Re: [Qemu-devel] [PATCH] Add support for fd: protocol

[Anthony Liguori <anthony@codemonkey.ws>]

On 05/23/2011 05:30 AM, Daniel P. Berrange wrote:
> It feels to me that turning the current block driver code which just does
> open(2) on files, into something which issues events&  asynchronously
> waits for a file would potentially be quite complex.
>
> You also need to be much more careful from a security POV if the mgmt
> app is accepting requests to open arbitrary files from QEMU, to ensure
> the filenames are correctly/strictly validated before opening them and
> giving them back to QEMU. An architecture where the mgmt app decides
> what FDs to supply upfront, has less potential for security errors.
>
> To me the ideal would thus be that we can supply FDs for the backing
> store with -blockdev syntax, and that places where QEMU re-opens files
> would be enhanced to avoid that need. If there are things we can't do
> without closing&  re-opening the same file, then perhaps we need some
> new ioctl()/fcntl() calls to change those file attributes on the fly.

I agree.  But my view of blockdev is that you wouldn't set an fd 
attribute but rather the backing file name and use the fd protocol.  For 
instance:

-blockdev id=foo-base,path=fd:4,format=raw
-blockdev id=foo,path=fd:3,format=qcow2,backing_file=foo



> Regards,
> Daniel