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

[Kevin Wolf <kwolf@redhat.com>]

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