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

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

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.

Den