[PATCH bpf-next 3/8] tools: bpftool: add probes for kernel configuration options

[Quentin Monnet <quentin.monnet@netronome.com>]

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",
            ...
        }
    }

    # bpftool feature probe kernel macros prefix BPFTOOL_
    /*** System configuration ***/
    ...
    #define BPFTOOL_CONFIG_BPF y
    #define BPFTOOL_CONFIG_BPF_SYSCALL y
    #define BPFTOOL_CONFIG_HAVE_EBPF_JIT y
    ...

Signed-off-by: Quentin Monnet <quentin.monnet@netronome.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
---
 tools/bpf/bpftool/feature.c | 144 ++++++++++++++++++++++++++++++++++++
 1 file changed, 144 insertions(+)

diff --git a/tools/bpf/bpftool/feature.c b/tools/bpf/bpftool/feature.c
index 9fa7016c7d21..4a3a45f44162 100644
--- a/tools/bpf/bpftool/feature.c
+++ b/tools/bpf/bpftool/feature.c
@@ -52,6 +52,37 @@ print_bool_feature(const char *feat_name, const char *define_name,
 		printf("%s is %savailable\n", plain_name, res ? "" : "NOT ");
 }
 
+static void
+print_kernel_option(const char *name, const char *value,
+		    const char *define_prefix)
+{
+	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 (define_prefix) {
+		if (value)
+			printf("#define %s%s %s\n", define_prefix, name, value);
+		else
+			printf("#define %sNO_%s\n", define_prefix, name);
+	} 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 *define_comment,
 		    const char *plain_title, const char *define_prefix)
@@ -298,6 +329,118 @@ static void probe_jit_kallsyms(const char *define_prefix)
 	}
 }
 
+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(const char *define_prefix)
+{
+	const char * const options[] = {
+		"CONFIG_BPF",
+		"CONFIG_BPF_SYSCALL",
+		"CONFIG_HAVE_EBPF_JIT",
+		"CONFIG_BPF_JIT",
+		"CONFIG_BPF_JIT_ALWAYS_ON",
+		"CONFIG_NET",
+		"CONFIG_XDP_SOCKETS",
+		"CONFIG_CGROUPS",
+		"CONFIG_CGROUP_BPF",
+		"CONFIG_CGROUP_NET_CLASSID",
+		"CONFIG_BPF_EVENTS",
+		"CONFIG_LWTUNNEL_BPF",
+		"CONFIG_NET_ACT_BPF",
+		"CONFIG_NET_CLS_ACT",
+		"CONFIG_NET_CLS_BPF",
+		"CONFIG_NET_SCH_INGRESS",
+		"CONFIG_XFRM",
+		"CONFIG_SOCK_CGROUP_DATA",
+		"CONFIG_IP_ROUTE_CLASSID",
+		"CONFIG_IPV6_SEG6_BPF",
+		"CONFIG_FUNCTION_ERROR_INJECTION",
+		"CONFIG_BPF_KPROBE_OVERRIDE",
+		"CONFIG_BPF_LIRC_MODE2",
+		"CONFIG_NETFILTER_XT_MATCH_BPF",
+		"CONFIG_TEST_BPF",
+		"CONFIG_BPFILTER",
+		"CONFIG_BPFILTER_UMH",
+		"CONFIG_BPF_STREAM_PARSER",
+	};
+	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) {
+		/* Sometimes config is at /proc/config */
+		fd = fopen("/proc/config", "r");
+	}
+	if (!fd) {
+		p_err("can't open kernel config file: %s", strerror(errno));
+		goto no_config;
+	}
+	/* Sanity checks */
+	ret = getline(&buf, &n, fd);
+	ret = getline(&buf, &n, fd);
+	if (!buf || !ret) {
+		p_err("can't read from kernel config file: %s",
+		      strerror(errno));
+		free(buf);
+		goto no_config;
+	}
+	if (strcmp(buf, "# Automatically generated file; DO NOT EDIT.\n")) {
+		p_err("can't find correct kernel config 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, define_prefix);
+		free(value);
+	}
+	fclose(fd);
+	return;
+
+no_config:
+	for (i = 0; i < ARRAY_SIZE(options); i++)
+		print_kernel_option(options[i], NULL, define_prefix);
+}
+
 static int probe_kernel_version(const char *define_prefix)
 {
 	int version, subversion, patchlevel, code = 0;
@@ -402,6 +545,7 @@ static int do_probe(int argc, char **argv)
 		} else {
 			p_info("/* procfs not mounted, skipping related probes */");
 		}
+		probe_kernel_image_config(define_prefix);
 		if (json_output)
 			jsonw_end_object(json_wtr);
 		else
-- 
2.17.1