From f0755442ff6747e72cda5a4b79a637f765a4086b Mon Sep 17 00:00:00 2001
From: Jesper Dangaard Brouer <netoptimizer@brouer.com>
Date: Tue, 30 Aug 2022 19:16:41 +0200
Subject: [PATCH] BTF-playground: Cannot get kernel BTF obj ID this way

Signed-off-by: Jesper Dangaard Brouer <netoptimizer@brouer.com>
---
 BTF-playground/btf_module_read.c | 40 ++++++++++++++++++++++++++++++--
 1 file changed, 38 insertions(+), 2 deletions(-)

diff --git a/BTF-playground/btf_module_read.c b/BTF-playground/btf_module_read.c
index 93a29e5..edf252f 100644
--- a/BTF-playground/btf_module_read.c
+++ b/BTF-playground/btf_module_read.c
@@ -11,6 +11,7 @@
 #include <unistd.h>
 #include <getopt.h>
 
+#include <bpf/bpf.h>
 #include <bpf/btf.h> /* Notice libbpf BTF include */
 
 #include <linux/err.h>
@@ -32,11 +33,42 @@ int print_all_levels(enum libbpf_print_level level,
 #define pr_err(fmt, ...) \
 	fprintf(stderr, "%s:%d - " fmt, __FILE__, __LINE__, ##__VA_ARGS__)
 
+int fail1_get_kernel_btf_obj_id(struct btf *btf_obj)
+{
+	/* *** DOES NOT WORK ***
+	 *
+	 * The struct btf returned from btf__load_module_btf() doesn't keep the
+	 * file descriptor open. Thus, we never reach bpf_obj_get_info_by_fd().
+	 *
+	 */
+	struct bpf_btf_info info;
+	__u32 len = sizeof(info);
+	int btf_fd;
+	int err;
+
+	memset(&info, 0, sizeof(info));
+
+	btf_fd = btf__fd(btf_obj);
+	if (btf_fd < 0) {
+		printf("ERR: No FD(%d) in btf_obj:%p\n", btf_fd, btf_obj);
+		return 0;
+	}
+
+	err = bpf_obj_get_info_by_fd(btf_fd, &info, &len); /* Privileged op */
+	if (err) {
+		printf("ERR(%d): Can't get BTF object info on FD(%d): %s\n",
+		       errno, btf_fd, strerror(errno));
+		return 0;
+	}
+
+	return info.id;
+}
 
 int main(int argc, char **argv)
 {
 	struct btf *vmlinux_btf, *module_btf = NULL;
 	int opt, longindex = 0;
+	__u32 btf_obj_id;
 	__s32 type_id;
 	int err = 0;
 
@@ -78,8 +110,12 @@ int main(int argc, char **argv)
 		goto out;
 	}
 
-	printf("Module:%s Symbol:%s have BTF id:%d\n",
-	       module_name, symbol_name, type_id);
+	/* Wanted to get BTF object ID used by kernel that ident BTF */
+	//btf_obj_id = fail1_get_kernel_btf_obj_id(vmlinux_btf);
+	btf_obj_id = fail1_get_kernel_btf_obj_id(module_btf);
+
+	printf("Module:%s (BTF-obj ID:%d) Symbol:%s have BTF type id:%d\n",
+	       module_name, btf_obj_id, symbol_name, type_id);
 
 out:
 	btf__free(module_btf);