[PATCH bpf-next v2] libbpf: fix compilation errors on ubuntu 16.04

[Yonghong Song <yhs@xxxxxx>]

libbpf is used as a submodule in bcc.
When importing latest libbpf repo in bcc, I observed the
following compilation errors when compiling on ubuntu 16.04.
  .../netlink.c:416:23: error: ‘TC_H_CLSACT’ undeclared (first use in this function)
     *parent = TC_H_MAKE(TC_H_CLSACT,
                         ^
  .../netlink.c:418:9: error: ‘TC_H_MIN_INGRESS’ undeclared (first use in this function)
           TC_H_MIN_INGRESS : TC_H_MIN_EGRESS);
           ^
  .../netlink.c:418:28: error: ‘TC_H_MIN_EGRESS’ undeclared (first use in this function)
           TC_H_MIN_INGRESS : TC_H_MIN_EGRESS);
                              ^
  .../netlink.c: In function ‘__get_tc_info’:
  .../netlink.c:522:11: error: ‘TCA_BPF_ID’ undeclared (first use in this function)
    if (!tbb[TCA_BPF_ID])
             ^

In ubuntu 16.04, TCA_BPF_* enumerator looks like below
  enum {
	TCA_BPF_UNSPEC,
	TCA_BPF_ACT,
	...
	TCA_BPF_NAME,
	TCA_BPF_FLAGS,
	__TCA_BPF_MAX,
  };
  #define TCA_BPF_MAX	(__TCA_BPF_MAX - 1)
while in latest bpf-next, the enumerator looks like
  enum {
	TCA_BPF_UNSPEC,
	...
	TCA_BPF_FLAGS,
	TCA_BPF_FLAGS_GEN,
	TCA_BPF_TAG,
	TCA_BPF_ID,
	__TCA_BPF_MAX,
  };

In this patch, TCA_BPF_ID is defined as a macro with proper value and this
works regardless of whether TCA_BPF_ID is defined in uapi header or not.

I also added a comparison "TCA_BPF_MAX < TCA_BPF_ID" in function __get_tc_info()
such that if the compare result if true, returns -EOPNOTSUPP. This is used to
prevent otherwise array overflows:
  .../netlink.c:538:10: warning: array subscript is above array bounds [-Warray-bounds]
    if (!tbb[TCA_BPF_ID])
            ^

Fixes: 715c5ce454a6 ("libbpf: Add low level TC-BPF management API")
Cc: Kumar Kartikeya Dwivedi <memxor@xxxxxxxxx>
Signed-off-by: Yonghong Song <yhs@xxxxxx>
---
 tools/lib/bpf/netlink.c | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

Changelog:
  v1 -> v2:
    - gcc 8.3 doesn't like macro condition
        (__TCA_BPF_MAX - 1) <= 10
      where __TCA_BPF_MAX is an enumerator value.
      So define TCA_BPF_ID macro without macro condition.

diff --git a/tools/lib/bpf/netlink.c b/tools/lib/bpf/netlink.c
index 39f25e09b51e..e00660e0b87a 100644
--- a/tools/lib/bpf/netlink.c
+++ b/tools/lib/bpf/netlink.c
@@ -22,6 +22,24 @@
 #define SOL_NETLINK 270
 #endif
 
+#ifndef TC_H_CLSACT
+#define TC_H_CLSACT TC_H_INGRESS
+#endif
+
+#ifndef TC_H_MIN_INGRESS
+#define TC_H_MIN_INGRESS 0xFFF2U
+#endif
+
+#ifndef TC_H_MIN_EGRESS
+#define TC_H_MIN_EGRESS 0xFFF3U
+#endif
+
+/* TCA_BPF_ID is an enumerate value in uapi/linux/pkt_cls.h.
+ * Declare it as a macro here so old system can still work
+ * without TCA_BPF_ID defined in pkt_cls.h.
+ */
+#define TCA_BPF_ID 11
+
 typedef int (*libbpf_dump_nlmsg_t)(void *cookie, void *msg, struct nlattr **tb);
 
 typedef int (*__dump_nlmsg_t)(struct nlmsghdr *nlmsg, libbpf_dump_nlmsg_t,
@@ -504,6 +522,8 @@ static int __get_tc_info(void *cookie, struct tcmsg *tc, struct nlattr **tb,
 		return -EINVAL;
 	if (!tb[TCA_OPTIONS])
 		return NL_CONT;
+	if (TCA_BPF_MAX < TCA_BPF_ID)
+		return -EOPNOTSUPP;
 
 	libbpf_nla_parse_nested(tbb, TCA_BPF_MAX, tb[TCA_OPTIONS], NULL);
 	if (!tbb[TCA_BPF_ID])
-- 
2.30.2