Re: [Qemu-devel] [PATCH 1/1] nbd: fix max_discard/max_transfer_length

["Denis V. Lunev" <den@openvz.org>]

On 06/02/15 15:22, Peter Lieven wrote:
> Am 06.02.2015 um 13:17 schrieb Denis V. Lunev:
>> On 06/02/15 15:07, Kevin Wolf wrote:
>>> Am 06.02.2015 um 12:59 hat Denis V. Lunev geschrieben:
>>>> On 06/02/15 14:53, Kevin Wolf wrote:
>>>>> Am 06.02.2015 um 12:24 hat Denis V. Lunev geschrieben:
>>>>>> nbd_co_discard calls nbd_client_session_co_discard which uses uint32_t
>>>>>> as the length in bytes of the data to discard due to the following
>>>>>> definition:
>>>>>>
>>>>>> struct nbd_request {
>>>>>>       uint32_t magic;
>>>>>>       uint32_t type;
>>>>>>       uint64_t handle;
>>>>>>       uint64_t from;
>>>>>>       uint32_t len; <-- the length of data to be discarded, in bytes
>>>>>> } QEMU_PACKED;
>>>>>>
>>>>>> Thus we should limit bl_max_discard to UINT32_MAX >> BDRV_SECTOR_BITS to
>>>>>> avoid overflow.
>>>>>>
>>>>>> NBD read/write code uses the same structure for transfers. Fix
>>>>>> max_transfer_length accordingly.
>>>>>>
>>>>>> Signed-off-by: Denis V. Lunev <den@openvz.org>
>>>>>> CC: Peter Lieven <pl@kamp.de>
>>>>>> CC: Kevin Wolf <kwolf@redhat.com>
>>>>> Thanks, I have applied both Peter's and your patch. Can you guys please
>>>>> check whether the current state of my block branch is correct or whether
>>>>> I forgot to include or remove some patch?
>>>> can you give me tree URL?
>>> Sure:
>>>
>>> git: git://repo.or.cz/qemu/kevin.git block
>>> Web: http://repo.or.cz/w/qemu/kevin.git/shortlog/refs/heads/block
>>>
>>>>> By the way, I don't think this NBD patch is strictly necessary as you'll
>>>>> have a hard time finding a platform where INT_MAX > UINT32_MAX, but I
>>>>> think it's good documentation at least and a safeguard if we ever decide
>>>>> to lift the general block layer restrictions.
>>>>>
>>>>> Kevin
>>>> nope, it is absolutely mandatory
>>>>
>>>> stdint.h:
>>>>
>>>> /* Limit of `size_t' type.  */
>>>> # if __WORDSIZE == 64
>>>> #  define SIZE_MAX              (18446744073709551615UL)
>>>> # else
>>>> #  define SIZE_MAX              (4294967295U)
>>>> # endif
>>> But Peter defined it like this:
>>>
>>> #define BDRV_REQUEST_MAX_SECTORS MIN(SIZE_MAX >> BDRV_SECTOR_BITS, \
>>>                                        INT_MAX >> BDRV_SECTOR_BITS)
>>>
>>> And having integers with more the 32 bits is at least unusual. I don't
>>> know of any platform that has them.
>>>
>>> Anyway, as I said, your patch is good documentation, so I'm happy to
>>> apply it nevertheless.
>>>
>>> Kevin
>> I have misinterpreted this.
>>
>> Actually I think then the limit should be MAX() rather then MIN()
>> as the stack is ready to size_t transfers. In the other case
>> there is no need at all to use this construction. INT_MAX will be
>> always less than SIZE_MAX. I do not know any platform
>> where this is violated.
> That doesn't work. All internal routines have int (32-bit) as type for nb_sectors
> whereas size_t is often 64-bit.
>
> I also think that INT_MAX is always less than SIZE_MAX, but I would leave it
> in just to be absolutely sure. Its evaluated at compile time and will not
> hurt.
>
> Peter
>
OK :) let it be. Staying on safe side is always good.

Kevin, all my stuff we have agreed on is applied.

Regards,
     Den