bpftool: introduce cgroup tree command

This commit introduces a new bpftool command: cgroup tree.
The idea is to iterate over the whole cgroup tree and print
all attached programs.

I was debugging a bpf/systemd issue, and found, that there is
no simple way to listen all bpf programs attached to cgroups.
I did master something in bash, but after some time got tired of it,
and decided, that adding a dedicated bpftool command could be
a better idea.

So, here it is:
  $ sudo ./bpftool cgroup tree
  CgroupPath
  ID       AttachType      AttachFlags     Name
  /sys/fs/cgroup/system.slice/systemd-machined.service
      18       ingress
      17       egress
  /sys/fs/cgroup/system.slice/systemd-logind.service
      20       ingress
      19       egress
  /sys/fs/cgroup/system.slice/systemd-udevd.service
      16       ingress
      15       egress
  /sys/fs/cgroup/system.slice/systemd-journald.service
      14       ingress
      13       egress

Signed-off-by: Roman Gushchin <guro@fb.com>
Acked-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Cc: Quentin Monnet <quentin.monnet@netronome.com>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

tools/bpf/bpftool/cgroup.c

@@ -2,7 +2,12 @@
 // Copyright (C) 2017 Facebook
 // Author: Roman Gushchin <guro@fb.com>
 
+#define _XOPEN_SOURCE 500
+#include <errno.h>
 #include <fcntl.h>
+#include <ftw.h>
+#include <mntent.h>
+#include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
 #include <sys/stat.h>
@@ -53,7 +58,8 @@ static enum bpf_attach_type parse_attach_type(const char *str)
 }
 
 static int show_bpf_prog(int id, const char *attach_type_str,
-			 const char *attach_flags_str)
+			 const char *attach_flags_str,
+			 int level)
 {
 	struct bpf_prog_info info = {};
 	__u32 info_len = sizeof(info);
@@ -78,7 +84,8 @@ static int show_bpf_prog(int id, const char *attach_type_str,
 		jsonw_string_field(json_wtr, "name", info.name);
 		jsonw_end_object(json_wtr);
 	} else {
-		printf("%-8u %-15s %-15s %-15s\n", info.id,
+		printf("%s%-8u %-15s %-15s %-15s\n", level ? "    " : "",
+		       info.id,
 		       attach_type_str,
 		       attach_flags_str,
 		       info.name);
@@ -88,7 +95,20 @@ static int show_bpf_prog(int id, const char *attach_type_str,
 	return 0;
 }
 
-static int show_attached_bpf_progs(int cgroup_fd, enum bpf_attach_type type)
+static int count_attached_bpf_progs(int cgroup_fd, enum bpf_attach_type type)
+{
+	__u32 prog_cnt = 0;
+	int ret;
+
+	ret = bpf_prog_query(cgroup_fd, type, 0, NULL, NULL, &prog_cnt);
+	if (ret)
+		return -1;
+
+	return prog_cnt;
+}
+
+static int show_attached_bpf_progs(int cgroup_fd, enum bpf_attach_type type,
+				   int level)
 {
 	__u32 prog_ids[1024] = {0};
 	char *attach_flags_str;
@@ -123,7 +143,7 @@ static int show_attached_bpf_progs(int cgroup_fd, enum bpf_attach_type type)
 
 	for (iter = 0; iter < prog_cnt; iter++)
 		show_bpf_prog(prog_ids[iter], attach_type_strings[type],
-			      attach_flags_str);
+			      attach_flags_str, level);
 
 	return 0;
 }
@@ -161,7 +181,7 @@ static int do_show(int argc, char **argv)
 		 * If we were able to get the show for at least one
 		 * attach type, let's return 0.
 		 */
-		if (show_attached_bpf_progs(cgroup_fd, type) == 0)
+		if (show_attached_bpf_progs(cgroup_fd, type, 0) == 0)
 			ret = 0;
 	}
 
@@ -173,6 +193,143 @@ exit:
 	return ret;
 }
 
+/*
+ * To distinguish nftw() errors and do_show_tree_fn() errors
+ * and avoid duplicating error messages, let's return -2
+ * from do_show_tree_fn() in case of error.
+ */
+#define NFTW_ERR		-1
+#define SHOW_TREE_FN_ERR	-2
+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 cgroup_fd;
+
+	if (typeflag != FTW_D)
+		return 0;
+
+	cgroup_fd = open(fpath, O_RDONLY);
+	if (cgroup_fd < 0) {
+		p_err("can't open cgroup %s: %s", fpath, strerror(errno));
+		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) {
+		close(cgroup_fd);
+		return 0;
+	}
+
+	if (json_output) {
+		jsonw_start_object(json_wtr);
+		jsonw_string_field(json_wtr, "cgroup", fpath);
+		jsonw_name(json_wtr, "programs");
+		jsonw_start_array(json_wtr);
+	} else {
+		printf("%s\n", fpath);
+	}
+
+	for (type = 0; type < __MAX_BPF_ATTACH_TYPE; type++)
+		show_attached_bpf_progs(cgroup_fd, type, ftw->level);
+
+	if (json_output) {
+		jsonw_end_array(json_wtr);
+		jsonw_end_object(json_wtr);
+	}
+
+	close(cgroup_fd);
+
+	return 0;
+}
+
+static char *find_cgroup_root(void)
+{
+	struct mntent *mnt;
+	FILE *f;
+
+	f = fopen("/proc/mounts", "r");
+	if (f == NULL)
+		return NULL;
+
+	while ((mnt = getmntent(f))) {
+		if (strcmp(mnt->mnt_type, "cgroup2") == 0) {
+			fclose(f);
+			return strdup(mnt->mnt_dir);
+		}
+	}
+
+	fclose(f);
+	return NULL;
+}
+
+static int do_show_tree(int argc, char **argv)
+{
+	char *cgroup_root;
+	int ret;
+
+	switch (argc) {
+	case 0:
+		cgroup_root = find_cgroup_root();
+		if (!cgroup_root) {
+			p_err("cgroup v2 isn't mounted");
+			return -1;
+		}
+		break;
+	case 1:
+		cgroup_root = argv[0];
+		break;
+	default:
+		p_err("too many parameters for cgroup tree");
+		return -1;
+	}
+
+
+	if (json_output)
+		jsonw_start_array(json_wtr);
+	else
+		printf("%s\n"
+		       "%-8s %-15s %-15s %-15s\n",
+		       "CgroupPath",
+		       "ID", "AttachType", "AttachFlags", "Name");
+
+	switch (nftw(cgroup_root, do_show_tree_fn, 1024, FTW_MOUNT)) {
+	case NFTW_ERR:
+		p_err("can't iterate over %s: %s", cgroup_root,
+		      strerror(errno));
+		ret = -1;
+		break;
+	case SHOW_TREE_FN_ERR:
+		ret = -1;
+		break;
+	default:
+		ret = 0;
+	}
+
+	if (json_output)
+		jsonw_end_array(json_wtr);
+
+	if (argc == 0)
+		free(cgroup_root);
+
+	return ret;
+}
+
 static int do_attach(int argc, char **argv)
 {
 	enum bpf_attach_type attach_type;
@@ -289,6 +446,7 @@ static int do_help(int argc, char **argv)
 
 	fprintf(stderr,
 		"Usage: %s %s { show | list } CGROUP\n"
+		"       %s %s tree [CGROUP_ROOT]\n"
 		"       %s %s attach CGROUP ATTACH_TYPE PROG [ATTACH_FLAGS]\n"
 		"       %s %s detach CGROUP ATTACH_TYPE PROG\n"
 		"       %s %s help\n"
@@ -298,6 +456,7 @@ static int do_help(int argc, char **argv)
 		"       " HELP_SPEC_PROGRAM "\n"
 		"       " HELP_SPEC_OPTIONS "\n"
 		"",
+		bin_name, argv[-2],
 		bin_name, argv[-2], bin_name, argv[-2],
 		bin_name, argv[-2], bin_name, argv[-2]);
 
@@ -307,6 +466,7 @@ static int do_help(int argc, char **argv)
 static const struct cmd cmds[] = {
 	{ "show",	do_show },
 	{ "list",	do_show },
+	{ "tree",       do_show_tree },
 	{ "attach",	do_attach },
 	{ "detach",	do_detach },
 	{ "help",	do_help },