libbpf: Suppress compiler warning when using SEC() macro with externs

When used on externs SEC() macro will trigger compilation warning about
inapplicable `__attribute__((used))`. That's expected for extern declarations,
so suppress it with the corresponding _Pragma.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20210423181348.1801389-4-andrii@kernel.org

diff --git a/tools/lib/bpf/bpf_helpers.h b/tools/lib/bpf/bpf_helpers.h
index b904128626c2a8a9c284ffe0dd9d82247aed0be8..75c7581b304c8168d779b705da6ce27a72a4217a 100644 (file)
--- a/tools/lib/bpf/bpf_helpers.h
+++ b/tools/lib/bpf/bpf_helpers.h
@@ -25,9 +25,16 @@
 /*
  * Helper macro to place programs, maps, license in
  * different sections in elf_bpf file. Section names
- * are interpreted by elf_bpf loader
+ * are interpreted by libbpf depending on the context (BPF programs, BPF maps,
+ * extern variables, etc).
+ * To allow use of SEC() with externs (e.g., for extern .maps declarations),
+ * make sure __attribute__((unused)) doesn't trigger compilation warning.
  */
-#define SEC(NAME) __attribute__((section(NAME), used))
+#define SEC(name) \
+       _Pragma("GCC diagnostic push")                                      \
+       _Pragma("GCC diagnostic ignored \"-Wignored-attributes\"")          \
+       __attribute__((section(name), used))                                \
+       _Pragma("GCC diagnostic pop")                                       \
 
 /* Avoid 'linux/stddef.h' definition of '__always_inline'. */
 #undef __always_inline