libbpf: Define BTF_KIND_* constants in btf.h to avoid compilation errors

The btf.h header included with libbpf contains inline helper functions to
check for various BTF kinds. These helpers directly reference the
BTF_KIND_* constants defined in the kernel header, and because the header
file is included in user applications, this happens in the user application
compile units.

This presents a problem if a user application is compiled on a system with
older kernel headers because the constants are not available. To avoid
this, add #defines of the constants directly in btf.h before using them.

Since the kernel header moved to an enum for BTF_KIND_*, the #defines can
shadow the enum values without any errors, so we only need #ifndef guards
for the constants that predates the conversion to enum. We group these so
there's only one guard for groups of values that were added together.

  [0] Closes: https://github.com/libbpf/libbpf/issues/436

Fixes: 223f903e9c83 ("bpf: Rename BTF_KIND_TAG to BTF_KIND_DECL_TAG")
Fixes: 5b84bd10363e ("libbpf: Add support for BTF_KIND_TAG")
Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Link: https://lore.kernel.org/bpf/20220118141327.34231-1-toke@redhat.com

diff --git a/tools/lib/bpf/btf.h b/tools/lib/bpf/btf.h
index 061839f0452558c0f9bf80ec56a86e5f0cef26de..51862fdee850b2f437ceab48b3e677af4d50fc26 100644 (file)
--- a/tools/lib/bpf/btf.h
+++ b/tools/lib/bpf/btf.h
@@ -375,8 +375,28 @@ btf_dump__dump_type_data(struct btf_dump *d, __u32 id,
                         const struct btf_dump_type_data_opts *opts);
 
 /*
- * A set of helpers for easier BTF types handling
+ * A set of helpers for easier BTF types handling.
+ *
+ * The inline functions below rely on constants from the kernel headers which
+ * may not be available for applications including this header file. To avoid
+ * compilation errors, we define all the constants here that were added after
+ * the initial introduction of the BTF_KIND* constants.
  */
+#ifndef BTF_KIND_FUNC
+#define BTF_KIND_FUNC          12      /* Function     */
+#define BTF_KIND_FUNC_PROTO    13      /* Function Proto       */
+#endif
+#ifndef BTF_KIND_VAR
+#define BTF_KIND_VAR           14      /* Variable     */
+#define BTF_KIND_DATASEC       15      /* Section      */
+#endif
+#ifndef BTF_KIND_FLOAT
+#define BTF_KIND_FLOAT         16      /* Floating point       */
+#endif
+/* The kernel header switched to enums, so these two were never #defined */
+#define BTF_KIND_DECL_TAG      17      /* Decl Tag */
+#define BTF_KIND_TYPE_TAG      18      /* Type Tag */
+
 static inline __u16 btf_kind(const struct btf_type *t)
 {
        return BTF_INFO_KIND(t->info);