bpf: Print error message for bpftool cgroup show

Currently, when bpftool cgroup show <path> has an error, no error
message is printed. This is confusing because the user may think the
result is empty.

Before the change:

$ bpftool cgroup show /sys/fs/cgroup
ID       AttachType      AttachFlags     Name
$ echo $?
255

After the change:
$ ./bpftool cgroup show /sys/fs/cgroup
Error: can't query bpf programs attached to /sys/fs/cgroup: Operation
not permitted

v2: Rename check_query_cgroup_progs to cgroup_has_attached_progs
Signed-off-by: Hechao Li <hechaol@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20191224011742.3714301-1-hechaol@fb.com

tools/bpf/bpftool/cgroup.c

@@ -117,6 +117,25 @@ static int count_attached_bpf_progs(int cgroup_fd, enum bpf_attach_type type)
 	return prog_cnt;
 }
 
+static int cgroup_has_attached_progs(int cgroup_fd)
+{
+	enum bpf_attach_type type;
+	bool no_prog = true;
+
+	for (type = 0; type < __MAX_BPF_ATTACH_TYPE; type++) {
+		int count = count_attached_bpf_progs(cgroup_fd, type);
+
+		if (count < 0 && errno != EINVAL)
+			return -1;
+
+		if (count > 0) {
+			no_prog = false;
+			break;
+		}
+	}
+
+	return no_prog ? 0 : 1;
+}
 static int show_attached_bpf_progs(int cgroup_fd, enum bpf_attach_type type,
 				   int level)
 {
@@ -161,6 +180,7 @@ static int show_attached_bpf_progs(int cgroup_fd, enum bpf_attach_type type,
 static int do_show(int argc, char **argv)
 {
 	enum bpf_attach_type type;
+	int has_attached_progs;
 	const char *path;
 	int cgroup_fd;
 	int ret = -1;
@@ -192,6 +212,16 @@ static int do_show(int argc, char **argv)
 		goto exit;
 	}
 
+	has_attached_progs = cgroup_has_attached_progs(cgroup_fd);
+	if (has_attached_progs < 0) {
+		p_err("can't query bpf programs attached to %s: %s",
+		      path, strerror(errno));
+		goto exit_cgroup;
+	} else if (!has_attached_progs) {
+		ret = 0;
+		goto exit_cgroup;
+	}
+
 	if (json_output)
 		jsonw_start_array(json_wtr);
 	else
@@ -212,6 +242,7 @@ static int do_show(int argc, char **argv)
 	if (json_output)
 		jsonw_end_array(json_wtr);
 
+exit_cgroup:
 	close(cgroup_fd);
 exit:
 	return ret;
@@ -228,7 +259,7 @@ static int do_show_tree_fn(const char *fpath, const struct stat *sb,
 			   int typeflag, struct FTW *ftw)
 {
 	enum bpf_attach_type type;
-	bool skip = true;
+	int has_attached_progs;
 	int cgroup_fd;
 
 	if (typeflag != FTW_D)
@@ -240,22 +271,13 @@ static int do_show_tree_fn(const char *fpath, const struct stat *sb,
 		return SHOW_TREE_FN_ERR;
 	}
 
-	for (type = 0; type < __MAX_BPF_ATTACH_TYPE; type++) {
-		int count = count_attached_bpf_progs(cgroup_fd, type);
-
-		if (count < 0 && errno != EINVAL) {
-			p_err("can't query bpf programs attached to %s: %s",
-			      fpath, strerror(errno));
-			close(cgroup_fd);
-			return SHOW_TREE_FN_ERR;
-		}
-		if (count > 0) {
-			skip = false;
-			break;
-		}
-	}
-
-	if (skip) {
+	has_attached_progs = cgroup_has_attached_progs(cgroup_fd);
+	if (has_attached_progs < 0) {
+		p_err("can't query bpf programs attached to %s: %s",
+		      fpath, strerror(errno));
+		close(cgroup_fd);
+		return SHOW_TREE_FN_ERR;
+	} else if (!has_attached_progs) {
 		close(cgroup_fd);
 		return 0;
 	}