flow_dissector: fix build failure without CONFIG_NET

If boolean CONFIG_BPF_SYSCALL is enabled, kernel/bpf/syscall.c will
call flow_dissector functions from net/core/flow_dissector.c.

This causes this build failure if CONFIG_NET is disabled:

    kernel/bpf/syscall.o: In function `__x64_sys_bpf':
    syscall.c:(.text+0x3278): undefined reference to
    `skb_flow_dissector_bpf_prog_attach'
    syscall.c:(.text+0x3310): undefined reference to
    `skb_flow_dissector_bpf_prog_detach'
    kernel/bpf/syscall.o:(.rodata+0x3f0): undefined reference to
    `flow_dissector_prog_ops'
    kernel/bpf/verifier.o:(.rodata+0x250): undefined reference to
    `flow_dissector_verifier_ops'

Analogous to other optional BPF program types in syscall.c, add stubs
if the relevant functions are not compiled and move the BPF_PROG_TYPE
definition in the #ifdef CONFIG_NET block.

Fixes: d58e468b1112 ("flow_dissector: implements flow dissector BPF hook")
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Willem de Bruijn <willemb@google.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org> # build-tested
Acked-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

include/linux/bpf_types.h

@@ -16,6 +16,7 @@ BPF_PROG_TYPE(BPF_PROG_TYPE_LWT_SEG6LOCAL, lwt_seg6local)
 BPF_PROG_TYPE(BPF_PROG_TYPE_SOCK_OPS, sock_ops)
 BPF_PROG_TYPE(BPF_PROG_TYPE_SK_SKB, sk_skb)
 BPF_PROG_TYPE(BPF_PROG_TYPE_SK_MSG, sk_msg)
+BPF_PROG_TYPE(BPF_PROG_TYPE_FLOW_DISSECTOR, flow_dissector)
 #endif
 #ifdef CONFIG_BPF_EVENTS
 BPF_PROG_TYPE(BPF_PROG_TYPE_KPROBE, kprobe)
@@ -32,7 +33,6 @@ BPF_PROG_TYPE(BPF_PROG_TYPE_LIRC_MODE2, lirc_mode2)
 #ifdef CONFIG_INET
 BPF_PROG_TYPE(BPF_PROG_TYPE_SK_REUSEPORT, sk_reuseport)
 #endif
-BPF_PROG_TYPE(BPF_PROG_TYPE_FLOW_DISSECTOR, flow_dissector)
 
 BPF_MAP_TYPE(BPF_MAP_TYPE_ARRAY, array_map_ops)
 BPF_MAP_TYPE(BPF_MAP_TYPE_PERCPU_ARRAY, percpu_array_map_ops)

include/linux/skbuff.h

@@ -1194,10 +1194,23 @@ void skb_flow_dissector_init(struct flow_dissector *flow_dissector,
 			     const struct flow_dissector_key *key,
 			     unsigned int key_count);
 
+#ifdef CONFIG_NET
 int skb_flow_dissector_bpf_prog_attach(const union bpf_attr *attr,
 				       struct bpf_prog *prog);
 
 int skb_flow_dissector_bpf_prog_detach(const union bpf_attr *attr);
+#else
+static inline int skb_flow_dissector_bpf_prog_attach(const union bpf_attr *attr,
+						     struct bpf_prog *prog)
+{
+	return -EOPNOTSUPP;
+}
+
+static inline int skb_flow_dissector_bpf_prog_detach(const union bpf_attr *attr)
+{
+	return -EOPNOTSUPP;
+}
+#endif
 
 bool __skb_flow_dissect(const struct sk_buff *skb,
 			struct flow_dissector *flow_dissector,