Commit e89d57d9 authored Apr 08, 2022 by Andrii Nakryiko Committed by Daniel Borkmann Apr 08, 2022

libbpf: Don't error out on CO-RE relos for overriden weak subprogs

During BPF static linking, all the ELF relocations and .BTF.ext
information (including CO-RE relocations) are preserved for __weak
subprograms that were logically overriden by either previous weak
subprogram instance or by corresponding "strong" (non-weak) subprogram.
This is just how native user-space linkers work, nothing new.

But libbpf is over-zealous when processing CO-RE relocation to error out
when CO-RE relocation belonging to such eliminated weak subprogram is
encountered. Instead of erroring out on this expected situation, log
debug-level message and skip the relocation.

Fixes: db2b8b06 ("libbpf: Support CO-RE relocations for multi-prog sections")
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20220408181425.2287230-2-andrii@kernel.org

parent 587323cf

tools/lib/bpf/libbpf.c

+11 −4

Original line number	Diff line number	Diff line
		@@ -5687,10 +5687,17 @@ bpf_object__relocate_core(struct bpf_object obj, const char targ_btf_path)
		insn_idx = rec->insn_off / BPF_INSN_SZ;
		prog = find_prog_by_sec_insn(obj, sec_idx, insn_idx);
		if (!prog) {
		pr_warn("sec '%s': failed to find program at insn #%d for CO-RE offset relocation #%d\n",
		sec_name, insn_idx, i);
		err = -EINVAL;
		goto out;
		/* When __weak subprog is "overridden" by another instance
		* of the subprog from a different object file, linker still
		* appends all the .BTF.ext info that used to belong to that
		* eliminated subprogram.
		* This is similar to what x86-64 linker does for relocations.
		* So just ignore such relocations just like we ignore
		* subprog instructions when discovering subprograms.
		*/
		pr_debug("sec '%s': skipping CO-RE relocation #%d for insn #%d belonging to eliminated weak subprogram\n",
		sec_name, i, insn_idx);
		continue;
		}
		/* no need to apply CO-RE relocation if the program is
		* not going to be loaded