C++ and SPARK as a continuum

SPARK is an Ada-inspired programming language for high-assurance computing. It was designed for the application of formal verification methods to real-world software. The prospect of bug-free software is music to our ears! I wonder, can there be a love affair between SPARK and Genode's predominant implementation language, C++?

At Genode Labs, our first encounter of SPARK was in 2010. Back then, we got commissioned a project for developing a small graphics library as well as device drivers for Intel graphics and PS/2 input in SPARK 2005. This was quite a ride. Usually, one started out with some nicely written code - only a few lines - carefully crafted with love and attention. The code would work on the first try. The final step would be to make the formal verification tools happy. How hard could that be? Well, the tools were not just picky. They were an insulting combination of picky and dumb. The picky part did not do any harm. I'm grateful whenever a tool calls out my mistakes. But thanks to the dumbness - like forgetting once proven verification conditions two steps down the road - a once beautiful piece of code could degenerate into a mess of repetitions and redundancies under your fingers. At the end of the day, one ended up with a formally proven (yay!) ball of mud. The project succeeded regardless, which made us immensely proud. But at the same time, we were quite happy to put the SPARK "examiner" to rest, I mean, to not ever look at it again. The only lasting trace of our engagement was the addition of a zero-footprint Ada runtime to Genode.

Over the years, we kept following SPARK from a distance. After all, the goals of the language still resonated very well with us, and we are friends with the developers of the Muen separation kernel who are enthusiastic users of the language. You know, social pressure. Eventually, the book "Building High Integrity Applications With Spark" reignited my interest. Admittedly, I'm not a book person. But this one, I enjoyed. The new version of SPARK as described in the book looks nothing like SPARK of olde thymes. The cultivation of proper encapsulation (unlike SPARK 2005), the sensibly added support for generics, and language-ergonomic tricks like disguising verification conditions as assertions, which a programmer like me can intuitively understand, left me impressed. I wondered, would there be a chance to leverage SPARK within Genode?

Genode uses C++ in a special flavour as predominant implementation language and this is unlikely to change anytime soon. However, there are certain areas in our code base where SPARK could shine, in particular when it comes to the parsing of data, the interaction with devices, or the implementation of water-tight policies. C++ code can be good but proven correct code would be better. But unlike formal verification purists, I don't see this as an all-or-nothing question but rather as a case-by-case economic consideration. Following this line of thinking, we have to embrace the idea of hybrid C++/SPARK components. Can this be a love affair? Let's find out.

Cohabitation rules

At the beginning of a new relationship, it is good to establish a few rules. Imagine this SPARK guy moving in into your C++ living community. You better tack some rules at the fridge! Here they are:

C++ code calls C++ code. That's hardly surprising.

SPARK code calls SPARK code. Seems fair.

C++ code calls SPARK code.

SPARK code never calls C++ code!

This implies that SPARK code implements functions and state machines but cannot block for I/O. From C++ perspective, SPARK code looks like a self-contained library.

Genode's API is designed to eliminate implicit side effects from the code. With side effects, I refer to I/O, memory allocations, inter-process communication, and global variables. To my surprise, SPARK is quite lax regarding the latter. The use of package variables and the reliance on so-called elaboration code (a form of global constructors) seems to be idiomatic in the SPARK universe. To harmonize our designated use of SPARK with Genode's philosophy, let's do away with package variables but keep state in records only. The allocation of such a record happens outside the package that defines the record and its operations. In other words, all SPARK packages must be pure. The cool thing is that SPARK allows us to enforce such additional rules easily using pragmas. You may wonder: Variables need to exist somewhere! If there is no SPARK code with any package variable, where are the variables located or allocated? In a Genode component, all variables used by SPARK would be allocated by the C++ code that invokes the SPARK code. A record defined in SPARK can of course aggregate other records in an arbitrarily nested fashion. The instantiation of the outer-most compound record, however, just happens outside of SPARK.

Modeling C++ objects in SPARK

A C++ object corresponds to an instance of a SPARK record. The backing store for the object/record is provided by an opaque object at the C++ side, represented by a Spark::Object as follows:

template <Genode::uint32_t BYTES> struct Spark::Object { static constexpr Genode::uint32_t bytes() { return BYTES; } struct Object_size_mismatch : Genode::Exception { }; long _space[(BYTES + sizeof(long) - 1)/sizeof(long)] { }; };

The BYTES template argument specifies the size of the opaque space reserved for the SPARK record. To illustrate the implementation of a C++ object using this utility, let's use an example class 'Machinery':

struct Machinery : Spark::Object<4> { Machinery(); void heat_up(); Genode::uint32_t temperature() const; };

In the "other" world at the SPARK side of the program, the Machinery type lives in the form of a private record inside a package of the same name. Here is the package specification:

package Machinery is pragma Pure; type Object_Size_Type is mod 2**32 with Size => 32; type Temperature_Type is mod 2**32 with Size => 32; type Machinery_Type is private; function Object_Size (Machinery : Machinery_Type) return Object_Size_Type with Export, Convention => C, External_Name => "_ZN5Spark11object_sizeERKNS_9MachineryE"; procedure Initialize (Machinery : out Machinery_Type) with Export, Convention => C, External_Name => "_ZN5Spark9MachineryC1Ev"; function Temperature (Machinery : Machinery_Type) return Temperature_Type with Export, Convention => C, External_Name => "_ZNK5Spark9Machinery11temperatureEv"; procedure Heat_up (Machinery : in out Machinery_Type) with Export, Convention => C, External_Name => "_ZN5Spark9Machinery7heat_upEv"; private type Machinery_Type is record Temperature : Temperature_Type; end record; end Machinery;

The C symbol names are mangled C++ symbols as obtained from the output of the nm tool for the compiled the C++ code.

It is fantastic that SPARK gives us the freedom to map a SPARK symbol to any C++ symbol we wish and that the Ada/SPARK calling convention is compatible with C and C++. This is very powerful. We use this power to our advantage to attain code that looks natural on each side of the fence. In particular, the Machinery constructor is mapped directly to the Initialize procedure that takes the record as out parameter. Also the other methods temperature and heat_up are mapped directly to their SPARK counterparts. Quite naturally, a C++ const & corresponds to a SPARK in parameter whereas a mutable & corresponds to an out or in-out parameter.

The body of the Machinery package may look like this:

package body Machinery is function Object_Size (Machinery : Machinery_Type) return Object_Size_Type is begin return Machinery'Size / 8; end Object_Size; procedure Initialize (Machinery : out Machinery_Type) is begin Machinery := ( Temperature => 25 ); end Initialize; function Temperature (Machinery : Machinery_Type) return Temperature_Type is begin return Machinery.Temperature; end Temperature; procedure Heat_Up (Machinery : in out Machinery_Type) is begin Machinery.Temperature := 77; end Heat_Up; end Machinery;

The Machinery C++ class is merely an opaque object with a method interface to interact with the object. It is similar in spirit to the so-called PIMPL idiom in C++ except that the backing store for the data members is allocated within the Spark::Object , not via a dynamic memory allocation.

Our notion of a class does not support inheritance or virtual functions. Of course, these concepts can be used on the C++ side. E.g., the Machinery class could have virtual methods in addition to being a SPARK object but this would be transparent to the SPARK world.

Object sizes matter

The above example raises one question though: If the space for the SPARK record is allocated as an opaque member variable of the C++ object, how does one know the number of bytes needed? Imagine that the SPARK record would actually be 12 bytes instead of 4. In this case, the SPARK code would happily operate beyond the bounds of the corresponding C++ object. To prevent such a disaster, the Machinery package provides the function Object_Size , which returns the size of the Machinery_Type record in bytes. By convention, this function is mapped to the C++ function overload object_size(<type>) . With this convention in place, we can safeguard the consistency of the C++ Spark::Object size with the underlying SPARK record via a function template like this:

template <typename T> static inline void Spark::assert_valid_object_size() { if (object_size(*(T *)nullptr) > T::bytes()) throw typename T::Object_size_mismatch(); }

At the construction time of the Genode component, we can add the following assertion:

Spark::assert_valid_object_size<Spark::Machinery>();

Granted, passing a nullptr as in argument into the object_size function looks a bit funny but I justify it with the following presumptions: First, the Object_Size functions are never supposed to actually access the argument. Only compile-time-generated meta data is returned. The object should remain untouched. Second, the functions are called in this way only at start time of the component. So even if the Ada compiler generated unreasonable code that accesses the object, the code would produce a null-pointer access right at the start time of the component when tested for the first time.

Caveats

I anticipate that - a few exceptions notwithstanding - object definitions will generally exist either on the SPARK side or the C++ side, but rarely on both sides at the same time. In the rare case where a type is shared by both worlds, we need to be extra careful. Such a type must be bit-precise. The C++ type must use __attribute__((packed)) and the SPARK record should be defined via the with Size => <NUMBER-OF-BITS> annotation to ensure the memory-layout consistency between both world. If such a binary interface uses enum values, we have to specify the underlying storage type and define the values of each member.

Prospects

The approach outlined above allows us to gradually "SPARKify" Genode components by successively and almost seamlessly replacing C++ objects with SPARK objects. Such objects can be used in the same way as C++ objects. E.g, they can be instantiated any number of times, allocated statically (BSS), or dynamically, or used as member variables, or as a stack variable.

By regarding C++ and SPARK as a continuum rather than an black-and-white decision, we can use SPARK at places where we regard formal verification as most valuable while not restricting Genode components to be entirely static. It gives us Genode developers the chance to slowly embrace the application of formal methods and recognize their benefit in practice.

Who knows where this takes us? I find it fascinating to explore. Furthermore, I learned at this year's FOSDEM about the planned addition of pointers to SPARK similar to the ownership model of Rust. This would eventually clear the way for using SPARK in many low-level areas of our code that would be clearly out of scope at the moment. Exciting times!