[Libguestfs] [PATCH nbdkit] luks: Don't advertise zero support

[Richard W.M. Jones]

On Fri, Jun 10, 2022 at 02:03:28PM +0200, Laszlo Ersek wrote:
> On 06/10/22 13:47, Richard W.M. Jones wrote:
> > Fix an assertion failure in nbdkit:
> > 
> > $ qemu-img create -f luks --object secret,data=LETMEPASS,id=sec0 -o key-secret=sec0 encrypted.img 100M
> > Formatting 'encrypted.img', fmt=luks size=104857600 key-secret=sec0
> > $ rm -f data.img
> > $ truncate -s 100M data.img
> > $ nbdkit file encrypted.img --filter=luks passphrase=LETMEPASS --run 'nbdcopy data.img $nbd' -v
> > ...
> > nbdkit: file.10: debug: luks: zero count=104857600 offset=0 may_trim=1 fua=0 fast=0
> > nbdkit: backend.c:718: backend_zero: Assertion `c->can_zero > NBDKIT_ZERO_NONE' failed.
> > 
> > It's always wrong for filters to try to return NBDKIT_ZERO_EMULATE, at
> > least with the current implementation of the server.  This is because
> > emulation is only done at the plugin layer (see server/plugins.c:
> > plugin_zero), and not at the filter layer.
> > 
> > We could adjust nbdkit's filter layer (server/filter.c:filter_zero) to
> > do similar emulation, but an easier solution here is not to advertise
> > it to the client at all.
> > 
> > In future we should adjust the LUKS plugin so it does whatever the
> > kernel does for trimming/zeroing.
> > ---
> >  filters/luks/luks.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/filters/luks/luks.c b/filters/luks/luks.c
> > index 8ad3f4ec..9c1a3389 100644
> > --- a/filters/luks/luks.c
> > +++ b/filters/luks/luks.c
> > @@ -189,7 +189,7 @@ luks_can_trim (nbdkit_next *next, void *handle)
> >  static int
> >  luks_can_zero (nbdkit_next *next, void *handle)
> >  {
> > -  return NBDKIT_ZERO_EMULATE;
> > +  return NBDKIT_ZERO_NONE;
> >  }
> >  
> >  static int
> > 
> 
> so this is withdrawn now, right?

Yup!

Rich.

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Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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