only
>> and the boundaries are checked in the only callsite, and it is unlikely
>> to change soon or ever.
>>
>> Rather that bothering with fixing that, may be just add (did not test):
>>
>> if (WARN_ON((
>> (tbl->it_reserved_start || tbl->it_reserved_end) && (it_offset != 0))
>> (tbl->it_reserved_start > it_offset && tbl->it_reserved_end < it_offset
>> + it_size) && (it_offset == 0)) )
>>  return true;
>>
>> Or simply always look for it_offset..it_reserved_start and
>> it_reserved_end..it_offset+it_size and if there is no reserved area,
>> initialize it_reserved_start=it_reserved_end=it_offset so the first
>> it_offset..it_reserved_start becomes a no-op.
> 
> The problem here is that the values of it_reserved_{start,end} are not
> necessarily valid. I mean, on iommu_table_reserve_pages() the values
> are stored however they are given (bit reserving is done only if they
> are valid). 
> 
> Having a it_reserved_{start,end} value outside the valid ranges would
> cause find_next_bit() to run over memory outside the bitmap.
> Even if the those values are < tbl->it_offset, the resulting
> subtraction on unsigned would cause it to become a big value and run
> over memory outside the bitmap.
> 
> But I think you are right. That is not the place to check if the
> reserved values are valid. It should just trust them here.
> I intent to change iommu_table_reserve_pages() to only store the
> parameters in it_reserved_{start,end} if they are in the range, and or
> it_offset in both of them if they are not.
> 
> What do you think?

This should work, yes.


> 
> Thanks for the feedback!
> Leonardo Bras
> 
> 
> 

-- 
Alexey