libbpf: sanitize VAR to conservative 1-byte INT

If VAR in non-sanitized BTF was size less than 4, converting such VAR
into an INT with size=4 will cause BTF validation failure due to
violationg of STRUCT (into which DATASEC was converted) member size.
Fix by conservatively using size=1.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

tools/lib/bpf/libbpf.c

@@ -1377,8 +1377,13 @@ static void bpf_object__sanitize_btf(struct bpf_object *obj)
 		if (!has_datasec && kind == BTF_KIND_VAR) {
 			/* replace VAR with INT */
 			t->info = BTF_INFO_ENC(BTF_KIND_INT, 0, 0);
-			t->size = sizeof(int);
-			*(int *)(t+1) = BTF_INT_ENC(0, 0, 32);
+			/*
+			 * using size = 1 is the safest choice, 4 will be too
+			 * big and cause kernel BTF validation failure if
+			 * original variable took less than 4 bytes
+			 */
+			t->size = 1;
+			*(int *)(t+1) = BTF_INT_ENC(0, 0, 8);
 		} else if (!has_datasec && kind == BTF_KIND_DATASEC) {
 			/* replace DATASEC with STRUCT */
 			struct btf_var_secinfo *v = (void *)(t + 1);