bpftool: Add libbpf's version number to "bpftool version" output

To help users check what version of libbpf is being used with bpftool,
print the number along with bpftool's own version number.

Output:

    $ ./bpftool version
    ./bpftool v5.16.0
    using libbpf v0.7
    features: libbfd, libbpf_strict, skeletons

    $ ./bpftool version --json --pretty
    {
        "version": "5.16.0",
        "libbpf_version": "0.7",
        "features": {
            "libbfd": true,
            "libbpf_strict": true,
            "skeletons": true
        }
    }

Note that libbpf does not expose its patch number.
Signed-off-by: Quentin Monnet <quentin@isovalent.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20220210104237.11649-2-quentin@isovalent.com

tools/bpf/bpftool/Documentation/common_options.rst

@@ -4,12 +4,13 @@
 	  Print short help message (similar to **bpftool help**).
 
 -V, --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).
+	  Print bpftool's version number (similar to **bpftool version**), the
+	  number of the libbpf version in use, 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

@@ -89,6 +89,9 @@ static int do_version(int argc, char **argv)
 
 		jsonw_name(json_wtr, "version");
 		jsonw_printf(json_wtr, "\"%s\"", BPFTOOL_VERSION);
+		jsonw_name(json_wtr, "libbpf_version");
+		jsonw_printf(json_wtr, "\"%d.%d\"",
+			     libbpf_major_version(), libbpf_minor_version());
 
 		jsonw_name(json_wtr, "features");
 		jsonw_start_object(json_wtr);	/* features */
@@ -102,6 +105,7 @@ static int do_version(int argc, char **argv)
 		unsigned int nb_features = 0;
 
 		printf("%s v%s\n", bin_name, BPFTOOL_VERSION);
+		printf("using libbpf %s\n", libbpf_version_string());
 		printf("features:");
 		if (has_libbfd) {
 			printf(" libbfd");