Re: [Qemu-devel] [PATCH 1/7] block: only force IO completion in .bdrv_truncate if we are shrinking

[Kevin Wolf <kwolf@redhat.com>]

Am 07.03.2013 um 09:56 hat Peter Lieven geschrieben:
> On 07.03.2013 09:50, Kevin Wolf wrote:
> >Am 06.03.2013 um 21:39 hat Paolo Bonzini geschrieben:
> >>Il 06/03/2013 20:03, Peter Lieven ha scritto:
> >>>Am 06.03.2013 19:48, schrieb Jeff Cody:
> >>>>On Wed, Mar 06, 2013 at 07:31:51PM +0100, Paolo Bonzini wrote:
> >>>>>Il 06/03/2013 19:14, Jeff Cody ha scritto:
> >>>>>>QCOW breaks with it using a normal raw posix file as a device.  As a
> >>>>>>test: qemu-img create -f qcow test.qcow 5G.  Now run qemu with that
> >>>>>>drive mounted, and try to partition and format it.  QEMU now asserts.
> >>>>>>
> >>>>>>The nicety of being able to using truncate during a write call,
> >>>>>>especially for VHDX (which can have relatively large block/cluster
> >>>>>>sizes), so to grow the file sparsely in a dynamically allocated file.
> >>>>>
> >>>>>Perhaps we need two APIs, "truncate" and "revalidate".
> >>>>>
> >>>>>Truncate should be a no-op if (!bs->growable).
> >>>>>
> >>>>>Revalidate could be called by the block_resize monitor command with no
> >>>>>size specified.
> >>>>>
> >>>>>Paolo
> >>>>
> >>>>I think that is a good solution.  Is it better to have "truncate" and
> >>>>"revalidate", or "truncate" and "grow", with grow being a subset of
> >>>>truncate, with fewer restrictions?  There may still be operations
> >>>>where it is OK to grow a file, but not OK to shrink it.
> >
> >What semantics would the both operations have? Is truncate the same as
> >it used to be? I don't really understand what "revalidate" would do, it
> >sounds like a read-only operation from its name?
> >
> >>>Or as a first step:
> >>>
> >>>a) Call brdv_drain_all() only if the device is shrinked (independently of !bs->growable)
> >>>b) Call brdv_drain_all() inside iscsi_truncate() because it is a special requirement there
> >>>c) Fix the value of bs->growable for all drivers
> >>
> >>Let's start from (c).  bdrv_file_open sets bs->growable = 1.  I think it
> >>should be removed and only the file protocol should set it.
> >
> >This is probably right.
> 
> If bs->growable is 1 for all drivers, whats the fix status of CVE-2008-0928? This
> flag was introduced as a fix for this problem.
> 
> bdrv_check_byte_request() does nothing useful if bs->growable is 1.

Don't ignore the difference between bdrv_open() and bdrv_file_open().
Typically you have two BDSes: On top there is e.g. a qcow2 BDS that is
opened through bdrv_open() and has bs->growable = false. Its bs->file is
using the file protocol (raw-posix driver) and opened by
bdrv_file_open(). This one has bs->file->growable = true so that qcow2
can write to newly allocated areas without calling bdrv_truncate()
first.

Kevin