[Libguestfs] [PATCH nbdkit 1/6] server: Add new plugin/filter .block_size callback

[Eric Blake]

On Wed, Feb 16, 2022 at 11:16:49AM -0600, Eric Blake wrote:

> > +int
> > +backend_block_size (struct context *c,
> > +                    uint32_t *minimum, uint32_t *preferred, uint32_t *maximum)
> > +{

> > +    r = b->block_size (c, minimum, preferred, maximum);
> > +    if (r == 0) {
> > +      c->minimum_block_size = *minimum;
> > +      c->preferred_block_size = *preferred;
> > +      c->maximum_block_size = *maximum;
> > +    }
> 
> We should probably ensure that NBD protocol constraints are met rather
> than just assuming the plugin gave us sane values: minimum must be
> power of 2 between 1 and 64k; preferred must be power of 2 between
> max(minsize,512) and 32M; maximum must be either -1 or a multiple of
> minsize (but not necessarily a power of 2).
> 
> /me reads on...
> 
> > +++ b/server/plugins.c
> >  
> > +static int
> > +plugin_block_size (struct context *c,
> > +                   uint32_t *minimum, uint32_t *preferred, uint32_t *maximum)
> > +{
> > +  struct backend *b = c->b;
> > +  struct backend_plugin *p = container_of (b, struct backend_plugin, backend);
> > +  int r;
> > +
> > +  if (p->plugin.block_size) {
> > +    r = p->plugin.block_size (c->handle, minimum, preferred, maximum);
> > +    if (r == 0) {
> > +      /* To make scripting easier, it's permitted to set
> > +       * minimum = preferred = maximum = 0 and return 0.
> > +       * That means "no information", and works the same
> > +       * way as the else clause below.
> > +       */
> > +      if (*minimum == 0 && *preferred == 0 && *maximum == 0)
> > +        return 0;
> > +
> > +      if (*minimum < 1) {
> > +        nbdkit_error ("plugin must set minimum block size >= 1");
> > +        r = -1;
> > +      }
> 
> In other words, either all three values are 0 (no info), or all three
> values are non-zero, ruling out partial info.  Makes sense.  We could
> instead decide to provide defaults to let plugins provide partial info
> (such as if minsize is nonzero but preferred is 0, then set preferred
> to min(minsize, 4k), but I don't know if it would be worth the extra
> complication.

...and then failed to complete my thought.  Okay, so instead of
validating that parameters are sane at the backend level, you only
enforce them to be sane at the plugin level (since all filters are
in-tree, we have a bit more control there).  Seems like a reasonable
tradeoff, although I'm still a bit worried that not checking in the
backend exposes us to a little more risk of writing a bad in-tree
filter.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
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