[PATCH 5.18 295/879] libbpf: Dont error out on CO-RE relos for overriden weak subprogs

Greg Kroah-Hartman
2022-06-07T22:27:21Z

From: Andrii Nakryiko <andrii@kernel.org>

[ Upstream commit e89d57d938c8fa80c457982154ed6110804814fe ]

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: db2b8b06423c ("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
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
tools/lib/bpf/libbpf.c | 15 +++++++++++----
1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index dabf9a1451c3..7af6805a863d 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -5665,10 +5665,17 @@ 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
--
2.35.1