Re: [PATCH] iommu: Up front sanity check in the arm_lpae_map

[zhukeqian]

Hi,

On 2020/12/7 20:05, Will Deacon wrote:
> On Mon, Dec 07, 2020 at 12:01:09PM +0000, Robin Murphy wrote:
>> On 2020-12-05 08:29, Keqian Zhu wrote:
>>> ... then we have more chance to detect wrong code logic.
>>
>> I don't follow that justification - it's still the same check with the same
>> outcome, so how does moving it have any effect on the chance to detect
>> errors?

>>
>> AFAICS the only difference it would make is to make some errors *less*
>> obvious - if a sufficiently broken caller passes an empty prot value
>> alongside an invalid size or already-mapped address, this will now quietly
>> hide the warnings from the more serious condition(s).
>>
>> Yes, it will bail out a bit faster in the specific case where the prot value
>> is the only thing wrong, but since when do we optimise for fundamentally
>> incorrect API usage?
> 
> I thought it was the other way round -- doesn't this patch move the "empty
> prot" check later, so we have a chance to check the size and addresses
> first?

Yes, this is my original idea.
For that we treat iommu_prot with no permission as success at early start, defer
this early return can expose hidden errors.

Thanks,
Keqian
> 
> Will
> 
>>> Signed-off-by: Keqian Zhu <zhukeqian1@xxxxxxxxxx>
>>> ---
>>>   drivers/iommu/io-pgtable-arm.c | 8 ++++----
>>>   1 file changed, 4 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/drivers/iommu/io-pgtable-arm.c b/drivers/iommu/io-pgtable-arm.c
>>> index a7a9bc08dcd1..8ade72adab31 100644
>>> --- a/drivers/iommu/io-pgtable-arm.c
>>> +++ b/drivers/iommu/io-pgtable-arm.c
>>> @@ -444,10 +444,6 @@ static int arm_lpae_map(struct io_pgtable_ops *ops, unsigned long iova,
>>>   	arm_lpae_iopte prot;
>>>   	long iaext = (s64)iova >> cfg->ias;
>>> -	/* If no access, then nothing to do */
>>> -	if (!(iommu_prot & (IOMMU_READ | IOMMU_WRITE)))
>>> -		return 0;
>>> -
>>>   	if (WARN_ON(!size || (size & cfg->pgsize_bitmap) != size))
>>>   		return -EINVAL;
>>> @@ -456,6 +452,10 @@ static int arm_lpae_map(struct io_pgtable_ops *ops, unsigned long iova,
>>>   	if (WARN_ON(iaext || paddr >> cfg->oas))
>>>   		return -ERANGE;
>>> +	/* If no access, then nothing to do */
>>> +	if (!(iommu_prot & (IOMMU_READ | IOMMU_WRITE)))
>>> +		return 0;
>>> +
>>>   	prot = arm_lpae_prot_to_pte(data, iommu_prot);
>>>   	ret = __arm_lpae_map(data, iova, paddr, size, prot, lvl, ptep, gfp);
>>>   	/*
>>>
> .
>