[PATCH bpf-next] libbpf: preserve empty DATASEC BTFs during static linking

[Andrii Nakryiko <andrii@kernel.org>]

Ensure that BPF static linker preserves all DATASEC BTF types, even if some of
them might not have any variable information at all. It's not completely clear
in which cases Clang chooses to not emit variable information, so adding
reliable repro is hard. But manual testing showed that this work correctly.

Reported-by: Alexei Starovoitov <ast@kernel.org>
Fixes: 8fd27bf69b86 ("libbpf: Add BPF static linker BTF and BTF.ext support")
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
---
 tools/lib/bpf/linker.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/tools/lib/bpf/linker.c b/tools/lib/bpf/linker.c
index 5e0aa2f2c0ca..2c43943da30c 100644
--- a/tools/lib/bpf/linker.c
+++ b/tools/lib/bpf/linker.c
@@ -94,6 +94,7 @@ struct dst_sec {
 	int sec_sym_idx;
 
 	/* section's DATASEC variable info, emitted on BTF finalization */
+	bool has_btf;
 	int sec_var_cnt;
 	struct btf_var_secinfo *sec_vars;
 
@@ -1436,6 +1437,15 @@ static int linker_append_btf(struct bpf_linker *linker, struct src_obj *obj)
 			continue;
 		dst_sec = &linker->secs[src_sec->dst_id];
 
+		/* Mark section as having BTF regardless of the presence of
+		 * variables. It seems to happen sometimes when BPF object
+		 * file has only static variables inside functions, not
+		 * globally, that DATASEC BTF with zero variables will be
+		 * emitted by Clang. We need to preserve such empty BTF and
+		 * just set correct section size.
+		 */
+		dst_sec->has_btf = true;
+
 		t = btf__type_by_id(obj->btf, src_sec->sec_type_id);
 		src_var = btf_var_secinfos(t);
 		n = btf_vlen(t);
@@ -1717,7 +1727,7 @@ static int finalize_btf(struct bpf_linker *linker)
 	for (i = 1; i < linker->sec_cnt; i++) {
 		struct dst_sec *sec = &linker->secs[i];
 
-		if (!sec->sec_var_cnt)
+		if (!sec->has_btf)
 			continue;
 
 		id = btf__add_datasec(btf, sec->sec_name, sec->sec_sz);
-- 
2.30.2