[libvirt] [PATCH] network: truncate bridges' dummy tap device names to IFNAMSIZ (15) chars

[Laine Stump]

On 04/13/2011 03:24 PM, Eric Blake wrote:
> On 04/13/2011 11:55 AM, Laine Stump wrote:
>> The solution is to shorten the part of the original name used to
>> generate the tap device name. However, simply truncating it is
>> insufficient, because the last few characters of an interface name are
>> often a number used to indicate one of a list of several similar
>> devices (for example, "verylongname123", "verylongname124", etc) and
>> simple truncation would lead to duplicate names. So instead we take
>> the first 8 characters of $bridgename ("verylong" in the example), add
>> on the final 3 bytes ("123"), then add "-nic" (so "verylong123-nic").
>> Not pretty, but it is much more likely to generate a unique name, and
>> is reproducible (unlike, say, a random number).
> Should we also minimize the truncation by adding just "-n" instead of
> "-nic", so that there are fewer user strings being butchered?  Or would
> that cause problems for existing users that already have bridge-nic and
> would now also have bridge-n?

Well, for someone who upgraded with a network that was still up, then 
restarted the network without rebooting, it would leave around an extra 
bridge-nic tap interface with the same MAC address as the new bridge-n 
interface. But that interface would be down (which also means it 
wouldn't show up in the ifconfig output unless you added "-a"), not 
connected to anything, and would disappear completely the next time the 
system was rebooted. (Also, I just checked to verify that an extra tap 
device with matching MAC address doesn't mess with proper networking of 
the guests, and it doesn't.)

So, is this minor artifact worth getting the extra two characters? I'll 
let the patch sit until midday, and if someone comes out in support of 
changing it, I'll reduce the suffix to "-n" and push, otherwise I'll 
push as-is.

>> -    virAsprintf(&nicname, "%s-nic", brname);
>> +    if (strlen(brname)>  (IFNAMSIZ - 5)) {
>> +        /* because the length of an ifname is limited to IFNAMSIZ-1
>> +         * (usually 15), and we're adding 4 more characters, we must
>> +         * truncate the original name to 11 to fit. In order to catch
>> +         * a possible numeric ending (eg virbr0, virbr1, etc), we grab
>> +         * the first 8 and last 3 characters of the string.
>> +         */
>> +         virAsprintf(&nicname, "%.*s%s-nic",
>> +                     IFNAMSIZ - 8, /* space for last 3 chars + "-nic" + NULL */
>> +                     brname, brname + strlen(brname) - 3);
>> +    } else {
>> +        virAsprintf(&nicname, "%s-nic", brname);
>> +    }
> ACK.
>