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

[Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>]

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