[ovs-dev] [PATCH] netlink-socket: Exit NL transaction loop when EINVAL is returned

[Nithin Raju]

> On Apr 14, 2015, at 2:39 PM, Ben Pfaff <blp at nicira.com> wrote:
> 
>> 
>> What is the genetlink semantics for returning an error from the OVS
>> module in Linux. I looked at the OVS code, and it returns -EINVAL if
>> it runs into a condition of invalid input (eg. flow with key & UFID
>> missing). If OVS module returns -EINVAL, does Linux???s netlink
>> layer massage the message to insert the error into a ???struct
>> nlmsgerr???, and return 0?
> 
> The netlink code turns that sort of return value into an
> nlmsghdr+nlmsgerr.
> 
>> We have a little confusion about that. We currently return the error
>> (and also the struct nlmsgerr) in the equivalent of recvmsg().
> 
> Linux would only return an error from recvmsg() if something really
> bad happened.  You can think of the difference between these two types
> of errors as being like a "real" network connection.  Suppose you're
> running a Netlink-like RPC protocol over a TCP socket instead of a
> Netlink socket.  You only get an error from recvmsg() if something
> really bad happens like the TCP connection dropping.  If the remote
> process wants to send you an error reply to your request, you get it
> as a message encapsulated inside the TCP connection data, not as an
> error returned by recvmsg().

Sounds good. Thanks for the clarifications Ben.

In addition to Sorin’s patch, we also need changes in the kernel to return STATUS_SUCCESS, and populate the ‘struct nlmsghdr+nlmsgerr’ appropriately.

thanks,
-- Nithin