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

Przemyslaw Marczak
2013-10-18T15:05:45Z

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@samsung.com>
>> Cc: Marek Vasut <marex@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?

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