bpftool: Refactor kernel config reading into common helper

Extract the kernel configuration file parsing logic from feature.c into
a new read_kernel_config() function in common.c. This includes:

1. Moving the config file handling and option parsing code
2. Adding required headers and struct definition
3. Keeping all existing functionality

The refactoring enables sharing this logic with other components while
maintaining current behavior. This will be used by subsequent patches
that need to check kernel config options.

Signed-off-by: Yuan Chen <[e-mail]>
Signed-off-by: Daniel Borkmann <[e-mail]>
Acked-by: Quentin Monnet <[e-mail]>
Acked-by: Yonghong Song <[e-mail]>
Acked-by: Jiri Olsa <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

tools/bpf/bpftool/common.c

···
 #include <sys/resource.h>
 #include <sys/stat.h>
 #include <sys/vfs.h>
+#include <sys/utsname.h>
 
 #include <linux/filter.h>
 #include <linux/limits.h>
···
 #include <bpf/hashmap.h>
 #include <bpf/libbpf.h> /* libbpf_num_possible_cpus */
 #include <bpf/btf.h>
+#include <zlib.h>
 
 #include "main.h"
 
···
 		return -ENAMETOOLONG;
 
 	return 0;
+}
+
+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;
+}
+
+int read_kernel_config(const struct kernel_config_option *requested_options,
+		       size_t num_options, char **out_values,
+		       const char *define_prefix)
+{
+	struct utsname utsn;
+	char path[PATH_MAX];
+	gzFile file = NULL;
+	char buf[4096];
+	char *value;
+	size_t i;
+	int ret = 0;
+
+	if (!requested_options || !out_values || num_options == 0)
+		return -1;
+
+	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));
+		return -1;
+	}
+
+	if (!gzgets(file, buf, sizeof(buf)) || !gzgets(file, buf, sizeof(buf))) {
+		p_info("skipping kernel config, can't read from file: %s",
+			strerror(errno));
+		ret = -1;
+		goto end_parse;
+	}
+
+	if (strcmp(buf, "# Automatically generated file; DO NOT EDIT.\n")) {
+		p_info("skipping kernel config, can't find correct file");
+		ret = -1;
+		goto end_parse;
+	}
+
+	while (read_next_kernel_config_option(file, buf, sizeof(buf), &value)) {
+		for (i = 0; i < num_options; i++) {
+			if ((define_prefix && !requested_options[i].macro_dump) ||
+			     out_values[i] || strcmp(buf, requested_options[i].name))
+				continue;
+
+			out_values[i] = strdup(value);
+		}
+	}
+
+end_parse:
+	gzclose(file);
+	return ret;
 }

tools/bpf/bpftool/feature.c

···
 #ifdef USE_LIBCAP
 #include <sys/capability.h>
 #endif
-#include <sys/utsname.h>
 #include <sys/vfs.h>
 
 #include <linux/filter.h>
···
 
 #include <bpf/bpf.h>
 #include <bpf/libbpf.h>
-#include <zlib.h>
 
 #include "main.h"
 
···
 	}
 }
 
-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(const char *define_prefix)
 {
-	static const struct {
-		const char * const name;
-		bool macro_dump;
-	} options[] = {
+	struct kernel_config_option options[] = {
 		/* Enable BPF */
 		{ "CONFIG_BPF", },
 		/* Enable bpf() syscall */
···
 		{ "CONFIG_HZ", true, }
 	};
 	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 ((define_prefix && !options[i].macro_dump) ||
-			    values[i] || strcmp(buf, options[i].name))
-				continue;
-
-			values[i] = strdup(value);
-		}
-	}
+	if (read_kernel_config(options, ARRAY_SIZE(options), values,
+			       define_prefix))
+		return;
 
 	for (i = 0; i < ARRAY_SIZE(options); i++) {
 		if (define_prefix && !options[i].macro_dump)
···
 		print_kernel_option(options[i].name, values[i], define_prefix);
 		free(values[i]);
 	}
-
-end_parse:
-	if (file)
-		gzclose(file);
 }
 
 static bool probe_bpf_syscall(const char *define_prefix)

tools/bpf/bpftool/main.h

···
 /* print netfilter bpf_link info */
 void netfilter_dump_plain(const struct bpf_link_info *info);
 void netfilter_dump_json(const struct bpf_link_info *info, json_writer_t *wtr);
+
+struct kernel_config_option {
+	const char *name;
+	bool macro_dump;
+};
+
+int read_kernel_config(const struct kernel_config_option *requested_options,
+		       size_t num_options, char **out_values,
+		       const char *define_prefix);
 #endif