bpf: Check scalar or invalid register in check_helper_mem_access

Move the check for a NULL or zero register to check_helper_mem_access. This
makes check_stack_boundary easier to understand.
Signed-off-by: Lorenz Bauer <lmb@cloudflare.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/20200921121227.255763-3-lmb@cloudflare.com

kernel/bpf/verifier.c

@@ -3641,18 +3641,6 @@ static int check_stack_boundary(struct bpf_verifier_env *env, int regno,
 	struct bpf_func_state *state = func(env, reg);
 	int err, min_off, max_off, i, j, slot, spi;
 
-	if (reg->type != PTR_TO_STACK) {
-		/* Allow zero-byte read from NULL, regardless of pointer type */
-		if (zero_size_allowed && access_size == 0 &&
-		    register_is_null(reg))
-			return 0;
-
-		verbose(env, "R%d type=%s expected=%s\n", regno,
-			reg_type_str[reg->type],
-			reg_type_str[PTR_TO_STACK]);
-		return -EACCES;
-	}
-
 	if (tnum_is_const(reg->var_off)) {
 		min_off = max_off = reg->var_off.value + reg->off;
 		err = __check_stack_boundary(env, regno, min_off, access_size,
@@ -3797,9 +3785,19 @@ static int check_helper_mem_access(struct bpf_verifier_env *env, int regno,
 					   access_size, zero_size_allowed,
 					   "rdwr",
 					   &env->prog->aux->max_rdwr_access);
-	default: /* scalar_value|ptr_to_stack or invalid ptr */
+	case PTR_TO_STACK:
 		return check_stack_boundary(env, regno, access_size,
 					    zero_size_allowed, meta);
+	default: /* scalar_value or invalid ptr */
+		/* Allow zero-byte read from NULL, regardless of pointer type */
+		if (zero_size_allowed && access_size == 0 &&
+		    register_is_null(reg))
+			return 0;
+
+		verbose(env, "R%d type=%s expected=%s\n", regno,
+			reg_type_str[reg->type],
+			reg_type_str[PTR_TO_STACK]);
+		return -EACCES;
 	}
 }