tools: bpftool: add probes for kernel configuration options

Add probes to dump a number of options set (or not set) for compiling
the kernel image. These parameters provide information about what BPF
components should be available on the system. A number of them are not
directly related to eBPF, but are in fact used in the kernel as
conditions on which to compile, or not to compile, some of the eBPF
helper functions.

Sample output:

    # bpftool feature probe kernel
    Scanning system configuration...
    ...
    CONFIG_BPF is set to y
    CONFIG_BPF_SYSCALL is set to y
    CONFIG_HAVE_EBPF_JIT is set to y
    ...

    # bpftool --pretty --json feature probe kernel
    {
        "system_config": {
            ...
            "CONFIG_BPF": "y",
            "CONFIG_BPF_SYSCALL": "y",
            "CONFIG_HAVE_EBPF_JIT": "y",
            ...
        }
    }

v5:
- Declare options[] array in probe_kernel_image_config() as static.

v4:
- Add some options to the list:
    - CONFIG_TRACING
    - CONFIG_KPROBE_EVENTS
    - CONFIG_UPROBE_EVENTS
    - CONFIG_FTRACE_SYSCALLS
- Add comments about those options in the source code.

v3:
- Add a comment about /proc/config.gz not being supported as a path for
  the config file at this time.
- Use p_info() instead of p_err() on failure to get options from config
  file, as bpftool keeps probing other parameters and that would
  possibly create duplicate "error" entries for JSON.

v2:
- Remove C-style macros output from this patch.
- NOT addressed: grouping of those config options into subsections
  (I don't see an easy way of grouping them at the moment, please see
  also the discussion on v1 thread).
Signed-off-by: Quentin Monnet <quentin.monnet@netronome.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

tools/bpf/bpftool/feature.c

@@ -48,6 +48,30 @@ print_bool_feature(const char *feat_name, const char *plain_name, bool res)
 		printf("%s is %savailable\n", plain_name, res ? "" : "NOT ");
 }
 
+static void print_kernel_option(const char *name, const char *value)
+{
+	char *endptr;
+	int res;
+
+	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)
 {
@@ -208,6 +232,163 @@ static void probe_jit_limit(void)
 	}
 }
 
+static char *get_kernel_config_option(FILE *fd, const char *option)
+{
+	size_t line_n = 0, optlen = strlen(option);
+	char *res, *strval, *line = NULL;
+	ssize_t n;
+
+	rewind(fd);
+	while ((n = getline(&line, &line_n, fd)) > 0) {
+		if (strncmp(line, option, optlen))
+			continue;
+		/* Check we have at least '=', value, and '\n' */
+		if (strlen(line) < optlen + 3)
+			continue;
+		if (*(line + optlen) != '=')
+			continue;
+
+		/* Trim ending '\n' */
+		line[strlen(line) - 1] = '\0';
+
+		/* Copy and return config option value */
+		strval = line + optlen + 1;
+		res = strdup(strval);
+		free(line);
+		return res;
+	}
+	free(line);
+
+	return NULL;
+}
+
+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 *value, *buf = NULL;
+	struct utsname utsn;
+	char path[PATH_MAX];
+	size_t i, n;
+	ssize_t ret;
+	FILE *fd;
+
+	if (uname(&utsn))
+		goto no_config;
+
+	snprintf(path, sizeof(path), "/boot/config-%s", utsn.release);
+
+	fd = fopen(path, "r");
+	if (!fd && errno == ENOENT) {
+		/* Some distributions put the config file at /proc/config, give
+		 * it a try.
+		 * Sometimes it is also at /proc/config.gz but we do not try
+		 * this one for now, it would require linking against libz.
+		 */
+		fd = fopen("/proc/config", "r");
+	}
+	if (!fd) {
+		p_info("skipping kernel config, can't open file: %s",
+		       strerror(errno));
+		goto no_config;
+	}
+	/* Sanity checks */
+	ret = getline(&buf, &n, fd);
+	ret = getline(&buf, &n, fd);
+	if (!buf || !ret) {
+		p_info("skipping kernel config, can't read from file: %s",
+		       strerror(errno));
+		free(buf);
+		goto no_config;
+	}
+	if (strcmp(buf, "# Automatically generated file; DO NOT EDIT.\n")) {
+		p_info("skipping kernel config, can't find correct file");
+		free(buf);
+		goto no_config;
+	}
+	free(buf);
+
+	for (i = 0; i < ARRAY_SIZE(options); i++) {
+		value = get_kernel_config_option(fd, options[i]);
+		print_kernel_option(options[i], value);
+		free(value);
+	}
+	fclose(fd);
+	return;
+
+no_config:
+	for (i = 0; i < ARRAY_SIZE(options); i++)
+		print_kernel_option(options[i], NULL);
+}
+
 static bool probe_bpf_syscall(void)
 {
 	bool res;
@@ -268,6 +449,7 @@ static int do_probe(int argc, char **argv)
 		} else {
 			p_info("/* procfs not mounted, skipping related probes */");
 		}
+		probe_kernel_image_config();
 		if (json_output)
 			jsonw_end_object(json_wtr);
 		else