introduce compile argument COMPAT_MODE for low version kernel

Rename 'MAP_CONFIG' with 'COMPAT_MODE' and use compatibility mode
for 4.X kernel.

Signed-off-by: Menglong Dong <imagedong@tencent.com>

Author: menglongdong

shared/bpf/skb_utils.h

@@ -41,7 +41,7 @@ struct {
 #define _C(src, a, ...)	BPF_CORE_READ(src, a, ##__VA_ARGS__)
 #endif
 
-#ifdef MAP_CONFIG
+#ifdef COMPAT_MODE
 struct {
 	__uint(type, BPF_MAP_TYPE_ARRAY);
 	__uint(key_size, sizeof(int));

shared/bpf_utils.h

@@ -18,7 +18,7 @@ typedef struct {
 
 extern long int syscall (long int __sysno, ...);
 
-#ifdef MAP_CONFIG
+#ifdef COMPAT_MODE
 #define bpf_set_config(skel, sec, value) do {		\
 	int fd = bpf_map__fd(skel->maps.m_config);	\
 	u8 buf[CONFIG_MAP_SIZE] = {};			\

src/Makefile

@@ -18,7 +18,7 @@ COMPAT ?= $(if $(KERNEL),$(shell cat $(HEADERS)/Makefile | grep \
 	  'VERSION = 4'),$(shell uname -r | grep ^4))
 
 ifneq ($(strip $(COMPAT)),)
-	CFLAGS += -DMAP_CONFIG
+	CFLAGS += -DCOMPAT_MODE
 endif
 
 NFT_HEADER	:= $(HEADERS)/include/net/netfilter/nf_tables.h

src/gen_trace.py

@@ -121,9 +121,10 @@ def gen_group(group, is_root=False):
             else:
                 trace_type = 'TRACE_FUNCTION'
                 if 'skb' in child:
-                    skb_str = '\n\t.skb = {},'.format(child["skb"])
+                    skb_index = int(child["skb"]) + 1
+                    skb_str = '\n\t.skb = {},'.format(skb_index)
                     probe_str += '\tFN({}, {})\t\\\n'.format(name,
-                                                             int(child["skb"]) + 1)
+                                                             skb_index)
             if 'analyzer' in child:
                 analyzer = '\n\t.analyzer = &ANALYZER({}),'.format(
                     child["analyzer"])