Re: Error reporting in Netlink (Re: Xtables2 Netlink spec)

[Pablo Neira Ayuso <pablo@netfilter.org>]

On 16/12/10 15:19, Thomas Graf wrote:
> On Thu, Dec 16, 2010 at 02:51:07PM +0100, Jan Engelhardt wrote:
>>> Why not use nlattr to encode the error string? It would make error
>>> messages easier to extend in the future. At some point we might want
>>> to add an offset field which points into the original netlink
>>> message describing the attribute which caused the failure.
>>
>> Is that a yes or a no?
> 
> The proposed solution at netconf involved appending the error string
> directly. Inspired by your comment I realizezd that encoding the
> error string as nlattr allow for additional attributes would be a
> better implementation.

Indeed, I'd prefer adding an extra netlink header with a new type like
NLM_ERROR2 followed by attributes like the string (or an int) that
contain the error.

This also can be added with breaking existing apps and libraries IIRC.

> As for size limitations, even though most netlink protocols do it, I
> don't see the point in appending the whole request message in a error
> message. The header would be completely sufficient for all request/reply
> based protocols. It is no problem for userspace to keep a copy of the
> last request sent.

At least, we require the netlink header of the request message in error
messages. This is useful for message batches that are sent to
kernel-space, to know which one triggered the error.