Subject: Re: [PATCH net-next v2] netlink: split up copies in the ack construction

From: Jakub Kicinski <[email-masked]>
Date: Mon, 14 Nov 2022 09:06:14 -0800

On Sun, 13 Nov 2022 19:39:27 -0700 David Ahern wrote:
> On Thu, Oct 27, 2022 at 02:25:53PM -0700, Jakub Kicinski wrote:
> > diff --git a/include/uapi/linux/netlink.h b/include/uapi/linux/netlink.h
> > index e2ae82e3f9f7..5da0da59bf01 100644
> > --- a/include/uapi/linux/netlink.h
> > +++ b/include/uapi/linux/netlink.h
> > @@ -48,6 +48,7 @@ struct sockaddr_nl {
> >   * @nlmsg_flags: Additional flags
> >   * @nlmsg_seq:   Sequence number
> >   * @nlmsg_pid:   Sending process port ID
> > + * @nlmsg_data:  Message payload
> >   */
> >  struct nlmsghdr {
> >  	__u32		nlmsg_len;
> > @@ -55,6 +56,7 @@ struct nlmsghdr {
> >  	__u16		nlmsg_flags;
> >  	__u32		nlmsg_seq;
> >  	__u32		nlmsg_pid;
> > +	__u8		nlmsg_data[];  
> 
> This breaks compile of iproute2 with clang. It does not like the
> variable length array in the middle of a struct. While I could re-do the
> structs in iproute2, I doubt it is alone in being affected by this
> change.

Kees, would you mind lending your expertise?

Not sure why something like (simplified):

struct top {
	struct nlmsghdr hdr;
	int tail;
}; 

generates a warning:

In file included from stat-mr.c:7:
In file included from ./res.h:9:
In file included from ./rdma.h:21:
In file included from ../include/utils.h:17:
../include/libnetlink.h:41:18: warning: field 'nlh' with variable sized type 'struct nlmsghdr' not at the end of a struct or class is a GNU extension [-Wgnu-variable-sized-type-not-at-end]
        struct nlmsghdr nlh;
                        ^

which is not confined to -Wpedantic. 
Seems like a useless warning :S