libbpf: Preserve empty DATASEC BTFs during static linking

Ensure that BPF static linker preserves all DATASEC BTF types, even if some of
them might not have any variable information at all. This may happen if the
compiler promotes local initialized variable contents into .rodata section and
there are no global or static functions in the program.

For example,

  $ cat t.c
  struct t { char a; char b; char c; };
  void bar(struct t*);
  void find() {
     struct t tmp = {1, 2, 3};
     bar(&tmp);
  }

  $ clang -target bpf -O2 -g -S t.c
         .long   104                             # BTF_KIND_DATASEC(id = 8)
         .long   251658240                       # 0xf000000
         .long   0

         .ascii  ".rodata"                       # string offset=104

  $ clang -target bpf -O2 -g -c t.c
  $ readelf -S t.o | grep data
     [ 4] .rodata           PROGBITS         0000000000000000  00000090

Fixes: 8fd27bf6 ("libbpf: Add BPF static linker BTF and BTF.ext support")
Reported-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20210326043036.3081011-1-andrii@kernel.org

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 +1437,16 @@ 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. In some cases compiler might generate empty BTF
+		 * with no variables information. E.g., when promoting local
+		 * array/structure variable initial values and BPF object
+		 * file otherwise has no read-only static variables in
+		 * .rodata. 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 +1728,7 @@ 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);