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

From: Denis V. Lunev
Subject: Re: [Qemu-devel] [PATCH 1/1] nbd: fix max_discard/max_transfer_length
Date: Fri, 6 Feb 2015 15:17:26 +0300
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.10; rv:31.0) Gecko/20100101 Thunderbird/31.4.0

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

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

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 <address@hidden>
CC: Peter Lieven <address@hidden>
CC: Kevin Wolf <address@hidden>
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?

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.

nope, it is absolutely mandatory


/* Limit of `size_t' type.  */
# if __WORDSIZE == 64
#  define SIZE_MAX              (18446744073709551615UL)
# else
#  define SIZE_MAX              (4294967295U)
# endif
But Peter defined it like this:

                                      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.

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.


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