selftests/bpf: Add check_lto_kernel() helper

Add check_lto_kernel() helper to detect whether the underlying kernel
enabled CONFIG_LTO or not. The function check_lto_kernel() can be
used by selftests to handle some lto-specific situations.

The code is heavily borrowed from libbpf function
bpf_object__read_kconfig_file().

Signed-off-by: Yonghong Song <[email protected]>

Author: Yonghong Song

tools/testing/selftests/bpf/testing_helpers.c

@@ -5,6 +5,8 @@
 #include <stdlib.h>
 #include <string.h>
 #include <errno.h>
+#include <zlib.h>
+#include <sys/utsname.h>
 #include <bpf/bpf.h>
 #include <bpf/libbpf.h>
 #include "test_progs.h"
@@ -475,3 +477,48 @@ bool is_jit_enabled(void)
 
 	return enabled;
 }
+
+int check_lto_kernel(void)
+{
+	static int check_lto = 2;
+	char buf[PATH_MAX];
+	struct utsname uts;
+	gzFile file;
+	int len;
+
+	if (check_lto != 2)
+		return check_lto;
+
+	uname(&uts);
+	len = snprintf(buf, PATH_MAX, "/boot/config-%s", uts.release);
+	if (len < 0) {
+		check_lto = -EINVAL;
+		goto out;
+	} else if (len >= PATH_MAX) {
+		check_lto = -ENAMETOOLONG;
+		goto out;
+	}
+
+	/* gzopen also accepts uncompressed files. */
+	file = gzopen(buf, "re");
+	if (!file)
+		file = gzopen("/proc/config.gz", "re");
+
+	if (!file) {
+		check_lto = -ENOENT;
+		goto out;
+	}
+
+	check_lto = 0;
+	while (gzgets(file, buf, sizeof(buf))) {
+		/* buf also contains '\n', skip it during comparison. */
+		if (!strncmp(buf, "CONFIG_LTO=y", 12)) {
+			check_lto = 1;
+			break;
+		}
+	}
+
+	gzclose(file);
+out:
+	return check_lto;
+}

tools/testing/selftests/bpf/testing_helpers.h

@@ -55,5 +55,6 @@ struct bpf_insn;
 int get_xlated_program(int fd_prog, struct bpf_insn **buf, __u32 *cnt);
 int testing_prog_flags(void);
 bool is_jit_enabled(void);
+int check_lto_kernel(void);
 
 #endif /* __TESTING_HELPERS_H */