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

[Sabrina Dubroca <sd@queasysnail.net>]

2025-03-21, 12:35:34 -0400, Willem de Bruijn wrote:
> 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?

It's already not user selectable?

config NET_UDP_TUNNEL
	tristate

(no string after bool/tristate, so no manual config)

But there are tunnels that don't do GRO, so they're not counted in
UDP_MAX_TUNNEL_TYPES, and if only those types are enabled, we'll have
CONFIG_NET_UDP_TUNNEL=y with UDP_MAX_TUNNEL_TYPES == 0.


> > ---
> >  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.

I think the alternative would be to add

    #if UDP_MAX_TUNNEL_TYPES > 0

around some of this code.

-- 
Sabrina