bpf: Hoist type checking for nullable arg types

check_func_arg has a plethora of weird if statements with empty branches.
They work around the fact that *_OR_NULL argument types should accept a
SCALAR_VALUE register, as long as it's value is 0. These statements make
it difficult to reason about the type checking logic.

Instead, skip more detailed type checking logic iff the register is 0,
and the function expects a nullable type. This allows simplifying the type
checking itself.
Signed-off-by: Lorenz Bauer <lmb@cloudflare.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/bpf/20200921121227.255763-11-lmb@cloudflare.com

kernel/bpf/verifier.c

@@ -435,6 +435,15 @@ static bool arg_type_may_be_refcounted(enum bpf_arg_type type)
 	return type == ARG_PTR_TO_SOCK_COMMON;
 }
 
+static bool arg_type_may_be_null(enum bpf_arg_type type)
+{
+	return type == ARG_PTR_TO_MAP_VALUE_OR_NULL ||
+	       type == ARG_PTR_TO_MEM_OR_NULL ||
+	       type == ARG_PTR_TO_CTX_OR_NULL ||
+	       type == ARG_PTR_TO_SOCKET_OR_NULL ||
+	       type == ARG_PTR_TO_ALLOC_MEM_OR_NULL;
+}
+
 /* Determine whether the function releases some resources allocated by another
  * function call. The first reference type argument will be assumed to be
  * released by release_reference().
@@ -3988,17 +3997,20 @@ static int check_func_arg(struct bpf_verifier_env *env, u32 arg,
 			return err;
 	}
 
+	if (register_is_null(reg) && arg_type_may_be_null(arg_type))
+		/* A NULL register has a SCALAR_VALUE type, so skip
+		 * type checking.
+		 */
+		goto skip_type_check;
+
 	if (arg_type == ARG_PTR_TO_MAP_KEY ||
 	    arg_type == ARG_PTR_TO_MAP_VALUE ||
 	    arg_type == ARG_PTR_TO_UNINIT_MAP_VALUE ||
 	    arg_type == ARG_PTR_TO_MAP_VALUE_OR_NULL) {
 		expected_type = PTR_TO_STACK;
-		if (register_is_null(reg) &&
-		    arg_type == ARG_PTR_TO_MAP_VALUE_OR_NULL)
-			/* final test in check_stack_boundary() */;
-		else if (!type_is_pkt_pointer(type) &&
-			 type != PTR_TO_MAP_VALUE &&
-			 type != expected_type)
+		if (!type_is_pkt_pointer(type) &&
+		    type != PTR_TO_MAP_VALUE &&
+		    type != expected_type)
 			goto err_type;
 	} else if (arg_type == ARG_CONST_SIZE ||
 		   arg_type == ARG_CONST_SIZE_OR_ZERO ||
@@ -4013,11 +4025,8 @@ static int check_func_arg(struct bpf_verifier_env *env, u32 arg,
 	} else if (arg_type == ARG_PTR_TO_CTX ||
 		   arg_type == ARG_PTR_TO_CTX_OR_NULL) {
 		expected_type = PTR_TO_CTX;
-		if (!(register_is_null(reg) &&
-		      arg_type == ARG_PTR_TO_CTX_OR_NULL)) {
-			if (type != expected_type)
-				goto err_type;
-		}
+		if (type != expected_type)
+			goto err_type;
 	} else if (arg_type == ARG_PTR_TO_SOCK_COMMON) {
 		expected_type = PTR_TO_SOCK_COMMON;
 		/* Any sk pointer can be ARG_PTR_TO_SOCK_COMMON */
@@ -4026,11 +4035,8 @@ static int check_func_arg(struct bpf_verifier_env *env, u32 arg,
 	} else if (arg_type == ARG_PTR_TO_SOCKET ||
 		   arg_type == ARG_PTR_TO_SOCKET_OR_NULL) {
 		expected_type = PTR_TO_SOCKET;
-		if (!(register_is_null(reg) &&
-		      arg_type == ARG_PTR_TO_SOCKET_OR_NULL)) {
-			if (type != expected_type)
-				goto err_type;
-		}
+		if (type != expected_type)
+			goto err_type;
 	} else if (arg_type == ARG_PTR_TO_BTF_ID) {
 		expected_type = PTR_TO_BTF_ID;
 		if (type != expected_type)
@@ -4041,27 +4047,16 @@ static int check_func_arg(struct bpf_verifier_env *env, u32 arg,
 			goto err_type;
 	} else if (arg_type_is_mem_ptr(arg_type)) {
 		expected_type = PTR_TO_STACK;
-		/* One exception here. In case function allows for NULL to be
-		 * passed in as argument, it's a SCALAR_VALUE type. Final test
-		 * happens during stack boundary checking.
-		 */
-		if (register_is_null(reg) &&
-		    (arg_type == ARG_PTR_TO_MEM_OR_NULL ||
-		     arg_type == ARG_PTR_TO_ALLOC_MEM_OR_NULL))
-			/* final test in check_stack_boundary() */;
-		else if (!type_is_pkt_pointer(type) &&
-			 type != PTR_TO_MAP_VALUE &&
-			 type != PTR_TO_MEM &&
-			 type != PTR_TO_RDONLY_BUF &&
-			 type != PTR_TO_RDWR_BUF &&
-			 type != expected_type)
+		if (!type_is_pkt_pointer(type) &&
+		    type != PTR_TO_MAP_VALUE &&
+		    type != PTR_TO_MEM &&
+		    type != PTR_TO_RDONLY_BUF &&
+		    type != PTR_TO_RDWR_BUF &&
+		    type != expected_type)
 			goto err_type;
 	} else if (arg_type_is_alloc_mem_ptr(arg_type)) {
 		expected_type = PTR_TO_MEM;
-		if (register_is_null(reg) &&
-		    arg_type == ARG_PTR_TO_ALLOC_MEM_OR_NULL)
-			/* final test in check_stack_boundary() */;
-		else if (type != expected_type)
+		if (type != expected_type)
 			goto err_type;
 	} else if (arg_type_is_int_ptr(arg_type)) {
 		expected_type = PTR_TO_STACK;
@@ -4098,6 +4093,7 @@ static int check_func_arg(struct bpf_verifier_env *env, u32 arg,
 			return err;
 	}
 
+skip_type_check:
 	if (reg->ref_obj_id) {
 		if (meta->ref_obj_id) {
 			verbose(env, "verifier internal error: more than one arg with ref_obj_id R%d %u %u\n",