bpf: Refactor verifier dynptr into get_dynptr_arg_reg

This commit refactors the logic for determining which register in a
function is the dynptr into "get_dynptr_arg_reg". This will be used
in the future when the dynptr reg for BPF_FUNC_dynptr_write will need
to be obtained in order to support writes for skb dynptrs.

Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
Link: https://lore.kernel.org/r/20230301154953.641654-6-joannelkoong@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/verifier.c

@@ -6689,6 +6689,28 @@ int check_func_arg_reg_off(struct bpf_verifier_env *env,
 	}
 }
 
+static struct bpf_reg_state *get_dynptr_arg_reg(struct bpf_verifier_env *env,
+						const struct bpf_func_proto *fn,
+						struct bpf_reg_state *regs)
+{
+	struct bpf_reg_state *state = NULL;
+	int i;
+
+	for (i = 0; i < MAX_BPF_FUNC_REG_ARGS; i++)
+		if (arg_type_is_dynptr(fn->arg_type[i])) {
+			if (state) {
+				verbose(env, "verifier internal error: multiple dynptr args\n");
+				return NULL;
+			}
+			state = &regs[BPF_REG_1 + i];
+		}
+
+	if (!state)
+		verbose(env, "verifier internal error: no dynptr arg found\n");
+
+	return state;
+}
+
 static int dynptr_id(struct bpf_verifier_env *env, struct bpf_reg_state *reg)
 {
 	struct bpf_func_state *state = func(env, reg);
@@ -8326,43 +8348,41 @@ static int check_helper_call(struct bpf_verifier_env *env, struct bpf_insn *insn
 		}
 		break;
 	case BPF_FUNC_dynptr_data:
-		for (i = 0; i < MAX_BPF_FUNC_REG_ARGS; i++) {
-			if (arg_type_is_dynptr(fn->arg_type[i])) {
-				struct bpf_reg_state *reg = &regs[BPF_REG_1 + i];
-				int id, ref_obj_id;
-
-				if (meta.dynptr_id) {
-					verbose(env, "verifier internal error: meta.dynptr_id already set\n");
-					return -EFAULT;
-				}
-
-				if (meta.ref_obj_id) {
-					verbose(env, "verifier internal error: meta.ref_obj_id already set\n");
-					return -EFAULT;
-				}
+	{
+		struct bpf_reg_state *reg;
+		int id, ref_obj_id;
 
-				id = dynptr_id(env, reg);
-				if (id < 0) {
-					verbose(env, "verifier internal error: failed to obtain dynptr id\n");
-					return id;
-				}
+		reg = get_dynptr_arg_reg(env, fn, regs);
+		if (!reg)
+			return -EFAULT;
 
-				ref_obj_id = dynptr_ref_obj_id(env, reg);
-				if (ref_obj_id < 0) {
-					verbose(env, "verifier internal error: failed to obtain dynptr ref_obj_id\n");
-					return ref_obj_id;
-				}
 
-				meta.dynptr_id = id;
-				meta.ref_obj_id = ref_obj_id;
-				break;
-			}
+		if (meta.dynptr_id) {
+			verbose(env, "verifier internal error: meta.dynptr_id already set\n");
+			return -EFAULT;
 		}
-		if (i == MAX_BPF_FUNC_REG_ARGS) {
-			verbose(env, "verifier internal error: no dynptr in bpf_dynptr_data()\n");
+		if (meta.ref_obj_id) {
+			verbose(env, "verifier internal error: meta.ref_obj_id already set\n");
 			return -EFAULT;
 		}
+
+		id = dynptr_id(env, reg);
+		if (id < 0) {
+			verbose(env, "verifier internal error: failed to obtain dynptr id\n");
+			return id;
+		}
+
+		ref_obj_id = dynptr_ref_obj_id(env, reg);
+		if (ref_obj_id < 0) {
+			verbose(env, "verifier internal error: failed to obtain dynptr ref_obj_id\n");
+			return ref_obj_id;
+		}
+
+		meta.dynptr_id = id;
+		meta.ref_obj_id = ref_obj_id;
+
 		break;
+	}
 	case BPF_FUNC_user_ringbuf_drain:
 		err = __check_func_call(env, insn, insn_idx_p, meta.subprogno,
 					set_user_ringbuf_callback_state);