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

[Peter Lieven]

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.

Peter