Re: [Qemu-devel] Question about total_sectors in block/vpc.c

From: Stefan Hajnoczi <stefanha@gmail.com>
To: Lyu Mitnick <mitnick.lyu@gmail.com>
Cc: Kevin Wolf <kwolf@redhat.com>,
	qemu-devel@nongnu.org, Christoph Hellwig <hch@lst.de>
Subject: Re: [Qemu-devel] Question about total_sectors in block/vpc.c
Date: Sat, 9 Apr 2011 21:05:41 +0100	[thread overview]
Message-ID: <BANLkTi=-x2AgNnQ8t9H=kaAT45Az3m782w@mail.gmail.com> (raw)
In-Reply-To: <BANLkTikWG=GvCMnY6daq4VWv2wnvJwZUDw@mail.gmail.com>

On Sat, Apr 9, 2011 at 5:51 PM, Lyu Mitnick <mitnick.lyu@gmail.com> wrote:
> Hell all,
> I have take a look of block/vpc.c and meet a question in vpc_create(). At
> the line
> 550, the code is:
> total_sectors = options->value.n / 512;
> I am wondering whether the size between total_sectors * 512
> and options->value.n
> would be discard.

Yes, it rounds down.  This reflects the assumption that a block device
cannot be addressed below 512 byte sectors.  Because of this block
devices size must be a multiple of 512 bytes.

I think a reasonable protection would be to have block.c:bdrv_create()
fail if size is not a multiple of BDRV_SECTOR_SIZE.  This way other
image formats are protected too.

Stefan