path: root/tools/bpf/bpftool/feature.c

// SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
/* Copyright (c) 2019 Netronome Systems, Inc. */

#include <ctype.h>
#include <errno.h>
#include <string.h>
#include <unistd.h>
#include <net/if.h>
#ifdef USE_LIBCAP
#include <sys/capability.h>
#endif
#include <sys/utsname.h>
#include <sys/vfs.h>

#include <linux/filter.h>
#include <linux/limits.h>

#include <bpf/bpf.h>
#include <bpf/libbpf.h>
#include <zlib.h>

#include "main.h"

#ifndef PROC_SUPER_MAGIC
# define PROC_SUPER_MAGIC	0x9fa0
#endif

enum probe_component {
	COMPONENT_UNSPEC,
	COMPONENT_KERNEL,
	COMPONENT_DEVICE,
};

#define BPF_HELPER_MAKE_ENTRY(name)	[BPF_FUNC_ ## name] = "bpf_" # name
static const char * const helper_name[] = {
	__BPF_FUNC_MAPPER(BPF_HELPER_MAKE_ENTRY)
};

#undef BPF_HELPER_MAKE_ENTRY

static bool full_mode;
#ifdef USE_LIBCAP
static bool run_as_unprivileged;
#endif

/* Miscellaneous utility functions */

static bool check_procfs(void)
{
	struct statfs st_fs;

	if (statfs("/proc", &st_fs) < 0)
		return false;
	if ((unsigned long)st_fs.f_type != PROC_SUPER_MAGIC)
		return false;

	return true;
}

static void uppercase(char *str, size_t len)
{
	size_t i;

	for (i = 0; i < len && str[i] != '\0'; i++)
		str[i] = toupper(str[i]);
}

/* Printing utility functions */

static void
print_bool_feature(const char *feat_name, const char *plain_name,
		   const char *define_name, bool res, const char *define_prefix)
{
	if (json_output)
		jsonw_bool_field(json_wtr, feat_name, res);
	else if (define_prefix)
		printf("#define %s%sHAVE_%s\n", define_prefix,
		       res ? "" : "NO_", define_name);
	else
		printf("%s is %savailable\n", plain_name, res ? "" : "NOT ");
}

static void print_kernel_option(const char *name, const char *value)
{
	char *endptr;
	int res;

	/* No support for C-style ouptut */

	if (json_output) {
		if (!value) {
			jsonw_null_field(json_wtr, name);
			return;
		}
		errno = 0;
		res = strtol(value, &endptr, 0);
		if (!errno && *endptr == '\n')
			jsonw_int_field(json_wtr, name, res);
		else
			jsonw_string_field(json_wtr, name, value);
	} else {
		if (value)
			printf("%s is set to %s\n", name, value);
		else
			printf("%s is not set\n", name);
	}
}

static void
print_start_section(const char *json_title, const char *plain_title,
		    const char *define_comment, const char *define_prefix)
{
	if (json_output) {
		jsonw_name(json_wtr, json_title);
		jsonw_start_object(json_wtr);
	} else if (define_prefix) {
		printf("%s\n", define_comment);
	} else {
		printf("%s\n", plain_title);
	}
}

static void print_end_section(void)
{
	if (json_output)
		jsonw_end_object(json_wtr);
	else
		printf("\n");
}

/* Probing functions */

static int read_procfs(const char *path)
{
	char *endptr, *line = NULL;
	size_t len = 0;
	FILE *fd;
	int res;

	fd = fopen(path, "r");
	if (!fd)
		return -1;

	res = getline(&line, &len, fd);
	fclose(fd);
	if (res < 0)
		return -1;

	errno = 0;
	res = strtol(line, &endptr, 10);
	if (errno || *line == '\0' || *endptr != '\n')
		res = -1;
	free(line);

	return res;
}

static void probe_unprivileged_disabled(void)
{
	int res;

	/* No support for C-style ouptut */

	res = read_procfs("/proc/sys/kernel/unprivileged_bpf_disabled");
	if (json_output) {
		jsonw_int_field(json_wtr, "unprivileged_bpf_disabled", res);
	} else {
		switch (res) {
		case 0:
			printf("bpf() syscall for unprivileged users is enabled\n");
			break;
		case 1:
			printf("bpf() syscall restricted to privileged users\n");
			break;
		case -1:
			printf("Unable to retrieve required privileges for bpf() syscall\n");
			break;
		default:
			printf("bpf() syscall restriction has unknown value %d\n", res);
		}
	}
}

static void probe_jit_enable(void)
{
	int res;

	/* No support for C-style ouptut */

	res = read_procfs("/proc/sys/net/core/bpf_jit_enable");
	if (json_output) {
		jsonw_int_field(json_wtr, "bpf_jit_enable", res);
	} else {
		switch (res) {
		case 0:
			printf("JIT compiler is disabled\n");
			break;
		case 1:
			printf("JIT compiler is enabled\n");
			break;
		case 2:
			printf("JIT compiler is enabled with debugging traces in kernel logs\n");
			break;
		case -1:
			printf("Unable to retrieve JIT-compiler status\n");
			break;
		default:
			printf("JIT-compiler status has unknown value %d\n",
			       res);
		}
	}
}

static void probe_jit_harden(void)
{
	int res;

	/* No support for C-style ouptut */

	res = read_procfs("/proc/sys/net/core/bpf_jit_harden");
	if (json_output) {
		jsonw_int_field(json_wtr, "bpf_jit_harden", res);
	} else {
		switch (res) {
		case 0:
			printf("JIT compiler hardening is disabled\n");
			break;
		case 1:
			printf("JIT compiler hardening is enabled for unprivileged users\n");
			break;
		case 2:
			printf("JIT compiler hardening is enabled for all users\n");
			break;
		case -1:
			printf("Unable to retrieve JIT hardening status\n");
			break;
		default:
			printf("JIT hardening status has unknown value %d\n",
			       res);
		}
	}
}

static void probe_jit_kallsyms(void)
{
	int res;

	/* No support for C-style ouptut */

	res = read_procfs("/proc/sys/net/core/bpf_jit_kallsyms");
	if (json_output) {
		jsonw_int_field(json_wtr, "bpf_jit_kallsyms", res);
	} else {
		switch (res) {
		case 0:
			printf("JIT compiler kallsyms exports are disabled\n");
			break;
		case 1:
			printf("JIT compiler kallsyms exports are enabled for root\n");
			break;
		case -1:
			printf("Unable to retrieve JIT kallsyms export status\n");
			break;
		default:
			printf("JIT kallsyms exports status has unknown value %d\n", res);
		}
	}
}

static void probe_jit_limit(void)
{
	int res;

	/* No support for C-style ouptut */

	res = read_procfs("/proc/sys/net/core/bpf_jit_limit");
	if (json_output) {
		jsonw_int_field(json_wtr, "bpf_jit_limit", res);
	} else {
		switch (res) {
		case -1:
			printf("Unable to retrieve global memory limit for JIT compiler for unprivileged users\n");
			break;
		default:
			printf("Global memory limit for JIT compiler for unprivileged users is %d bytes\n", res);
		}
	}
}

static bool read_next_kernel_config_option(gzFile file, char *buf, size_t n,
					   char **value)
{
	char *sep;

	while (gzgets(file, buf, n)) {
		if (strncmp(buf, "CONFIG_", 7))
			continue;

		sep = strchr(buf, '=');
		if (!sep)
			continue;

		/* Trim ending '\n' */
		buf[strlen(buf) - 1] = '\0';

		/* Split on '=' and ensure that a value is present. */
		*sep = '\0';
		if (!sep[1])
			continue;

		*value = sep + 1;
		return true;
	}

	return false;
}

static void probe_kernel_image_config(void)
{
	static const char * const options[] = {
		/* Enable BPF */
		"CONFIG_BPF",
		/* Enable bpf() syscall */
		"CONFIG_BPF_SYSCALL",
		/* Does selected architecture support eBPF JIT compiler */
		"CONFIG_HAVE_EBPF_JIT",
		/* Compile eBPF JIT compiler */
		"CONFIG_BPF_JIT",
		/* Avoid compiling eBPF interpreter (use JIT only) */
		"CONFIG_BPF_JIT_ALWAYS_ON",

		/* cgroups */
		"CONFIG_CGROUPS",
		/* BPF programs attached to cgroups */
		"CONFIG_CGROUP_BPF",
		/* bpf_get_cgroup_classid() helper */
		"CONFIG_CGROUP_NET_CLASSID",
		/* bpf_skb_{,ancestor_}cgroup_id() helpers */
		"CONFIG_SOCK_CGROUP_DATA",

		/* Tracing: attach BPF to kprobes, tracepoints, etc. */
		"CONFIG_BPF_EVENTS",
		/* Kprobes */
		"CONFIG_KPROBE_EVENTS",
		/* Uprobes */
		"CONFIG_UPROBE_EVENTS",
		/* Tracepoints */
		"CONFIG_TRACING",
		/* Syscall tracepoints */
		"CONFIG_FTRACE_SYSCALLS",
		/* bpf_override_return() helper support for selected arch */
		"CONFIG_FUNCTION_ERROR_INJECTION",
		/* bpf_override_return() helper */
		"CONFIG_BPF_KPROBE_OVERRIDE",

		/* Network */
		"CONFIG_NET",
		/* AF_XDP sockets */
		"CONFIG_XDP_SOCKETS",
		/* BPF_PROG_TYPE_LWT_* and related helpers */
		"CONFIG_LWTUNNEL_BPF",
		/* BPF_PROG_TYPE_SCHED_ACT, TC (traffic control) actions */
		"CONFIG_NET_ACT_BPF",
		/* BPF_PROG_TYPE_SCHED_CLS, TC filters */
		"CONFIG_NET_CLS_BPF",
		/* TC clsact qdisc */
		"CONFIG_NET_CLS_ACT",
		/* Ingress filtering with TC */
		"CONFIG_NET_SCH_INGRESS",
		/* bpf_skb_get_xfrm_state() helper */
		"CONFIG_XFRM",
		/* bpf_get_route_realm() helper */
		"CONFIG_IP_ROUTE_CLASSID",
		/* BPF_PROG_TYPE_LWT_SEG6_LOCAL and related helpers */
		"CONFIG_IPV6_SEG6_BPF",
		/* BPF_PROG_TYPE_LIRC_MODE2 and related helpers */
		"CONFIG_BPF_LIRC_MODE2",
		/* BPF stream parser and BPF socket maps */
		"CONFIG_BPF_STREAM_PARSER",
		/* xt_bpf module for passing BPF programs to netfilter  */
		"CONFIG_NETFILTER_XT_MATCH_BPF",
		/* bpfilter back-end for iptables */
		"CONFIG_BPFILTER",
		/* bpftilter module with "user mode helper" */
		"CONFIG_BPFILTER_UMH",

		/* test_bpf module for BPF tests */
		"CONFIG_TEST_BPF",
	};
	char *values[ARRAY_SIZE(options)] = { };
	struct utsname utsn;
	char path[PATH_MAX];
	gzFile file = NULL;
	char buf[4096];
	char *value;
	size_t i;

	if (!uname(&utsn)) {
		snprintf(path, sizeof(path), "/boot/config-%s", utsn.release);

		/* gzopen also accepts uncompressed files. */
		file = gzopen(path, "r");
	}

	if (!file) {
		/* Some distributions build with CONFIG_IKCONFIG=y and put the
		 * config file at /proc/config.gz.
		 */
		file = gzopen("/proc/config.gz", "r");
	}
	if (!file) {
		p_info("skipping kernel config, can't open file: %s",
		       strerror(errno));
		goto end_parse;
	}
	/* Sanity checks */
	if (!gzgets(file, buf, sizeof(buf)) ||
	    !gzgets(file, buf, sizeof(buf))) {
		p_info("skipping kernel config, can't read from file: %s",
		       strerror(errno));
		goto end_parse;
	}
	if (strcmp(buf, "# Automatically generated file; DO NOT EDIT.\n")) {
		p_info("skipping kernel config, can't find correct file");
		goto end_parse;
	}

	while (read_next_kernel_config_option(file, buf, sizeof(buf), &value)) {
		for (i = 0; i < ARRAY_SIZE(options); i++) {
			if (values[i] || strcmp(buf, options[i]))
				continue;

			values[i] = strdup(value);
		}
	}

end_parse:
	if (file)
		gzclose(file);

	for (i = 0; i < ARRAY_SIZE(options); i++) {
		print_kernel_option(options[i], values[i]);
		free(values[i]);
	}
}

static bool probe_bpf_syscall(const char *define_prefix)
{
	bool res;

	bpf_load_program(BPF_PROG_TYPE_UNSPEC, NULL, 0, NULL, 0, NULL, 0);
	res = (errno != ENOSYS);

	print_bool_feature("have_bpf_syscall",
			   "bpf() syscall",
			   "BPF_SYSCALL",
			   res, define_prefix);

	return res;
}

static void
probe_prog_type(enum bpf_prog_type prog_type, bool *supported_types,
		const char *define_prefix, __u32 ifindex)
{
	char feat_name[128], plain_desc[128], define_name[128];
	const char *plain_comment = "eBPF program_type ";
	size_t maxlen;
	bool res;

	if (ifindex)
		/* Only test offload-able program types */
		switch (prog_type) {
		case BPF_PROG_TYPE_SCHED_CLS:
		case BPF_PROG_TYPE_XDP:
			break;
		default:
			return;
		}

	res = bpf_probe_prog_type(prog_type, ifindex);
#ifdef USE_LIBCAP
	/* Probe may succeed even if program load fails, for unprivileged users
	 * check that we did not fail because of insufficient permissions
	 */
	if (run_as_unprivileged && errno == EPERM)
		res = false;
#endif

	supported_types[prog_type] |= res;

	maxlen = sizeof(plain_desc) - strlen(plain_comment) - 1;
	if (strlen(prog_type_name[prog_type]) > maxlen) {
		p_info("program type name too long");
		return;
	}

	sprintf(feat_name, "have_%s_prog_type", prog_type_name[prog_type]);
	sprintf(define_name, "%s_prog_type", prog_type_name[prog_type]);
	uppercase(define_name, sizeof(define_name));
	sprintf(plain_desc, "%s%s", plain_comment, prog_type_name[prog_type]);
	print_bool_feature(feat_name, plain_desc, define_name, res,
			   define_prefix);
}

static void
probe_map_type(enum bpf_map_type map_type, const char *define_prefix,
	       __u32 ifindex)
{
	char feat_name[128], plain_desc[128], define_name[128];
	const char *plain_comment = "eBPF map_type ";
	size_t maxlen;
	bool res;

	res = bpf_probe_map_type(map_type, ifindex);

	/* Probe result depends on the success of map creation, no additional
	 * check required for unprivileged users
	 */

	maxlen = sizeof(plain_desc) - strlen(plain_comment) - 1;
	if (strlen(map_type_name[map_type]) > maxlen) {
		p_info("map type name too long");
		return;
	}

	sprintf(feat_name, "have_%s_map_type", map_type_name[map_type]);
	sprintf(define_name, "%s_map_type", map_type_name[map_type]);
	uppercase(define_name, sizeof(define_name));
	sprintf(plain_desc, "%s%s", plain_comment, map_type_name[map_type]);
	print_bool_feature(feat_name, plain_desc, define_name, res,
			   define_prefix);
}

static void
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)
{
	bool res = false;

	if (supported_type) {
		res = bpf_probe_helper(id, prog_type, ifindex);
#ifdef USE_LIBCAP
		/* Probe may succeed even if program load fails, for
		 * unprivileged users check that we did not fail because of
		 * insufficient permissions
		 */
		if (run_as_unprivileged && errno == EPERM)
			res = false;
#endif
	}

	if (json_output) {
		if (res)
			jsonw_string(json_wtr, helper_name[id]);
	} else if (define_prefix) {
		printf("#define %sBPF__PROG_TYPE_%s__HELPER_%s %s\n",
		       define_prefix, ptype_name, helper_name[id],
		       res ? "1" : "0");
	} else {
		if (res)
			printf("\n\t- %s", helper_name[id]);
	}
}

static void
probe_helpers_for_progtype(enum bpf_prog_type prog_type, bool supported_type,
			   const char *define_prefix, __u32 ifindex)
{
	const char *ptype_name = prog_type_name[prog_type];
	char feat_name[128];
	unsigned int id;

	if (ifindex)
		/* Only test helpers for offload-able program types */
		switch (prog_type) {
		case BPF_PROG_TYPE_SCHED_CLS:
		case BPF_PROG_TYPE_XDP:
			break;
		default:
			return;
		}

	if (json_output) {
		sprintf(feat_name, "%s_available_helpers", ptype_name);
		jsonw_name(json_wtr, feat_name);
		jsonw_start_array(json_wtr);
	} else if (!define_prefix) {
		printf("eBPF helpers supported for program type %s:",
		       ptype_name);
	}

	for (id = 1; id < ARRAY_SIZE(helper_name); id++) {
		/* Skip helper functions which emit dmesg messages when not in
		 * the full mode.
		 */
		switch (id) {
		case BPF_FUNC_trace_printk:
		case BPF_FUNC_probe_write_user:
			if (!full_mode)
				continue;
			/* fallthrough */
		default:
			probe_helper_for_progtype(prog_type, supported_type,
						  define_prefix, id, ptype_name,
						  ifindex);
		}
	}

	if (json_output)
		jsonw_end_array(json_wtr);
	else if (!define_prefix)
		printf("\n");
}

static void
probe_large_insn_limit(const char *define_prefix, __u32 ifindex)
{
	bool res;

	res = bpf_probe_large_insn_limit(ifindex);
	print_bool_feature("have_large_insn_limit",
			   "Large program size limit",
			   "LARGE_INSN_LIMIT",
			   res, define_prefix);
}

static void
section_system_config(enum probe_component target, const char *define_prefix)
{
	switch (target) {
	case COMPONENT_KERNEL:
	case COMPONENT_UNSPEC:
		if (define_prefix)
			break;

		print_start_section("system_config",
				    "Scanning system configuration...",
				    NULL, /* define_comment never used here */
				    NULL); /* define_prefix always NULL here */
		if (check_procfs()) {
			probe_unprivileged_disabled();
			probe_jit_enable();
			probe_jit_harden();
			probe_jit_kallsyms();
			probe_jit_limit();
		} else {
			p_info("/* procfs not mounted, skipping related probes */");
		}
		probe_kernel_image_config();
		print_end_section();
		break;
	default:
		break;
	}
}

static bool section_syscall_config(const char *define_prefix)
{
	bool res;

	print_start_section("syscall_config",
			    "Scanning system call availability...",
			    "/*** System call availability ***/",
			    define_prefix);
	res = probe_bpf_syscall(define_prefix);
	print_end_section();

	return res;
}

static void
section_program_types(bool *supported_types, const char *define_prefix,
		      __u32 ifindex)
{
	unsigned int i;

	print_start_section("program_types",
			    "Scanning eBPF program types...",
			    "/*** eBPF program types ***/",
			    define_prefix);

	for (i = BPF_PROG_TYPE_UNSPEC + 1; i < ARRAY_SIZE(prog_type_name); i++)
		probe_prog_type(i, supported_types, define_prefix, ifindex);

	print_end_section();
}

static void section_map_types(const char *define_prefix, __u32 ifindex)
{
	unsigned int i;

	print_start_section("map_types",
			    "Scanning eBPF map types...",
			    "/*** eBPF map types ***/",
			    define_prefix);

	for (i = BPF_MAP_TYPE_UNSPEC + 1; i < map_type_name_size; i++)
		probe_map_type(i, define_prefix, ifindex);

	print_end_section();
}

static void
section_helpers(bool *supported_types, const char *define_prefix, __u32 ifindex)
{
	unsigned int i;

	print_start_section("helpers",
			    "Scanning eBPF helper functions...",
			    "/*** eBPF helper functions ***/",
			    define_prefix);

	if (define_prefix)
		printf("/*\n"
		       " * Use %sHAVE_PROG_TYPE_HELPER(prog_type_name, helper_name)\n"
		       " * to determine if <helper_name> is available for <prog_type_name>,\n"
		       " * e.g.\n"
		       " *	#if %sHAVE_PROG_TYPE_HELPER(xdp, bpf_redirect)\n"
		       " *		// do stuff with this helper\n"
		       " *	#elif\n"
		       " *		// use a workaround\n"
		       " *	#endif\n"
		       " */\n"
		       "#define %sHAVE_PROG_TYPE_HELPER(prog_type, helper)	\\\n"
		       "	%sBPF__PROG_TYPE_ ## prog_type ## __HELPER_ ## helper\n",
		       define_prefix, define_prefix, define_prefix,
		       define_prefix);
	for (i = BPF_PROG_TYPE_UNSPEC + 1; i < ARRAY_SIZE(prog_type_name); i++)
		probe_helpers_for_progtype(i, supported_types[i], define_prefix,
					   ifindex);

	print_end_section();
}

static void section_misc(const char *define_prefix, __u32 ifindex)
{
	print_start_section("misc",
			    "Scanning miscellaneous eBPF features...",
			    "/*** eBPF misc features ***/",
			    define_prefix);
	probe_large_insn_limit(define_prefix, ifindex);
	print_end_section();
}

static int handle_perms(void)
{
#ifdef USE_LIBCAP
	cap_value_t cap_list[1] = { CAP_SYS_ADMIN };
	bool has_sys_admin_cap = false;
	cap_flag_value_t val;
	int res = -1;
	cap_t caps;

	caps = cap_get_proc();
	if (!caps) {
		p_err("failed to get capabilities for process: %s",
		      strerror(errno));
		return -1;
	}

	if (cap_get_flag(caps, CAP_SYS_ADMIN, CAP_EFFECTIVE, &val)) {
		p_err("bug: failed to retrieve CAP_SYS_ADMIN status");
		goto exit_free;
	}
	if (val == CAP_SET)
		has_sys_admin_cap = true;

	if (!run_as_unprivileged && !has_sys_admin_cap) {
		p_err("full feature probing requires CAP_SYS_ADMIN, run as root or use 'unprivileged'");
		goto exit_free;
	}

	if ((run_as_unprivileged && !has_sys_admin_cap) ||
	    (!run_as_unprivileged && has_sys_admin_cap)) {
		/* We are all good, exit now */
		res = 0;
		goto exit_free;
	}

	/* if (run_as_unprivileged && has_sys_admin_cap), drop CAP_SYS_ADMIN */

	if (cap_set_flag(caps, CAP_EFFECTIVE, ARRAY_SIZE(cap_list), cap_list,
			 CAP_CLEAR)) {
		p_err("bug: failed to clear CAP_SYS_ADMIN from capabilities");
		goto exit_free;
	}

	if (cap_set_proc(caps)) {
		p_err("failed to drop CAP_SYS_ADMIN: %s", strerror(errno));
		goto exit_free;
	}

	res = 0;

exit_free:
	if (cap_free(caps) && !res) {
		p_err("failed to clear storage object for capabilities: %s",
		      strerror(errno));
		res = -1;
	}

	return res;
#else
	/* Detection assumes user has sufficient privileges (CAP_SYS_ADMIN).
	 * We do not use libpcap so let's approximate, and restrict usage to
	 * root user only.
	 */
	if (geteuid()) {
		p_err("full feature probing requires root privileges");
		return -1;
	}

	return 0;
#endif /* USE_LIBCAP */
}

static int do_probe(int argc, char **argv)
{
	enum probe_component target = COMPONENT_UNSPEC;
	const char *define_prefix = NULL;
	bool supported_types[128] = {};
	__u32 ifindex = 0;
	char *ifname;

	set_max_rlimit();

	while (argc) {
		if (is_prefix(*argv, "kernel")) {
			if (target != COMPONENT_UNSPEC) {
				p_err("component to probe already specified");
				return -1;
			}
			target = COMPONENT_KERNEL;
			NEXT_ARG();
		} else if (is_prefix(*argv, "dev")) {
			NEXT_ARG();

			if (target != COMPONENT_UNSPEC || ifindex) {
				p_err("component to probe already specified");
				return -1;
			}
			if (!REQ_ARGS(1))
				return -1;

			target = COMPONENT_DEVICE;
			ifname = GET_ARG();
			ifindex = if_nametoindex(ifname);
			if (!ifindex) {
				p_err("unrecognized netdevice '%s': %s", ifname,
				      strerror(errno));
				return -1;
			}
		} else if (is_prefix(*argv, "full")) {
			full_mode = true;
			NEXT_ARG();
		} else if (is_prefix(*argv, "macros") && !define_prefix) {
			define_prefix = "";
			NEXT_ARG();
		} else if (is_prefix(*argv, "prefix")) {
			if (!define_prefix) {
				p_err("'prefix' argument can only be use after 'macros'");
				return -1;
			}
			if (strcmp(define_prefix, "")) {
				p_err("'prefix' already defined");
				return -1;
			}
			NEXT_ARG();

			if (!REQ_ARGS(1))
				return -1;
			define_prefix = GET_ARG();
		} else if (is_prefix(*argv, "unprivileged")) {
#ifdef USE_LIBCAP
			run_as_unprivileged = true;
			NEXT_ARG();
#else
			p_err("unprivileged run not supported, recompile bpftool with libcap");
			return -1;
#endif
		} else {
			p_err("expected no more arguments, 'kernel', 'dev', 'macros' or 'prefix', got: '%s'?",
			      *argv);
			return -1;
		}
	}

	/* Full feature detection requires CAP_SYS_ADMIN privilege.
	 * Let's approximate, and warn if user is not root.
	 */
	if (handle_perms())
		return -1;

	if (json_output) {
		define_prefix = NULL;
		jsonw_start_object(json_wtr);
	}

	section_system_config(target, define_prefix);
	if (!section_syscall_config(define_prefix))
		/* bpf() syscall unavailable, don't probe other BPF features */
		goto exit_close_json;
	section_program_types(supported_types, define_prefix, ifindex);
	section_map_types(define_prefix, ifindex);
	section_helpers(supported_types, define_prefix, ifindex);
	section_misc(define_prefix, ifindex);

exit_close_json:
	if (json_output)
		/* End root object */
		jsonw_end_object(json_wtr);

	return 0;
}

static int do_help(int argc, char **argv)
{
	if (json_output) {
		jsonw_null(json_wtr);
		return 0;
	}

	fprintf(stderr,
		"Usage: %s %s probe [COMPONENT] [full] [unprivileged] [macros [prefix PREFIX]]\n"
		"       %s %s help\n"
		"\n"
		"       COMPONENT := { kernel | dev NAME }\n"
		"",
		bin_name, argv[-2], bin_name, argv[-2]);

	return 0;
}

static const struct cmd cmds[] = {
	{ "probe",	do_probe },
	{ "help",	do_help },
	{ 0 }
};

int do_feature(int argc, char **argv)
{
	return cmd_select(cmds, argc, argv, do_help);
}