Re: [PATCH bpf-next v4 1/3] libbpf: Introduce 'btf_custom_path' to 'bpf_obj_open_opts'

btf_custom_path allows developers to load custom BTF, and subsequent
CO-RE will use custom BTF for relocation.

Learn from Andrii's comments in [0], add the btf_custom_path parameter
to bpf_obj_open_opts, you can directly use the skeleton's
<objname>_bpf__open_opts function to pass in the btf_custom_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 (local)
        [1]https://yhbt.net/lore/all/CAEf4Bzbgw49w2PtowsrzKQNcxD4fZRE6AKByX-5-dMo-+oWHHA@mail.gmail.com/

Signed-off-by: Shuyi Cheng <redacted>
---
 tools/lib/bpf/libbpf.c | 36 ++++++++++++++++++++++++++++++------
 tools/lib/bpf/libbpf.h |  9 ++++++++-
 2 files changed, 38 insertions(+), 7 deletions(-)

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 1e04ce7..6e11a7b 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c

@@ -498,6 +498,13 @@ struct bpf_object {
         * it at load time.
         */
        struct btf *btf_vmlinux;
+       /* Path to the custom BTF to be used for BPF CO-RE relocations.
+        * This custom BTF completely replaces the use of vmlinux BTF
+        * for the purpose of CO-RE relocations.
+        * NOTE: any other BPF feature (e.g., fentry/fexit programs,
+        * struct_ops, etc) will need actual kernel BTF at /sys/kernel/btf/vmlinux.
+        */

+       char *btf_custom_path;
        /* vmlinux BTF override for CO-RE relocations */
        struct btf *btf_vmlinux_override;
        /* Lazily initialized kernel module BTFs */

@@ -2645,10 +2652,6 @@ static bool obj_needs_vmlinux_btf(const struct bpf_object *obj)
        struct bpf_program *prog;
        int i;

-       /* CO-RE relocations need kernel BTF */
-       if (obj->btf_ext && obj->btf_ext->core_relo_info.len)
-               return true;
-
        /* Support for typed ksyms needs kernel BTF */
        for (i = 0; i < obj->nr_extern; i++) {
                const struct extern_desc *ext;

@@ -2665,6 +2668,13 @@ static bool obj_needs_vmlinux_btf(const struct bpf_object *obj)
                        return true;
        }

+       /* CO-RE relocations need kernel BTF, only when btf_custom_path
+        * is not specified
+        */
+       if (obj->btf_ext && obj->btf_ext->core_relo_info.len
+               && !obj->btf_custom_path)
+               return true;