[U-Boot] [PATCH 3/4] usb: ums: fix bug in partition capacity computation.

[Przemyslaw Marczak]

Hello Marek,

On 10/19/2013 02:57 AM, Marek Vasut wrote:
> Dear Przemyslaw Marczak,
>
>> Hi Marek,
>>
>> On 10/17/2013 07:41 PM, Marek Vasut wrote:
>>> Dear Przemyslaw Marczak,
>>>
>>>> Before this change ums disk capacity was miscalculated because
>>>> of integer overflow.
>>>>
>>>> Signed-off-by: Przemyslaw Marczak <p.marczak at samsung.com>
>>>> Cc: Marek Vasut <marex at denx.de>
>>>> ---
>>>>
>>>>    board/samsung/common/ums.c |   16 ++++++++++++----
>>>>    1 file changed, 12 insertions(+), 4 deletions(-)
>>>>
>>>> diff --git a/board/samsung/common/ums.c b/board/samsung/common/ums.c
>>>> index 1f28590..6c4e6c4 100644
>>>> --- a/board/samsung/common/ums.c
>>>> +++ b/board/samsung/common/ums.c
>>>> @@ -37,11 +37,19 @@ static int ums_write_sector(struct ums *ums_dev,
>>>>
>>>>    static void ums_get_capacity(struct ums *ums_dev, long long int
>>>>    *capacity) {
>>>>
>>>> -	long long int tmp_capacity;
>>>> +	int64_t mmc_capacity = (int64_t)ums_dev->mmc->capacity;
>>>
>>> Why are these casts here?
>>>
>>>> +	int64_t ums_capacity = (int64_t)ums_dev->part_size * SECTOR_SIZE;
>>>> +	int64_t ums_offset = (int64_t)ums_dev->offset * SECTOR_SIZE;
>>>
>>> And here all around? And why are these values signed, can there ever be
>>> negative value in them?
>>
>> I tried to fix it without changes in ums driver because it works fine.
>> Of course capacity can't be a negative value.
>>
>> When we set some offset and some part size we have an integer overflow
>>
>> at this line, just before cast to long long int:
>>>> -	tmp_capacity = (long long int)((ums_dev->offset + ums_dev->part_size)
>>>> -				       * SECTOR_SIZE);
>>>> -	*capacity = ums_dev->mmc->capacity - tmp_capacity;
>>
>> In the best case of overflow - ums partition capacity will have the same
>> value as mmc cap, but if offset was set, then the partition size will be
>> exceeded.
>>
>>>> +	if (ums_capacity && ((ums_capacity + ums_offset) < mmc_capacity))
>>>> +		*capacity = ums_capacity;
>>>> +	else
>>>> +		*capacity = mmc_capacity - ums_offset;
>>>
>>> Urgh, what exactly does this code achieve again?
>>
>> This code above avoids situation when tmp_capacity value is bigger  than
>> real mmc capacity. I don't check next the offset but this is also the
>> reason why I put printf here. I assume that developer should know how to
>> define UMS_START_BLOCK and UMS_PART_SIZE if no, some information will be
>> printed.
>>
>>>> +	printf("UMS: partition capacity: %#llx blocks\n"
>>>> +	       "UMS: partition start block: %#x\n",
>>>> +	       *capacity / SECTOR_SIZE,
>>>> +	       ums_dev->offset);
>>>>
>>>>    }
>>>>
>>>>    static struct ums ums_dev = {
>>>
>>> Best regards,
>>> Marek Vasut
>>
>> In summary I will change signed variables to unsigned here and few in
>> the ums gadget driver.
>> Moreover now I think that it will be better to replace part_size from
>> the struct ums_dev with part_blk_num and compute its value at ums_init
>> function. And then pointer to ums_get_capacity is not needed in ums
>> structure.
>>
>> What do you think about this?
>
> I think the first screaming thing here is ... why is this all multiplied by
> SECTOR_SIZE before doing the comparisons and stuffs ? You can do that later
> (that does mean do it later, yes).

You're right, but actually we don't need to use real card capacity but 
only sector count. Patch v2 will include this.

>
> Try this:
>
> u64 mmc_cap = ums_dev->mmc->capacity / SECTOR_SIZE;
> u64 ums_start = ums_dev->offset;
> u64 ums_end = ums_start + ums_dev->part_size;
>
> /* Start past MMC size. */
> if (ums_start >= mmc_cap)
> 	return -EINVAL;
>
> /* End past MMC size. */
> if (ums_end > mmc_cap) {
> 	puts("UMS region larger than MMC device, capping\n");
> 	ums_end = mmc_cap;
> }
>
> *capacity = (ums_end - ums_start) * SECTOR_SIZE;
>
> Does this work? You'd need to add debug.
>

It will only work if UMS_PART_SIZE and UMS_START_BLOCK are set 
correctly. In default case when both values are defined as 0 - function 
returns null capacity but we don't want this.

Patch v2 will include cases for default, valid and bad definitions of 
UMS_PART_SIZE and UMS_START_BLOCK. I will also remove unnecessary code 
around capacity validation from ums gadget driver.
Next patch set will be send soon.

Regards
-- 
Przemyslaw Marczak
Samsung R&D Institute Poland
Samsung Electronics
p.marczak at samsung.com