bpftool: Enable libbpf's strict mode by default

Otherwise, attaching with bpftool doesn't work with strict section names.

Also:

- Add --legacy option to switch back to pre-1.0 behavior
- Print a warning when program fails to load in strict mode to
point to --legacy flag
- By default, don't append / to the section name; in strict
mode it's relevant only for a small subset of prog types

+ bpftool --legacy prog loadall tools/testing/selftests/bpf/test_cgroup_link.o /sys/fs/bpf/kprobe type kprobe
libbpf: failed to pin program: File exists
Error: failed to pin all programs
+ bpftool prog loadall tools/testing/selftests/bpf/test_cgroup_link.o /sys/fs/bpf/kprobe type kprobe

v1 -> v2:
- strict by default (Quentin Monnet)
- add more info to --legacy description (Quentin Monnet)
- add bash completion (Quentin Monnet)

Signed-off-by: Stanislav Fomichev <[e-mail]>
Signed-off-by: Daniel Borkmann <[e-mail]>
Reviewed-by: Quentin Monnet <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

tools/bpf/bpftool/Documentation/common_options.rst

···
 	  Print all logs available, even debug-level information. This includes
 	  logs from libbpf as well as from the verifier, when attempting to
 	  load programs.
+
+-l, --legacy
+	  Use legacy libbpf mode which has more relaxed BPF program
+	  requirements. By default, bpftool has more strict requirements
+	  about section names, changes pinning logic and doesn't support
+	  some of the older non-BTF map declarations.
+
+	  See https://github.com/libbpf/libbpf/wiki/Libbpf:-the-road-to-v1.0
+	  for details.

tools/bpf/bpftool/bash-completion/bpftool

···
     # Deal with options
     if [[ ${words[cword]} == -* ]]; then
         local c='--version --json --pretty --bpffs --mapcompat --debug \
-	       --use-loader --base-btf'
+	       --use-loader --base-btf --legacy'
         COMPREPLY=( $( compgen -W "$c" -- "$cur" ) )
         return 0
     fi

tools/bpf/bpftool/main.c

···
 bool verifier_logs;
 bool relaxed_maps;
 bool use_loader;
+bool legacy_libbpf;
 struct btf *base_btf;
 struct hashmap *refs_table;
 
···
 		{ "debug",	no_argument,	NULL,	'd' },
 		{ "use-loader",	no_argument,	NULL,	'L' },
 		{ "base-btf",	required_argument, NULL, 'B' },
+		{ "legacy",	no_argument,	NULL,	'l' },
 		{ 0 }
 	};
 	int opt, ret;
···
 	bin_name = argv[0];
 
 	opterr = 0;
-	while ((opt = getopt_long(argc, argv, "VhpjfLmndB:",
+	while ((opt = getopt_long(argc, argv, "VhpjfLmndB:l",
 				  options, NULL)) >= 0) {
 		switch (opt) {
 		case 'V':
···
 		case 'L':
 			use_loader = true;
 			break;
+		case 'l':
+			legacy_libbpf = true;
+			break;
 		default:
 			p_err("unrecognized option '%s'", argv[optind - 1]);
 			if (json_output)
···
 			else
 				usage();
 		}
+	}
+
+	if (!legacy_libbpf) {
+		ret = libbpf_set_strict_mode(LIBBPF_STRICT_ALL);
+		if (ret)
+			p_err("failed to enable libbpf strict mode: %d", ret);
 	}
 
 	argc -= optind;

tools/bpf/bpftool/main.h

···
 #define HELP_SPEC_PROGRAM						\
 	"PROG := { id PROG_ID | pinned FILE | tag PROG_TAG | name PROG_NAME }"
 #define HELP_SPEC_OPTIONS						\
-	"OPTIONS := { {-j|--json} [{-p|--pretty}] | {-d|--debug}"
+	"OPTIONS := { {-j|--json} [{-p|--pretty}] | {-d|--debug} | {-l|--legacy}"
 #define HELP_SPEC_MAP							\
 	"MAP := { id MAP_ID | pinned FILE | name MAP_NAME }"
 #define HELP_SPEC_LINK							\
···
 extern bool verifier_logs;
 extern bool relaxed_maps;
 extern bool use_loader;
+extern bool legacy_libbpf;
 extern struct btf *base_btf;
 extern struct hashmap *refs_table;
 

tools/bpf/bpftool/prog.c

···
 
 	while (argc) {
 		if (is_prefix(*argv, "type")) {
-			char *type;
-
 			NEXT_ARG();
 
 			if (common_prog_type != BPF_PROG_TYPE_UNSPEC) {
···
 			if (!REQ_ARGS(1))
 				goto err_free_reuse_maps;
 
-			/* Put a '/' at the end of type to appease libbpf */
-			type = malloc(strlen(*argv) + 2);
-			if (!type) {
-				p_err("mem alloc failed");
-				goto err_free_reuse_maps;
-			}
-			*type = 0;
-			strcat(type, *argv);
-			strcat(type, "/");
+			err = libbpf_prog_type_by_name(*argv, &common_prog_type,
+						       &expected_attach_type);
+			if (err < 0) {
+				/* Put a '/' at the end of type to appease libbpf */
+				char *type = malloc(strlen(*argv) + 2);
 
-			err = get_prog_type_by_name(type, &common_prog_type,
-						    &expected_attach_type);
-			free(type);
-			if (err < 0)
-				goto err_free_reuse_maps;
+				if (!type) {
+					p_err("mem alloc failed");
+					goto err_free_reuse_maps;
+				}
+				*type = 0;
+				strcat(type, *argv);
+				strcat(type, "/");
+
+				err = get_prog_type_by_name(type, &common_prog_type,
+							    &expected_attach_type);
+				free(type);
+				if (err < 0)
+					goto err_free_reuse_maps;
+			}
 
 			NEXT_ARG();
 		} else if (is_prefix(*argv, "map")) {
···
 	else
 		bpf_object__unpin_programs(obj, pinfile);
 err_close_obj:
+	if (!legacy_libbpf) {
+		p_info("Warning: bpftool is now running in libbpf strict mode and has more stringent requirements about BPF programs.\n"
+		       "If it used to work for this object file but now doesn't, see --legacy option for more details.\n");
+	}
+
 	bpf_object__close(obj);
 err_free_reuse_maps:
 	for (i = 0; i < old_map_fds; i++)