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Change sanitization process to preserve original BTF, which might be used by libbpf itself for Kconfig externs, CO-RE relocs, etc, even if kernel is old and doesn't support BTF. To achieve that, if libbpf detects the need for BTF sanitization, it would clone original BTF, sanitize it in-place, attempt to load it into kernel, and if successful, will preserve loaded BTF FD in original `struct btf`, while freeing sanitized local copy. If kernel doesn't support any BTF, original btf and btf_ext will still be preserved to be used later for CO-RE relocation and other BTF-dependent libbpf features, which don't dependon kernel BTF support. Patch takes care to not specify BTF and BTF.ext features when loading BPF programs and/or maps, if it was detected that kernel doesn't support BTF features. Signed-off-by: Andrii Nakryiko <andriin@fb.com> Signed-off-by: Daniel Borkmann <daniel@iogearbox.net> Link: https://lore.kernel.org/bpf/20200708015318.3827358-4-andriin@fb.com