[PATCH bpf-next] libbpf: Introduce 'custom_btf_path' to 'bpf_obj_open_opts'.

[Shuyi Cheng <chengshuyi@linux.alibaba.com>]

In order to enable the older kernel to use the CO-RE feature, load the
vmlinux btf of the specified path.

Learn from Andrii's comments in [0], add the custom_btf_path parameter
to bpf_obj_open_opts, you can directly use the skeleton's
<objname>_bpf__open_opts function to pass in the custom_btf_path
parameter.

Prior to this, there was also a developer who provided a patch with
similar functions. It is a pity that the follow-up did not continue to
advance. See [1].

	[0]https://lore.kernel.org/bpf/CAEf4BzbJZLjNoiK8_VfeVg_Vrg=9iYFv+po-38SMe=UzwDKJ=Q@mail.gmail.com/#t
	[1]https://yhbt.net/lore/all/CAEf4Bzbgw49w2PtowsrzKQNcxD4fZRE6AKByX-5-dMo-+oWHHA@mail.gmail.com/

Signed-off-by: Shuyi Cheng <chengshuyi@linux.alibaba.com>
---
 tools/lib/bpf/libbpf.c | 23 ++++++++++++++++++++---
 tools/lib/bpf/libbpf.h |  6 +++++-
 2 files changed, 25 insertions(+), 4 deletions(-)

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 1e04ce7..518b19f 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -509,6 +509,8 @@ struct bpf_object {
 	void *priv;
 	bpf_object_clear_priv_t clear_priv;
 
+	char *custom_btf_path;
+
 	char path[];
 };
 #define obj_elf_valid(o)	((o)->efile.elf)
@@ -2679,8 +2681,15 @@ static int bpf_object__load_vmlinux_btf(struct bpf_object *obj, bool force)
 	if (!force && !obj_needs_vmlinux_btf(obj))
 		return 0;
 
-	obj->btf_vmlinux = libbpf_find_kernel_btf();
-	err = libbpf_get_error(obj->btf_vmlinux);
+	if (obj->custom_btf_path) {
+		obj->btf_vmlinux = btf__parse(obj->custom_btf_path, NULL);
+		err = libbpf_get_error(obj->btf_vmlinux);
+		pr_debug("loading custom vmlinux BTF '%s': %d\n", obj->custom_btf_path, err);
+	} else {
+		obj->btf_vmlinux = libbpf_find_kernel_btf();
+		err = libbpf_get_error(obj->btf_vmlinux);
+	}
+
 	if (err) {
 		pr_warn("Error loading vmlinux BTF: %d\n", err);
 		obj->btf_vmlinux = NULL;
@@ -7554,7 +7563,7 @@ int bpf_program__load(struct bpf_program *prog, char *license, __u32 kern_ver)
 __bpf_object__open(const char *path, const void *obj_buf, size_t obj_buf_sz,
 		   const struct bpf_object_open_opts *opts)
 {
-	const char *obj_name, *kconfig;
+	const char *obj_name, *kconfig, *tmp_btf_path;
 	struct bpf_program *prog;
 	struct bpf_object *obj;
 	char tmp_name[64];
@@ -7584,6 +7593,13 @@ int bpf_program__load(struct bpf_program *prog, char *license, __u32 kern_ver)
 	obj = bpf_object__new(path, obj_buf, obj_buf_sz, obj_name);
 	if (IS_ERR(obj))
 		return obj;
+
+	tmp_btf_path = OPTS_GET(opts, custom_btf_path, NULL);
+	if (tmp_btf_path && strlen(tmp_btf_path) < PATH_MAX) {
+		obj->custom_btf_path = strdup(tmp_btf_path);
+		if (!obj->custom_btf_path)
+			return ERR_PTR(-ENOMEM);
+	}
 
 	kconfig = OPTS_GET(opts, kconfig, NULL);
 	if (kconfig) {
@@ -8702,6 +8718,7 @@ void bpf_object__close(struct bpf_object *obj)
 	for (i = 0; i < obj->nr_maps; i++)
 		bpf_map__destroy(&obj->maps[i]);
 
+	zfree(&obj->custom_btf_path);
 	zfree(&obj->kconfig);
 	zfree(&obj->externs);
 	obj->nr_extern = 0;
diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
index 6e61342..16e0f01 100644
--- a/tools/lib/bpf/libbpf.h
+++ b/tools/lib/bpf/libbpf.h
@@ -94,8 +94,12 @@ struct bpf_object_open_opts {
 	 * system Kconfig for CONFIG_xxx externs.
 	 */
 	const char *kconfig;
+	/* Specify the path of vmlinux btf to facilitate the use of CO-RE features
+	 * in the old kernel.
+	 */
+	char *custom_btf_path;
 };
-#define bpf_object_open_opts__last_field kconfig
+#define bpf_object_open_opts__last_field custom_btf_path
 
 LIBBPF_API struct bpf_object *bpf_object__open(const char *path);
 LIBBPF_API struct bpf_object *
-- 
1.8.3.1