Re: [PATCH V10 06/12] of: device: Fix overflow of coherent_dma_mask

[Sricharan R <sricharan@codeaurora.org>]

Hi Frank,

On 4/6/2017 12:31 PM, Frank Rowand wrote:
> On 04/04/17 03:18, Sricharan R wrote:
>> Size of the dma-range is calculated as coherent_dma_mask + 1
>> and passed to arch_setup_dma_ops further. It overflows when
>> the coherent_dma_mask is set for full 64 bits 0xFFFFFFFFFFFFFFFF,
>> resulting in size getting passed as 0 wrongly. Fix this by
>> passsing in max(mask, mask + 1). Note that in this case
>> when the mask is set to full 64bits, we will be passing the mask
>> itself to arch_setup_dma_ops instead of the size. The real fix
>> for this should be to make arch_setup_dma_ops receive the
>> mask and handle it, to be done in the future.
>>
>> Signed-off-by: Sricharan R <sricharan@codeaurora.org>
>> ---
>>  drivers/of/device.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/drivers/of/device.c b/drivers/of/device.c
>> index c17c19d..c2ae6bb 100644
>> --- a/drivers/of/device.c
>> +++ b/drivers/of/device.c
>> @@ -107,7 +107,7 @@ void of_dma_configure(struct device *dev, struct device_node *np)
>>  	ret = of_dma_get_range(np, &dma_addr, &paddr, &size);
>>  	if (ret < 0) {
>>  		dma_addr = offset = 0;
>> -		size = dev->coherent_dma_mask + 1;
>> +		size = max(dev->coherent_dma_mask, dev->coherent_dma_mask + 1);
>>  	} else {
>>  		offset = PFN_DOWN(paddr - dma_addr);
>>  		dev_dbg(dev, "dma_pfn_offset(%#08lx)\n", offset);
>>
>
> NACK.
>
> Passing an invalid size to arch_setup_dma_ops() is only part of the problem.
> size is also used in of_dma_configure() before calling arch_setup_dma_ops():
>
>         dev->coherent_dma_mask = min(dev->coherent_dma_mask,
>                                      DMA_BIT_MASK(ilog2(dma_addr + size)));
>         *dev->dma_mask = min((*dev->dma_mask),
>                              DMA_BIT_MASK(ilog2(dma_addr + size)));
>
> which would be incorrect for size == 0xffffffffffffffffULL when
> dma_addr != 0.  So the proposed fix really is not papering over
> the base problem very well.
>

Ok, but with your fix for of_dma_get_range and the above fix,
dma_addr will be '0' when size = 0xffffffffffffffffULL,
but DMA_BIT_MASK(ilog2(dma_addr + size)) would be wrong though,
making coherent_dma_mask to be smaller 0x7fffffffffffffffULL.

Regards,
  Sricharan

> I agree that the proper solution involves passing a mask instead
> of a size to arch_setup_dma_ops().
>

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