Re: [RFC for-6.2] block/nbd: forbid incompatible change of server options on reconnect

[Vladimir Sementsov-Ogievskiy]

29.11.2021 22:16, Eric Blake wrote:
> On Wed, Nov 24, 2021 at 03:09:51PM +0100, Vladimir Sementsov-Ogievskiy wrote:
> > Reconnect feature was never prepared to handle server options changed
> > on reconnect. Let's be stricter and check what exactly is changed. If
> > server capabilities just got richer don't worry. Otherwise fail and
> > drop the established connection.
> >
> > Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> > ---
> > +    /*
> > +     * No worry if rotational status changed. But other flags are feature 
> > flags,
> > +     * they should not degrade.
> > +     */
> > +    dropped_flags = (old->flags & ~new->flags) & ~NBD_FLAG_ROTATIONAL;
> > +    if (dropped_flags) {
> > +        error_setg(errp, "Server options degrade after reconnect: flags 0x%"
> > +                   PRIx32 " are not reported anymore", dropped_flags);
> > +        return false;
> > +    }
>
> Your logic is good for most flags, but somewhat wrong for
> NBD_FLAG_READ_ONLY_BIT.  For cases where we are only using the block
> device read-only, we don't care about changes of that bit, in either
> direction.  But for cases where we want to use the block device
> read-write, the bit changing from clear in the old to set in the new
> server is an incompatible change that your logic failed to flag.

Oh right! Will fix it and resend soon.

--
Best regards,
Vladimir