Re: [PATCH net-next v2 2/5] udp_tunnel: fix compile warning

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

Paolo Abeni wrote:
> Nathan reported that the compiler is not happy to use a zero
> size udp_tunnel_gro_types array:
> 
>   net/ipv4/udp_offload.c:130:8: warning: array index 0 is past the end of the array (that has type 'struct udp_tunnel_type_entry[0]') [-Warray-bounds]
>     130 |                                    udp_tunnel_gro_types[0].gro_receive);
>         |                                    ^                    ~
>   include/linux/static_call.h:154:42: note: expanded from macro 'static_call_update'
>     154 |         typeof(&STATIC_CALL_TRAMP(name)) __F = (func);                  \
>         |                                                 ^~~~
>   net/ipv4/udp_offload.c:47:1: note: array 'udp_tunnel_gro_types' declared here
>      47 | static struct udp_tunnel_type_entry udp_tunnel_gro_types[UDP_MAX_TUNNEL_TYPES];
>         | ^
>   1 warning generated.
> 
> In such (unusual) configuration we should skip entirely the
> static call optimization accounting.
> 
> Reported-by: syzbot+8c469a2260132cd095c1@syzkaller.appspotmail.com
> Closes: https://lore.kernel.org/netdev/cover.1741718157.git.pabeni@redhat.com/T/#m6e309a49f04330de81a618c3c166368252ba42e4
> Fixes: 311b36574ceac ("udp_tunnel: use static call for GRO hooks when possible")
> Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Should CONFIG_NET_UDP_TUNNEL just not be user selectable and only
enabled by implementations of UDP tunnels like vxlan and geneve?

> ---
>  net/ipv4/udp_offload.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/net/ipv4/udp_offload.c b/net/ipv4/udp_offload.c
> index 02365b818f1af..fd2b8e3830beb 100644
> --- a/net/ipv4/udp_offload.c
> +++ b/net/ipv4/udp_offload.c
> @@ -83,7 +83,7 @@ void udp_tunnel_update_gro_rcv(struct sock *sk, bool add)
>  	struct udp_sock *up = udp_sk(sk);
>  	int i, old_gro_type_nr;
>  
> -	if (!up->gro_receive)
> +	if (!UDP_MAX_TUNNEL_TYPES || !up->gro_receive)
>  		return;

If that is too risky, I suppose this workaround is sufficient.
But having a zero length array seems a bit odd.