[Libguestfs] [PATCH] lib: Autodetect backing format and specify it explicitly.

[Daniel P. Berrangé]

On Mon, Mar 09, 2020 at 11:19:39AM +0000, Richard W.M. Jones wrote:
> On Mon, Mar 09, 2020 at 12:07:08PM +0100, Pino Toscano wrote:
> > On Monday, 9 March 2020 11:40:46 CET Richard W.M. Jones wrote:
> > > On Mon, Mar 09, 2020 at 11:07:20AM +0100, Pino Toscano wrote:
> > > > On Thursday, 6 February 2020 18:20:19 CET Richard W.M. Jones wrote:
> > > > > In the guestfs_disk_create API we have traditionally allowed you to
> > > > > set backingfile without setting backingformat.  The meaning of this is
> > > > > to let qemu autodetect the backing format when opening the overlay
> > > > > disk.
> > > > > 
> > > > > However libvirt >= 6.0 refuses to even pass such disks to qemu (see
> > > > > https://bugzilla.redhat.com/show_bug.cgi?id=1798148).
> > > > > 
> > > > > For this reason, move the autodetection earlier and make it explicit.
> > > > > We now autodetect the format of the backing disk at the time of
> > > > > creation of the overlay, and set that as the backing format in the
> > > > > overlay disk itself, allowing libvirt to open the disk later.
> > > > > ---
> > > > >  lib/create.c | 13 +++++++++++++
> > > > >  1 file changed, 13 insertions(+)
> > > > > 
> > > > > diff --git a/lib/create.c b/lib/create.c
> > > > > index e2a59b88d..e30286c39 100644
> > > > > --- a/lib/create.c
> > > > > +++ b/lib/create.c
> > > > > @@ -255,6 +255,7 @@ disk_create_qcow2 (guestfs_h *g, const char *filename, int64_t size,
> > > > >                     const struct guestfs_disk_create_argv *optargs)
> > > > >  {
> > > > >    const char *backingformat = NULL;
> > > > > +  CLEANUP_FREE char *backingformat_free = NULL;
> > > > >    const char *preallocation = NULL;
> > > > >    const char *compat = NULL;
> > > > >    int clustersize = -1;
> > > > > @@ -302,6 +303,18 @@ disk_create_qcow2 (guestfs_h *g, const char *filename, int64_t size,
> > > > >      }
> > > > >    }
> > > > >  
> > > > > +  /* With libvirt >= 6.0 the backing format must be specified. */
> > > > > +  if (backingfile != NULL && backingformat == NULL) {
> > > > > +    backingformat = backingformat_free = guestfs_disk_format (g, backingfile);
> > > > > +    if (!backingformat)
> > > > > +      return -1;
> > > > > +    if (STREQ (backingformat, "unknown")) {
> > > > > +      error (g, _("could not auto-detect the format of the backing file %s"),
> > > > > +             backingfile);
> > > > > +      return -1;
> > > > > +    }
> > > > > +  }
> > > > 
> > > > I see this patch was pushed, even if it was not reviewed, and in
> > > > general the problem was more (not totally) on libvirt side.
> > > > 
> > > > Was it intentional?
> > > 
> > > Yes, I pushed it intentionally.  Peter has a partial fix on the
> > > libvirt side, but it seems to me they are heading in the direction of
> > > requiring that the format of backing files in the chain being
> > > specified.  Since our API specifies that a NULL backing format means
> > > the format will be autodetected (but not by what), I moved the
> > > detection to here.
> > 
> > One of the arguments was that using qemu-img to detect the format of
> > an image was unsafe in any case, and thus libvirt does not use it
> > (it has own parsing code).
> > 
> > To sum up the situation:
> > - if using qemu-img is safe (with more or less hardening on top), then
> >   there is no reason to not do that in libvirt
> > - if using qemu-img is not safe, then I do not see why libguestfs does
> >   it while libvirt avoids it
> > Hence, I think this patch in libguestfs is not correct, no matter which
> > POV/course of action we choose.
> 
> Well we can certainly revert the patch if you think this is wrong.
> Dan, any views on this?

Libvirt doesn't do format probing in all cases, but since the 6.1.0
release we do it in enough cases to fix the regressions wrt previous
libvirt behaviour. So IIUC, there shouldn't be a need for libguestfs
(or other apps) to work around this, except for with 6.0.0 specifically.
That one release could be blacklisted, or left as a docs issue.

Regards,
Daniel
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