bpf: add support for fixed-size memory pointer returns for kfuncs

Support direct fixed-size (and for now, read-only) memory access when
kfunc's return type is a pointer to non-struct type. Calculate type size
and let BPF program access that many bytes directly. This is crucial for
numbers iterator.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20230302235015.2044271-13-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/verifier.c

@@ -10336,6 +10336,14 @@ static int check_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
 				return -EFAULT;
 			}
 		} else if (!__btf_type_is_struct(ptr_type)) {
+			if (!meta.r0_size) {
+				__u32 sz;
+
+				if (!IS_ERR(btf_resolve_size(desc_btf, ptr_type, &sz))) {
+					meta.r0_size = sz;
+					meta.r0_rdonly = true;
+				}
+			}
 			if (!meta.r0_size) {
 				ptr_type_name = btf_name_by_offset(desc_btf,
 								   ptr_type->name_off);