tools: bpftool: Print optional built-in features along with version

Bpftool has a number of features that can be included or left aside
during compilation. This includes:

- Support for libbfd, providing the disassembler for JIT-compiled
  programs.
- Support for BPF skeletons, used for profiling programs or iterating on
  the PIDs of processes associated with BPF objects.

In order to make it easy for users to understand what features were
compiled for a given bpftool binary, print the status of the two
features above when showing the version number for bpftool ("bpftool -V"
or "bpftool version"). Document this in the main manual page. Example
invocations:

    $ bpftool version
    ./bpftool v5.9.0-rc1
    features: libbfd, skeletons

    $ bpftool -p version
    {
        "version": "5.9.0-rc1",
        "features": {
            "libbfd": true,
            "skeletons": true
        }
    }

Some other parameters are optional at compilation
("DISASM_FOUR_ARGS_SIGNATURE", LIBCAP support) but they do not impact
significantly bpftool's behaviour from a user's point of view, so their
status is not reported.

Available commands and supported program types depend on the version
number, and are therefore not reported either. Note that they are
already available, albeit without JSON, via bpftool's help messages.

v3:
- Use a simple list instead of boolean values for plain output.

v2:
- Fix JSON (object instead or array for the features).

Signed-off-by: Quentin Monnet <quentin@isovalent.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/20200909162500.17010-2-quentin@isovalent.com

tools/bpf/bpftool/Documentation/bpftool.rst

@@ -50,7 +50,13 @@ OPTIONS
 		  Print short help message (similar to **bpftool help**).
 
 	-V, --version
-		  Print version number (similar to **bpftool version**).
+		  Print version number (similar to **bpftool version**), and
+		  optional features that were included when bpftool was
+		  compiled. Optional features include linking against libbfd to
+		  provide the disassembler for JIT-ted programs (**bpftool prog
+		  dump jited**) and usage of BPF skeletons (some features like
+		  **bpftool prog profile** or showing pids associated to BPF
+		  objects may rely on it).
 
 	-j, --json
 		  Generate JSON output. For commands that cannot produce JSON, this

tools/bpf/bpftool/main.c

@@ -70,13 +70,42 @@ static int do_help(int argc, char **argv)
 
 static int do_version(int argc, char **argv)
 {
+#ifdef HAVE_LIBBFD_SUPPORT
+	const bool has_libbfd = true;
+#else
+	const bool has_libbfd = false;
+#endif
+#ifdef BPFTOOL_WITHOUT_SKELETONS
+	const bool has_skeletons = false;
+#else
+	const bool has_skeletons = true;
+#endif
+
 	if (json_output) {
-		jsonw_start_object(json_wtr);
+		jsonw_start_object(json_wtr);	/* root object */
+
 		jsonw_name(json_wtr, "version");
 		jsonw_printf(json_wtr, "\"%s\"", BPFTOOL_VERSION);
-		jsonw_end_object(json_wtr);
+
+		jsonw_name(json_wtr, "features");
+		jsonw_start_object(json_wtr);	/* features */
+		jsonw_bool_field(json_wtr, "libbfd", has_libbfd);
+		jsonw_bool_field(json_wtr, "skeletons", has_skeletons);
+		jsonw_end_object(json_wtr);	/* features */
+
+		jsonw_end_object(json_wtr);	/* root object */
 	} else {
+		unsigned int nb_features = 0;
+
 		printf("%s v%s\n", bin_name, BPFTOOL_VERSION);
+		printf("features:");
+		if (has_libbfd) {
+			printf(" libbfd");
+			nb_features++;
+		}
+		if (has_skeletons)
+			printf("%s skeletons", nb_features++ ? "," : "");
+		printf("\n");
 	}
 	return 0;
 }