Merge branch 'bpftool: fix feature output when helper probes fail'

Milan Landaverde says:

====================

Currently in bpftool's feature probe, we incorrectly tell the user that
all of the helper functions are supported for program types where helper
probing fails or is explicitly unsupported[1]:

$ bpftool feature probe
...
eBPF helpers supported for program type tracing:
	- bpf_map_lookup_elem
	- bpf_map_update_elem
	- bpf_map_delete_elem
	...
	- bpf_redirect_neigh
	- bpf_check_mtu
	- bpf_sys_bpf
	- bpf_sys_close

This patch adjusts bpftool to relay to the user when helper support
can't be determined:

$ bpftool feature probe
...
eBPF helpers supported for program type lirc_mode2:
    Program type not supported
eBPF helpers supported for program type tracing:
    Could not determine which helpers are available
eBPF helpers supported for program type struct_opts:
    Could not determine which helpers are available
eBPF helpers supported for program type ext:
    Could not determine which helpers are available

Rather than imply that no helpers are available for the program type, we
let the user know that helper function probing failed entirely.

[1] https://lore.kernel.org/bpf/20211217171202.3352835-2-andrii@kernel.org/
====================
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>

tools/bpf/bpftool/feature.c

@@ -638,7 +638,7 @@ probe_map_type(enum bpf_map_type map_type, const char *define_prefix,
 
 		res = probe_map_type_ifindex(map_type, ifindex);
 	} else {
-		res = libbpf_probe_bpf_map_type(map_type, NULL);
+		res = libbpf_probe_bpf_map_type(map_type, NULL) > 0;
 	}
 
 	/* Probe result depends on the success of map creation, no additional
@@ -690,7 +690,7 @@ probe_helper_ifindex(enum bpf_func_id id, enum bpf_prog_type prog_type,
 	return res;
 }
 
-static void
+static bool
 probe_helper_for_progtype(enum bpf_prog_type prog_type, bool supported_type,
 			  const char *define_prefix, unsigned int id,
 			  const char *ptype_name, __u32 ifindex)
@@ -701,7 +701,7 @@ probe_helper_for_progtype(enum bpf_prog_type prog_type, bool supported_type,
 		if (ifindex)
 			res = probe_helper_ifindex(id, prog_type, ifindex);
 		else
-			res = libbpf_probe_bpf_helper(prog_type, id, NULL);
+			res = libbpf_probe_bpf_helper(prog_type, id, NULL) > 0;
 #ifdef USE_LIBCAP
 		/* Probe may succeed even if program load fails, for
 		 * unprivileged users check that we did not fail because of
@@ -723,6 +723,8 @@ probe_helper_for_progtype(enum bpf_prog_type prog_type, bool supported_type,
 		if (res)
 			printf("\n\t- %s", helper_name[id]);
 	}
+
+	return res;
 }
 
 static void
@@ -732,6 +734,7 @@ probe_helpers_for_progtype(enum bpf_prog_type prog_type, bool supported_type,
 	const char *ptype_name = prog_type_name[prog_type];
 	char feat_name[128];
 	unsigned int id;
+	bool probe_res = false;
 
 	if (ifindex)
 		/* Only test helpers for offload-able program types */
@@ -764,7 +767,7 @@ probe_helpers_for_progtype(enum bpf_prog_type prog_type, bool supported_type,
 				continue;
 			/* fallthrough */
 		default:
-			probe_helper_for_progtype(prog_type, supported_type,
+			probe_res |= probe_helper_for_progtype(prog_type, supported_type,
 						  define_prefix, id, ptype_name,
 						  ifindex);
 		}
@@ -772,8 +775,17 @@ probe_helpers_for_progtype(enum bpf_prog_type prog_type, bool supported_type,
 
 	if (json_output)
 		jsonw_end_array(json_wtr);
-	else if (!define_prefix)
+	else if (!define_prefix) {
 		printf("\n");
+		if (!probe_res) {
+			if (!supported_type)
+				printf("\tProgram type not supported\n");
+			else
+				printf("\tCould not determine which helpers are available\n");
+		}
+	}
+
+
 }
 
 static void