libbpf: Make DECLARE_LIBBPF_OPTS macro strictly a variable declaration

LIBBPF_OPTS is implemented as a mix of field declaration and memset
+ assignment. This makes it neither variable declaration nor purely
statements, which is a problem, because you can't mix it with either
other variable declarations nor other function statements, because C90
compiler mode emits warning on mixing all that together.

This patch changes LIBBPF_OPTS into a strictly declaration of variable
and solves this problem, as can be seen in case of bpftool, which
previously would emit compiler warning, if done this way (LIBBPF_OPTS as
part of function variables declaration block).

This patch also renames LIBBPF_OPTS into DECLARE_LIBBPF_OPTS to follow
kernel convention for similar macros more closely.

v1->v2:
- rename LIBBPF_OPTS into DECLARE_LIBBPF_OPTS (Jakub Sitnicki).
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Toke Høiland-Jørgensen <toke@redhat.com>
Link: https://lore.kernel.org/bpf/20191022172100.3281465-1-andriin@fb.com

tools/bpf/bpftool/prog.c

@@ -1091,8 +1091,11 @@ static int do_run(int argc, char **argv)
 
 static int load_with_options(int argc, char **argv, bool first_prog_only)
 {
-	struct bpf_object_load_attr load_attr = { 0 };
 	enum bpf_prog_type common_prog_type = BPF_PROG_TYPE_UNSPEC;
+	DECLARE_LIBBPF_OPTS(bpf_object_open_opts, open_opts,
+		.relaxed_maps = relaxed_maps,
+	);
+	struct bpf_object_load_attr load_attr = { 0 };
 	enum bpf_attach_type expected_attach_type;
 	struct map_replace *map_replace = NULL;
 	struct bpf_program *prog = NULL, *pos;
@@ -1106,9 +1109,6 @@ static int load_with_options(int argc, char **argv, bool first_prog_only)
 	const char *file;
 	int idx, err;
 
-	LIBBPF_OPTS(bpf_object_open_opts, open_opts,
-		.relaxed_maps = relaxed_maps,
-	);
 
 	if (!REQ_ARGS(2))
 		return -1;

tools/lib/bpf/libbpf.c

@@ -3678,7 +3678,7 @@ __bpf_object__open(const char *path, const void *obj_buf, size_t obj_buf_sz,
 static struct bpf_object *
 __bpf_object__open_xattr(struct bpf_object_open_attr *attr, int flags)
 {
-	LIBBPF_OPTS(bpf_object_open_opts, opts,
+	DECLARE_LIBBPF_OPTS(bpf_object_open_opts, opts,
 		.relaxed_maps = flags & MAPS_RELAX_COMPAT,
 	);
 
@@ -3730,7 +3730,7 @@ struct bpf_object *
 bpf_object__open_buffer(const void *obj_buf, size_t obj_buf_sz,
 			const char *name)
 {
-	LIBBPF_OPTS(bpf_object_open_opts, opts,
+	DECLARE_LIBBPF_OPTS(bpf_object_open_opts, opts,
 		.object_name = name,
 		/* wrong default, but backwards-compatible */
 		.relaxed_maps = true,

tools/lib/bpf/libbpf.h

@@ -75,14 +75,19 @@ struct bpf_object_open_attr {
  * have all the padding bytes initialized to zero. It's not guaranteed though,
  * when copying literal, that compiler won't copy garbage in literal's padding
  * bytes, but that's the best way I've found and it seems to work in practice.
+ *
+ * Macro declares opts struct of given type and name, zero-initializes,
+ * including any extra padding, it with memset() and then assigns initial
+ * values provided by users in struct initializer-syntax as varargs.
  */
-#define LIBBPF_OPTS(TYPE, NAME, ...)					    \
-	struct TYPE NAME;						    \
+#define DECLARE_LIBBPF_OPTS(TYPE, NAME, ...)				    \
+	struct TYPE NAME = ({ 						    \
 		memset(&NAME, 0, sizeof(struct TYPE));			    \
-	NAME = (struct TYPE) {						    \
+		(struct TYPE) {						    \
 			.sz = sizeof(struct TYPE),			    \
 			__VA_ARGS__					    \
-	}
+		};							    \
+	})
 
 struct bpf_object_open_opts {
 	/* size of this struct, for forward/backward compatiblity */

tools/testing/selftests/bpf/prog_tests/attach_probe.c

@@ -50,7 +50,7 @@ void test_attach_probe(void)
 	const int kprobe_idx = 0, kretprobe_idx = 1;
 	const int uprobe_idx = 2, uretprobe_idx = 3;
 	const char *obj_name = "attach_probe";
-	LIBBPF_OPTS(bpf_object_open_opts, open_opts,
+	DECLARE_LIBBPF_OPTS(bpf_object_open_opts, open_opts,
 		.object_name = obj_name,
 		.relaxed_maps = true,
 	);

tools/testing/selftests/bpf/prog_tests/core_reloc.c

@@ -377,7 +377,7 @@ void test_core_reloc(void)
 		if (!test__start_subtest(test_case->case_name))
 			continue;
 
-		LIBBPF_OPTS(bpf_object_open_opts, opts,
+		DECLARE_LIBBPF_OPTS(bpf_object_open_opts, opts,
 			.relaxed_core_relocs = test_case->relaxed_core_relocs,
 		);
 

tools/testing/selftests/bpf/prog_tests/reference_tracking.c

@@ -5,7 +5,7 @@ void test_reference_tracking(void)
 {
 	const char *file = "test_sk_lookup_kern.o";
 	const char *obj_name = "ref_track";
-	LIBBPF_OPTS(bpf_object_open_opts, open_opts,
+	DECLARE_LIBBPF_OPTS(bpf_object_open_opts, open_opts,
 		.object_name = obj_name,
 		.relaxed_maps = true,
 	);