[Libguestfs] [nbdkit PATCH v2 5/7] server: Allow longer NBD_OPT

[Eric Blake]

On 9/30/19 11:30 AM, Eric Blake wrote:

>>>   /* Maximum length of any option data (bytes). */
>>> -#define MAX_OPTION_LENGTH 4096
>>> +#define MAX_OPTION_LENGTH (NBD_MAX_STRING * 4)
>>
>> I may have missed it - why was * 4 chosen?
> 
> NBD_OPT_SET_META_CONTEXT allows two strings plus a few glue bytes, so 
> more than 8k of data from a compliant client.  16k is the next power of 
> 2.  We can bump it larger if we want, especially since 16k pales in 
> comparison to our 32M limit on NBD_CMD_WRITE, but for now, there is 
> nothing in the NBD protocol larger than NBD_OPT_SET_META_CONTEXT.

Actually, having just written that, I now realize that 
NBD_OPT_SET_META_CONTEXT allows you to request more than one context at 
a time.  It's very easy to provoke a request larger than 16k by 
requesting 3 contexts at once (nbdsh can do so), even if we only ever 
respond to a single recognized context.  So I should probably just go 
whole-hog and cap this at the same limit as NBD_CMD_WRITE, rather than 
having two independent limits.  And I probably ought to beef up the 
testsuite to actually demonstrate nbdsh provoking that large of an option.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org