[Libguestfs] [PATCH nbdkit] tls-fallback: Fix filter for new .block_size callback

[Richard W.M. Jones]

On Thu, Feb 17, 2022 at 02:20:28PM +0100, Laszlo Ersek wrote:
> On 02/16/22 22:41, Richard W.M. Jones wrote:
> > This filter doesn't call the next_open function in the non-TLS case,
> > and therefore it never opens the plugin.  This leaves the internal
> > state of nbdkit a bit strange.  There is no plugin context allocated,
> > and the last filter in the chain has a context c_next pointer of NULL.
> > 
> > This works, provided we intercept every possible callback, check the
> > non-TLS case, and prevent it from calling the next function (because
> > it would dereference the NULL c_next).
> > 
> > To avoid a crash in backend_block_size we must therefore provide a
> > .block_size callback in this filter.
> > ---
> >  filters/tls-fallback/tls-fallback.c | 15 +++++++++++++++
> >  1 file changed, 15 insertions(+)
> > 
> > diff --git a/filters/tls-fallback/tls-fallback.c b/filters/tls-fallback/tls-fallback.c
> > index fab9e58b..b34e0431 100644
> > --- a/filters/tls-fallback/tls-fallback.c
> > +++ b/filters/tls-fallback/tls-fallback.c
> > @@ -138,6 +138,20 @@ tls_fallback_get_size (nbdkit_next *next,
> >    return next->get_size (next);
> >  }
> >  
> > +static int
> > +tls_fallback_block_size (nbdkit_next *next,
> > +                         void *handle,
> > +                         uint32_t *minimum,
> > +                         uint32_t *preferred,
> > +                         uint32_t *maximum)
> > +{
> > +  if (NOT_TLS) {
> > +    *minimum = *preferred = *maximum = 0;
> > +    return 0;
> > +  }
> > +  return next->block_size (next, minimum, preferred, maximum);
> > +}
> > +
> >  static int
> >  tls_fallback_can_write (nbdkit_next *next,
> >                          void *handle)
> > @@ -215,6 +229,7 @@ static struct nbdkit_filter filter = {
> >    .open               = tls_fallback_open,
> >    .export_description = tls_fallback_export_description,
> >    .get_size           = tls_fallback_get_size,
> > +  .block_size         = tls_fallback_block_size,
> >    .can_write          = tls_fallback_can_write,
> >    .can_flush          = tls_fallback_can_flush,
> >    .is_rotational      = tls_fallback_is_rotational,
> > 
> 
> This depends on the series
> 
> [Libguestfs] [PATCH nbdkit 0/6] UNFINISHED Advertise block size constraints
> 
> right? (I thought I'd skip reviewing that series until the dust settles
> on the design, between you and Eric).

Yes, that's right.

I could not make this filter fail except when used with the block size
constraints series.

Rich.

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