[RFC bpf-next v3 3/9] 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",
            ...
        }
    }

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>
---
 tools/bpf/bpftool/feature.c | 142 ++++++++++++++++++++++++++++++++++++
 1 file changed, 142 insertions(+)

diff --git a/tools/bpf/bpftool/feature.c b/tools/bpf/bpftool/feature.c
index 37fe79f59015..05c16fe67005 100644
--- a/tools/bpf/bpftool/feature.c
+++ b/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)
 {
@@ -190,6 +214,123 @@ static void probe_jit_kallsyms(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)
+{
+	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) {
+		/* 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;
@@ -249,6 +390,7 @@ 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
-- 
2.17.1