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

[Eric Blake]

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?

>  
> -    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.

-- 
Eric Blake   eblake at redhat.com    +1-801-349-2682
Libvirt virtualization library http://libvirt.org

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