Re: [Qemu-devel] [PATCH for-2.6] nbd: Don't fail handshake on NBD_OPT_LIST descriptions

[Eric Blake]

On 04/14/2016 03:31 PM, Max Reitz wrote:
> On 08.04.2016 03:09, Eric Blake wrote:
>> The NBD Protocol states that NBD_REP_SERVER may set
>> 'length > sizeof(namelen) + namelen'; in which case the rest
>> of the packet is a UTF-8 description of the export.  While we
>> don't know of any NBD servers that send this description yet,
>> we had better consume the data so we don't choke when we start
>> to talk to such a server.
>>

>> @@ -214,6 +219,20 @@ static int nbd_receive_list(QIOChannel *ioc, char 
>> **name, Error **errp)
>>              return -1;
>>          }
>>          (*name)[namelen] = '\0';
>> +        len -= namelen;
>> +        if (len) {
>> +            char *buf = g_malloc(len + 1);
>> +            if (read_sync(ioc, buf, len) != len) {
>> +                error_setg(errp, "failed to read export description");
>> +                g_free(*name);
>> +                g_free(buf);
>> +                *name = NULL;
>> +                return -1;
>> +            }
>> +            buf[len] = '\0';
>> +            TRACE("Ignoring export description: %s", buf);
> 
> I find this funny, somehow.
> 
> Perhaps it's because this may explicitly print something while
> explaining that it's being ignored.

The server.c code had a nice function for skipping unwanted bytes; and
in a 2.7 series, I borrowed that idea:

https://lists.gnu.org/archive/html/qemu-devel/2016-04/msg01597.html

but for the purpose of minimal churn in 2.6, I don't mind the
(temporary) oddity.

> 
>> +            g_free(buf);
>> +        }
>>      } else {
>>          error_setg(errp, "Unexpected reply type %x expected %x",
>>                     type, NBD_REP_SERVER);
>>
> 
> Thanks Eric, I applied this patch to my block branch (for 2.6). If this
> was not your intention, please speak up. :-)
> 
> https://github.com/XanClic/qemu/commits/block

You did the right thing.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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