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

[Eric Blake]

On 9/28/19 3:07 PM, Richard W.M. Jones wrote:
> On Fri, Sep 27, 2019 at 11:48:47PM -0500, Eric Blake wrote:
>> Fixes the fact that clients could not request the maximum string
>> length except with NBD_OPT_EXPORT_LEN.  Updates the testsuite to
>> match.
>>
>> Signed-off-by: Eric Blake <eblake at redhat.com>
>> ---
>>   server/protocol-handshake-newstyle.c | 12 +++++++-----
>>   tests/test-long-name.sh              | 10 ++++------
>>   2 files changed, 11 insertions(+), 11 deletions(-)
>>
>> diff --git a/server/protocol-handshake-newstyle.c b/server/protocol-handshake-newstyle.c
>> index 34958360..3b5d144e 100644
>> --- a/server/protocol-handshake-newstyle.c
>> +++ b/server/protocol-handshake-newstyle.c
>> @@ -48,7 +48,7 @@
>>   #define MAX_NR_OPTIONS 32
>>
>>   /* 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.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
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