\documentclass{llncs}
\usepackage{graphicx}

\title{A Road to a Formally Verified General-Purpose Operating System}
\author{Martin D\v{e}ck\'{y}}
\institute{Department of Distributed and Dependable Systems\\
           Faculty of Mathematics and Physics, Charles University\\
           Malostransk\'{e} n\'{a}m\v{e}st\'{i} 25, Prague 1, 118~00, Czech Republic\\
           \email{martin.decky@d3s.mff.cuni.cz}}

\begin{document}
	\maketitle
	
	\begin{abstract}
		Methods of formal description and verification represent a viable way for achieving
		fundamentally bug-free software. However, in reality only a small subset of the existing operating
		systems were ever formally verified, despite the fact that an operating system is a critical part
		of almost any other software system. This paper points out several key design choices which
		should make the formal verification of an operating system easier and presents a work-in-progress
		and initial experiences with formal verification of HelenOS, a state-of-the-art microkernel-based
		operating system, which, however, was not designed specifically with formal verification in mind,
		as this is mostly prohibitive due to time and budget constrains.
		
		The contribution of this paper is the shift of focus from attempts to use a single ``silver-bullet''
		formal verification method which would be able to verify everything to a combination of multiple
		formalisms and techniques to successfully cover various aspects of the operating system.
		A reliable and dependable operating system is the emerging property of the combination,
		thanks to the suitable architecture of the operating system.
	\end{abstract}
	
	\section{Introduction}
		\label{introduction}
		Operating systems (OSes for short) have a somewhat special position among all software.
		OSes are usually designed to run on bare hardware. This means that they do not require
		any special assumptions on the environment except the assumptions on the properties and
		behavior of hardware. In many cases it is perfectly valid to consider the hardware
		as \emph{idealized hardware} (zero mathematical probability of failure, perfect compliance
		with the specifications, etc.). This means that it is solely the OS that defines the
		environment for other software.
		
		OSes represent the lowest software layer and provide services to essentially all other
		software. Considering the principle of recursion, the properties of an OS form the
		assumptions for the upper layers of software. Thus the dependability of end-user and
		enterprise software systems is always limited by the dependability of the OS.
		
		Finally, OSes are non-trivial software on their own. The way they are generally designed
		and programmed (spanning both the kernel and user mode, manipulating execution contexts
		and concurrency, handling critical hardware-related operations) represent significant
		and interesting challenges for software analysis.
		
		\medskip
		
		These are probably the most important reasons that led to several research initiatives
		in the recent years which target the creation of a formally verified OSes from scratch
		(e.g. \cite{seL4}). Methods of formal description and verification provide fundamentally
		better guarantees of desirable properties than non-exhaustive engineering methods such
		as testing.
		
		However, 98~\%\footnote{98~\% of client computers connected to the Internet as of January
		2010~\cite{marketshare}.} of the market share of general-purpose OSes is taken
		by Windows, Mac~OS~X and Linux. These systems were clearly not designed with formal
		verification in mind from the very beginning. The situation on the embedded, real-time
		and special-purpose OSes market is probably different, but it is unlikely that the
		segmentation of the desktop and server OSes market is going to change very rapidly
		in the near future.
		
		The architecture of these major desktop and server OSes is monolithic, which makes
		any attempts to do formal verification on them extremely challenging due to the large
		state space. Fortunately we can observe that aspects of several novel approaches from
		the OS research from the late 1980s and early 1990s (microkernel design, user space
		file system and device drivers, etc.) are slowly emerging in these originally purely
		monolithic implementations.
		
		\medskip
		
		In this paper we show how specific design choices can markedly improve the feasibility
		of verification of an OS, even if the particular OS was not designed
		specifically with formal verification in mind. These design choices can be gradually
		introduced (and in fact some of them have already been introduced) to mainstream
		general-purpose OSes.
		
		Our approach is not based on using a single ``silver-bullet'' formalism, methodology or
		tool, but on combining various engineering, semi-formal and formal approaches.
		While the lesser formal approaches give lesser guarantees, they can complement
		the formal approaches on their boundaries and increase the coverage of the set of
		all hypothetical interesting properties of the system.
		
		We also demonstrate work-in-progress case study of an general-purpose research OS
		that was not created specifically with formal verification in mind from the very
		beginning, but that was designed according to state-of-the-art OS principles.
		
		\medskip
		
		\noindent\textbf{Structure of the Paper.} In Section \ref{design} we introduce
		the design choices and our case study in more detail. In Section \ref{analysis} we
		discuss our approach of the combination of methods and tools. In Section \ref{evaluation}
		we present a preliminary evaluation of our efforts and propose the imminent next steps
		to take. Finally, in Section \ref{conclusion} we present the conclusion of the paper.
	
	\section{Operating Systems Design}
		\label{design}
		Two very common schemes of OS design are \emph{monolithic design} and \emph{microkernel design}.
		Without going into much detail of any specific implementation, we can define the monolithic design as
		a preference to put numerous aspects of the core OS functionality into the kernel, while microkernel
		design is a preference to keep the kernel small, with just a minimal set of features.
		
		The features which are missing from the kernel in the microkernel design are implemented
		in user space, usually by means of libraries, servers (e.g. processes/tasks) and/or software components.
		
		\subsection{HelenOS}
			\label{helenos}
			\emph{HelenOS} is a general-purpose research OS which is being developed at Charles
			University in Prague. The source code is available under the BSD open source license and can be
			freely downloaded from the project web site~\cite{helenos}. The authors of the code base are
			both from the academia and from the open source community (several contributors are employed
			as Solaris kernel developers and many are freelance IT professionals).
			
			HelenOS uses a preemptive priority-feedback scheduler, it supports SMP hardware and it is
			designed to be highly portable. Currently it runs on 7 distinct hardware architectures, including the
			most common IA-32, x86-64 (AMD64), IA-64, SPARC~v9 and PowerPC. It also runs on ARMv7 and MIPS,
			but currently only in simulators and not on physical hardware.
			
			Although HelenOS is still far from being an everyday replacement for Linux or Windows due to the lack
			of end-user applications (whose development is extremely time-consuming, but unfortunately of
			no scientific value), the essential foundations such as file system support and TCP/IP networking
			are already in place.
			
			HelenOS does not currently target embedded devices (although the ARMv7 port can be very easily
			modified to run on various embedded boards) and does not implement real-time features.
			Many development projects such as task snapshoting and migration, support for MMU-less
			platforms and performance monitoring are currently underway.
			
			\medskip
			
			HelenOS can be briefly described as microkernel multiserver OS. However, the actual design
			guiding principles of the HelenOS are more elaborate:
			
			\begin{description}
				\item[Microkernel principle] Every functionality of the OS that does not
				      have to be necessary implemented in the kernel should be implemented in user space. This
				      implies that subsystems such as the file system, device drivers (except those which are
				      essential for the basic kernel functionality), naming and trading services, networking,
				      human interface and similar features should be implemented in user space.
				\item[Full-fledged principle] Features which need to be placed in kernel should
				      be implemented by full-fledged algorithms and data structures. In contrast
				      to several other microkernel OSes, where the authors have deliberately chosen
				      the most simplistic approach (static memory allocation, na\"{\i}ve algorithms, simple data
				      structures), HelenOS microkernel tries to use the most advanced and suitable means available.
				      It contains features such as AVL and B+ trees, hashing tables, SLAB memory allocator, multiple
				      in-kernel synchronization primitives, fine-grained locking and so on.
				\item[Multiserver principle] Subsystems in user space should be decomposed with the smallest
				      reasonable granularity. Each unit of decomposition should be encapsulated in a separate task.
				      The tasks represent software components with isolated address spaces. From the design point of
				      view the kernel can be seen as a separate software component, too.
				\item[Split of mechanism and policy] The kernel should only provide low-level me\-chanisms,
				      while the high-level policies which are built upon these mechanisms should be defined in
				      user space. This also implies that the policies should be easily replaceable while keeping
				      the low-level mechanisms intact.
				\item[Encapsulation principle] The communication between the tasks/components should be
				      implemented only via a set of well-defined interfaces. In the user-to-user case the preferred
				      communication mechanism is HelenOS IPC, which provides reasonable mix of abstraction and
				      performance (RPC-like primitives combined with implicit memory sharing for large data
				      transfers). In case of synchronous user-to-kernel communication the usual syscalls are used.
				      HelenOS IPC is used again for asynchronous kernel-to-user communication.
				\item[Portability principle] The design and implementation should always maintain a high
				      level of platform neutrality and portability. Platform-specific code in the kernel, core
				      libraries and tasks implementing device drivers should be clearly separated from the
				      generic code (either by component decomposition or at least by internal compile-time APIs).
			\end{description}
			
			In Section \ref{analysis} we argue that several of these design principles significantly improve
			the feasibility of formal verification of the entire system. On the other hand, other design principles
			induce new interesting challenges for formal description and verification.
			
			The run-time architecture of HelenOS is inherently dynamic. The bindings between the components are
			not created at compile-time, but during bootstrap and can be modified to a large degree also during
			normal operation mode of the system (via human interaction and external events).
			
			The design of the ubiquitous HelenOS IPC mechanism and the associated threading model present
			the possibility to significantly reduce the size of the state space which needs to be explored
			by formal verification tools, but at the same time it is quite hard to express these
			constrains in many formalisms. The IPC is inherently asynchronous with constant message buffers
			in the kernel and dynamic buffers in user space. It uses the notions of uni-directional bindings,
			mandatory pairing of requests and replies, binding establishment and abolishment handshakes,
			memory sharing and fast message forwarding.
			
			For easier management of the asynchronous messages and the possibility to process multiple
			messages from different peers without the usual kernel threading overhead, the core user space
			library manages the execution flow by so-called \emph{fibrils}. A fibril is a user-space-managed
			thread with cooperative scheduling. A different fibril is scheduled every time the current fibril
			is about to be blocked while sending out IPC requests (because the kernel buffers of the addressee
			are full) or while waiting on an IPC reply. This allows different execution flows within the
			same thread to process multiple requests and replies. To safeguard proper sequencing of IPC
			messages and provide synchronization, special fibril-aware synchronization primitives
			(mutexes, condition variables, etc.) are available.
			
			Because of the cooperative nature of fibrils, they might cause severe performance under-utilization
			in SMP configurations and system-wide bottlenecks. As multicore processors are more and more
			common nowadays, that would be a substantial design flaw. Therefore the fibrils can be also freely
			(and to some degree even automatically) combined with the usual kernel threads, which provide
			preemptive scheduling and true parallelism on SMP machines. Needless to say, this combination is
			also a grand challenge for the formal reasoning.
			
			\medskip
			
			Incidentally, the \emph{full-fledged principle} causes that the size of the HelenOS microkernel is
			considerably larger compared to other ``scrupulous'' microkernel implementations. The average
			footprint of the kernel on IA-32 ranges from 569~KiB when all logging messages, asserts, symbol
			resolution and the debugging kernel console are compiled in, down to 198~KiB for a non-debugging
			production build. But we do not believe that the raw size of the microkernel is a relevant quality
			criterion per se, without taking the actual feature set into account.
			
			\medskip
			
			To sum up, the choice of HelenOS as our case study is based on the fact that it was not designed
			in advance with formal verification in mind (some of the design principles might be beneficial,
			but others might be disadvantageous), but the design of HelenOS is also non-trivial and not obsolete.
			
		\subsection{The C Programming Language}
			A large majority of OSes is coded in the C programming language (HelenOS is no exception
			to this). The choice of C in the case of kernel is usually well-motivated, since the C language was designed
			specifically for implementing system software~\cite{c}: It is reasonably low-level in the sense that it allows
			to access the memory and other hardware resources with similar effectiveness as from assembler;
			It also requires almost no run-time support and it exports many features of the von Neumann hardware
			architecture to the programmer in a very straightforward, but still relatively portable way.
			
			However, what is the biggest advantage of C in terms of run-time performance is also the biggest weakness
			for formal reasoning. The permissive memory access model of C, the lack of any reference safety
			enforcement, the weak type system and generally little semantic information in the code -- all these
			properties do not allow to make many general assumptions about the code.
			
			Programming languages which target controlled environments such as Java and C\(\sharp\) are
			generally easier for formal reasoning because they provide a well-known set of primitives
			and language constructs for object ownership, threading and synchronization. Many non-imperative
			programming languages can be even considered to be a form of ``executable specification'' and thus
			very suitable for formal reasoning. In C, almost everything is left to the programmer who
			is free to set the rules.
			
			\medskip
			
			The reasons for frequent use of C in the user space of many established OSes (and HelenOS) is
			probably much more questionable. In the case of HelenOS, except for the core libraries and tasks
			(such as device drivers), C might be easily replaced by any high-level and perhaps even
			non-imperative programming language. The reasons for using C in this context are mostly historical.
			
			However, as we have stated in Section \ref{introduction}, the way general-purpose OSes
			are implemented changes only slowly and therefore any propositions which require radical modification
			of the existing code base before committing to the formal verification are not realistic.
		
	\section{Analysis}
		\label{analysis}
		
		\begin{figure}[t]
			\begin{center}
				\resizebox*{125mm}{!}{\includegraphics{diag}}
				\caption{Overview of the formalisms and tools proposed.}
				\label{fig:diag}
			\end{center}
		\end{figure}
		
		In this section, we analyze the properties we would like to check in a general-purpose
		OS. Each set of properties usually requires a specific verification method, tool or toolchain.
		
		Our approach will be mostly bottom-up, or, in other words, from the lower levels of abstraction
		to the higher levels of abstraction. If the verification fails on a lower level, it usually
		does not make much sense to continue with the higher levels of abstraction until the issues
		are tackled. A structured overview of the formalisms, methods and tools can be seen on
		Figure \ref{fig:diag}.
		
		\medskip
		
		Some of the proposed methods cannot be called ``formal methods'' in the rigorous understanding
		of the term. However, even methods which are based on semi-formal reasoning and non-exhaustive
		testing provide some limited guarantees in their specific context. A valued property
		of the formal methods is to preserve these limited guarantees even on the higher levels
		of abstraction, thus allowing the semi-formal methods to complement the big picture
		where the formal methods do not provide any feasible verification so far. This increases
		the coverage of the set of all hypothetical interesting properties of the system (although
		it is probably impossible to formally define this entire set).
		
		\medskip
		
		Please note that the titles of the following sections do not follow any particular established
		taxonomy. We have simply chosen the names to be intuitively descriptive.
		
		\subsection{C Language Compiler and Continuous Integration Tool}
			\label{clang}
			The initial levels of abstraction do not go far from the C source code and common engineering
			approaches. First, we would certainly like to know whether our code base is compliant with the
			programming language specification and passes only the basic semantic checks (proper number
			and types of arguments passed to functions, etc.). It is perhaps not very surprising that
			these decisions can be made by any plain C compiler. However, with the current implementation
			of HelenOS even this is not quite trivial.
			
			Besides the requirement to support 7 hardware platforms, the system's com\-pile-time configuration
			can be also affected by approximately 65 configuration options, most of which are booleans,
			the rest are enumerated types.
			
			These configuration options are bound by logical propositions in conjunctive or disjunctive
			normal forms and while some options are freely configurable, the value of others gets inferred
			by the build system of HelenOS. The overall number of distinct configurations in which
			HelenOS can be compiled is at least one order of magnitude larger than the plain number
			of supported hardware platforms.
			
			Various configuration options affect conditional compilation and linking. The programmers
			are used to make sure that the source code compiles and links fine with respect to the
			most common and obvious configurations, but the unforeseen interaction of the less common
			configuration options might cause linking or even compilation errors.
			
			\medskip
			
			A straightforward solution is to generate all distinct configurations, starting from the
			open variables and inferring the others. This can be part of the continuous integration
			process which would try to compile and link the sources in all distinct configurations.
			
			If we want to be really pedantic, we should also make sure that we run all higher
			level verification methods on all configurations generated by this step. That would certainly
			require to multiply the time required by the verification methods at least by the number
			of the distinct configurations. Constraining the set of configurations to just the most
			representative ones is perhaps a reasonable compromise to make the verification realistic.
		
		\subsection{Regression and Unit Tests}
			Running regression and unit tests which are part of HelenOS code base in the continuous
			integration process is fairly easy. The only complication lies in the technicalities:
			We need to setup an automated network of physical machines and simulators which can run the
			appropriate compilation outputs for the specific platforms. We need to be able to reboot
			them remotely and distribute the boot images to them. And last but not least, we need to be
			able to gather the results from them.
			
			Testing is always non-exhaustive, thus the guarantees provided by tests are strictly limited
			to the use cases and contexts which are being explicitly tested. However, it is arguably
			easier to express many common use cases in the primary programming language than in some
			different formalism. As we follow the bottom-up approach, filtering out the most obvious
			bugs by testing can save us a lot of valuable time which would be otherwise waisted by
			a futile verification by more formal (and more time-consuming) methods.
		
		\subsection{Instrumentation}
			Instrumentation tools for detecting memory leaks, performance bottlenecks and soft-deadlocks
			are also not usually considered to be formal verification tools (since it is hard to define
			exact formal properties which are being verified by the non-exhaustive nature of these tools).
			They are also rarely utilized on regular basis as part of the continuous integration process.
			But again, it might be helpful to just mention them in the big picture.
			
			If some regression or unit tests fail, they sometimes do not give sufficient information to
			tell immediately what is the root cause of the issue. In that case running the faulting tests
			on manually or automatically instrumented executable code might provide more data and point
			more directly to the actual bug.
		
		\subsection{Verifying C Language Compiler}
			C language compilers are traditionally also not considered to be formal verification tools.
			Many people just say that C compilers are good at generating executable code, but do not
			care much about the semantics of the source code (on the other hand, formal verification
			tools usually do not generate any executable code at all). However, with recent development
			in the compiler domain, the old paradigms are shifting.
			
			As the optimization passes and general maturity of the compilers improve over time,
			the compilers try to extract and use more and more semantic information from the source code.
			The C language is quite poor on explicit semantic information, but the verifying compilers
			try to rely on vendor-specific language extensions and on the fact that some semantic information
			can be added to the source code without changing the resulting executable code.
			
			The checks done by the verifying compilers cannot result in fatal errors in the usual cases (they
			are just warnings). Firstly, the compilers still need to successfully compile a well-formed C source
			code compliant to some older standard (e.g. C89) even when it is not up with the current quality
			expectations. Old legacy source code should still pass the compilation as it did decades ago.
			
			Secondly, the checks run by the verifying compilers are usually not based on abstract interpretation.
			They are mostly realized as abstract syntax tree transformations much in the line with the supporting
			routines of the compilation process (data and control flow graph analysis, dead code elimination,
			register allocation, etc.) and the evaluation function is basically the matching of antipatterns
			of common programming bugs.
			
			The checks are usually conservative. The verifying compilers identify code constructs which are suspicious,
			which might arise out of programmer's bad intuition and so on, but even these code snippets cannot be
			tagged as definitive bugs (since the programmer can be simply in a position where he/she really wants to
			do something very strange, but nevertheless legitimate). It is upon the programmer
			to examine the root cause of the compiler warning, tell whether it is really a bug or just a false
			positive and fix the issue by either amending some additional semantic information (e.g. adding an
			explicit typecast or a vendor-specific language extension) or rewriting the code.
			
			Although this level of abstraction is coarse-grained and conservative, it can be called semi-formal,
			since the properties which are being verified can be actually defined quite exactly and they
			are reasonably general. They do not deal with single traces of methods, runs and use
			cases like tests, but they deal with all possible contexts in which the code can run.
		
		\subsection{Static Analyzer}
			Static analyzers try to go deeper than verifying compilers. Besides detecting common antipatterns of
			bugs, they also use techniques such as abstract interpretation to check for more complex properties.
			
			Most commercial static analyzers come with a predefined set of properties which cannot be easily changed.
			They are coupled with the commonly used semantics of the environment and generate domain-specific models
			of the software based not only on the syntax of the source code, but also based on the assumptions derived
			from the memory access model, allocation and deallocation rules, tracking of references and tracking of
			concurrency locks.
			
			The biggest advantage of static analyzers is that they can be easily included in the development or
			continuous integration process as an additional automated step, very similar to the verifying compilers.
			No manual definition of code-specific properties is needed and false positives can be relatively easily
			eliminated by amending some explicit additional information to the source code within the boundaries
			of the programming language.
			
			The authors of static analyzers claim large quantities of bugs detected or prevented~\cite{billion},
			but static analyzers are still relatively limited by the kind of bugs they are designed to detect.
			They are usually good at pointing out common issues with security implications (specific types of
			buffer and stack overruns, usage of well-known functions in an unsafe way, clear cases of forgotten
			deallocation of resources and release of locks, etc.). Unfortunately, many static analyzers
			only analyze a single-threaded control flow and are thus unable to detect concurrency issues
			such as deadlocks.
		
		\subsection{Static Verifier}
			There is one key difference between a static analyzer and a static verifier: Static verifiers
			allow the user to specify one's own properties, in terms of preconditions, postconditions and
			invariants in the code. Many static verifiers also target true multithreaded usage patterns
			and have the capability to check proper locking order, hand-over-hand locking and even liveliness.
			
			In the context of an OS kernel and core libraries two kinds of properties are common:
			
			\begin{description}
				\item[Consistency constrains] These properties define the correct way how data is supposed
				      to be manipulated by some related set of subroutines. Checking for these
				      properties ensures that data structures and internal states will not get corrupt due
				      to bugs in the functions and methods which are designed to manipulate them.
				\item[Interface enforcements] These properties define the correct semantics by which
				      a set of subroutines should be used by the rest of the code. Checking for these properties
				      ensures that some API is always used by the rest of the code in a specified way
				      and all reported exceptions are handled by the client code.
			\end{description}
		
		\subsection{Model Checker}
			\label{modelcheck}
			On the first sight it does not seem to be reasonable to consider general model checkers as
			relevant independent tools for formal verification of an existing OS. While many different
			tools use model checkers as their backends, verifying a complete model of the entire
			system created by hand seems to be infeasible both in the sense of time required for the model
			creation and resources required by the checker to finish the exhaustive traversal of the model's
			state space.
			
			Nevertheless, model checkers on their own can still serve a good job verifying abstract
			properties of key algorithms without dealing with the technical details of the implementation.
			Various properties of synchronization algorithms, data structures and communication protocols
			can be verified in the most generic conditions by model checkers, answering the
			question whether they are designed properly in theory.
			
			If the implementation of these algorithms and protocols do not behave correctly, we can be sure
			that the root cause is in the non-compliance between the design and implementation and not a
			fundamental flaw of the design itself.
		
		\subsection{Architecture and Behavior Checker}
			All previously mentioned verification methods were targeting internal properties of the OS
			components. If we are moving to a higher-level abstraction in order to specify correct
			interaction of the encapsulated components in terms of interface compatibility and communication,
			we can utilize \emph{Behavior Protocols}~\cite{bp} or some other formalism describing correct
			interaction between software components.
			
			To gain the knowledge about the architecture of the whole OS in terms of software
			component composition and bindings, we can use \emph{Architecture Description Language}~\cite{adl}
			as the specification of the architecture of the system. This language has the possibility to capture
			interface types (with method signatures), primitive components (in terms of provided and required
			interfaces), composite components (an architectural compositions of primitive components) and the
			bindings between the respective interfaces of the components.
			
			It is extremely important to define the right role of the behavior and architecture description.
			A flawed approach would be to reverse-engineer this description from the source code (either manually
			or via some sophisticated tool) and then verify the compliance between the description and
			the implementation. However, different directions can give more interesting results:
			
			\begin{description}
				\item[Description as specification] Behavior and architecture description created independently
				      on the source code serves the role of specification. This has the following primary
				      goals of formal verification:
				      \begin{description}
				      	\item[Horizontal compliance] Also called \emph{compatibility}. The goal is to check
				      	     whether the specifications of components that are bound together are semantically
				      	     compatible. All required interfaces need to be bound to provided interfaces and
				      	     the communication between the components cannot lead to \emph{no activity} (a deadlock),
				      	     \emph{bad activity} (a livelock) or other communication and synchronization errors.
				      	\item[Vertical compliance] Also called \emph{substituability}. The goal is to check whe\-ther
				      	     it is possible to replace a set of primitive components that are nested inside a composite
				      	     component by the composite component itself. In other words, this compliance can answer the
				      	     question whether the architecture description of the system is sound with respect to the hierarchical
				      	     composition of the components.
				      	\item[Specification and implementation compliance] Using various means
				      	     of generating artificial environments for an isolated component a checker is able to
				      	     partially answer the question whether the implementation of the component is an instantiation
				      	     of the component specification.
				      \end{description}
				\item[Description as abstraction] Generating the behavior and architecture description from the
				      source code by means of abstract interpretation can serve the purpose of verifying various
				      properties of the implementation such as invariants, preconditions and postconditions.
				      This is similar to static verification, but on the level of component interfaces.
			\end{description}
			
			Unfortunately, most of the behavior and architecture formalisms are static, which is not quite suitable
			for the dynamic nature of most OSes. This limitation can be circumvented by considering a relevant
			snapshot of the dynamic run-time architecture. This snapshot fixed in time is equivalent to
			a statically defined architecture.
			
			\medskip
			
			The key features of software systems with respect to behavior and architecture checkers are the granularity
			of the individual primitive components, the level of isolation and complexity of the communication mechanism
			between them. Large monolithic OSes created in semantic-poor C present a severe challenge because the
			isolation of the individual components is vague and the communication between them is basically unlimited
			(function calls, shared resources, etc.).
			
			OSes with explicit component architecture and fine-grained components (such as microkernel multiserver
			systems) make the feasibility of the verification much easier, since the degrees of freedom (and thus
			the state space) is limited.
			
			Horizontal and vertical compliance checking can be done exhaustively. This is a fundamental property
			which allows the reasoning about the dependability of the entire component-based OS.
			Assuming that the lower-level verification methods (described in Sections \ref{clang} to \ref{modelcheck})
			prove some specific properties of the primitive components, we can be sure that the composition of
			the primitive components into composite components and ultimately into the whole OS
			does not break these properties.
			
			The feasibility of many lower-level verification methods from Sections \ref{clang} to \ref{modelcheck}
			depends largely on the size and complexity of the code under verification. If the entire OS
			is decomposed into primitive components with a fine granularity, it is more likely that the
			individual primitive components can be verified against a large number of properties. Thanks to the
			recursive component composition we can then be sure that these properties also hold for the entire system.
			
			\medskip
			
			The compliance between the behavior specification and the actual behavior of the implementation is, unfortunately,
			the missing link in the chain. This compliance cannot be easily verified in an exhaustive manner. If there is
			a discrepancy between the specified and the actual behavior of the components, we cannot conclude anything about
			the properties holding in the entire system.
			
			However, there is one way how to improve the situation: \emph{code generation}. If we generate implementation
			from the specification, the compliance between them is axiomatic. If we are able to generate enough
			code from the specification to run into the hand-written ``business code'' where we check for
			the lower-level properties, the conclusions about the component composition are going to hold.
		
		\subsection{Behavior Description Generator}
			To conclude our path towards higher abstractions we can utilize tools that can
			generate the behavior descriptions from \emph{textual use cases} written in a domain-constrained English.
			These generated artifacts can be then compared (e.g. via vertical compliance checking) with the formal
			specification. Although the comparison might not provide clean-cut results, it can still be
			helpful to confront the more-or-less informal user expectations on the system with the exact formal description.
		
		\subsection{Summary}
			\label{missing}
			So far, we have proposed a compact combination of engineering, semi-formal and formal methods which
			start at the level of C programming language, offer the possibility to check for the presence of various
			common antipatterns, check for generic algorithm-related properties, consistency constrains, interface
			enforcements and conclude with a framework to make these properties hold even in the case of a large
			OS composed from many components of compliant behavior.
			
			We do not claim that there are no missing pieces in the big picture or that the semi-formal verifications
			might provide more guarantees in this setup. However, state-of-the-art OS design guidelines can push
			further the boundaries of practical feasibility of the presented methods. The limited guarantees
			of the low-level methods hold even in the composition and the high-level formal methods do not have
			to deal with unlimited degrees of freedom of the primitive component implementation.
			
			\medskip
			
			We have spoken only about the functional properties. In general, we cannot apply the same formalisms
			and methods on extra-functional properties (e.g. timing properties, performance properties, etc.).
			And although it probably does make a good sense to reason about component composition for the extra-functi\-onal
			properties, the exact relation might be different compared to the functional properties.
			
			The extra-functional properties need to be tackled by our future work.
		
	\section{Evaluation}
		\label{evaluation}
		This section copies the structure of the previous Section \ref{analysis} and adds HelenOS-specific
		evaluation of the the proposed formalisms and tools. As this is still largely a work-in-progress,
		in many cases just the initial observations can be made.
		
		The choice of the specific methods, tools and formalisms in this initial phase is mostly motivated
		by their perceived commonality and author's claims about fitness for the given purpose. An important
		part of further evaluation would certainly be to compare multiple particular approaches, tools
		and formalisms to find the optimal combination.
		
		\subsection{Verifying C Language Compiler and Continuous Integration Tool}
			The primary C compiler used by HelenOS is \emph{GNU GCC 4.4.3} (all platforms)~\cite{gcc} and \emph{Clang 2.6.0}
			(IA-32)~\cite{clang}. We have taken some effort to support also \emph{ICC} and \emph{Sun Studio} C compilers,
			but the compatibility with these compilers in not guaranteed.
			
			The whole code base is compiled with the \texttt{-Wall} and \texttt{-Wextra} compilation options. These options turn on
			most of the verification checks of the compilers. The compilers trip on common bug antipatterns such
			as implicit typecasting of pointer types, comparison of signed and unsigned integer values (danger
			of unchecked overflows), the usage of uninitialized variables, the presence of unused local variables,
			NULL-pointer dereferencing (determined by conservative local control flow analysis), functions
			with non-void return typed that do not return any value and so on. We treat all compilation warnings
			as fatal errors (\texttt{-Werror}), thus the code base must pass without any warnings.
			
			We also turn on several more specific and strict checks. These checks helped to discover several
			latent bugs in the source code:
			
			\begin{description}
				\item[\texttt{-Wfloat-equal}] Check for exact equality comparison between floating point values. The
				      usage of equal comparator on floats is usually misguided due to the inherent computational errors
				      of floats.
				\item[\texttt{-Wcast-align}] Check for code which casts pointers to a type with a stricter alignment
				      requirement. On many RISC-based platforms this can cause run-time unaligned access exceptions.
				\item[\texttt{-Wconversion}] Check for code where the implicit type conversion (e.g. from float to integer,
				      between signed and unsigned integers or between integers with different number of bits) can
				      cause the actual value to change.
			\end{description}
			
			To enhance the semantic information in the source code, we use GCC-specific language extensions to annotate
			some particular kernel and core library routines:
			
			\begin{description}
				\item[\texttt{\_\_attribute\_\_((noreturn))}] Functions marked in this way never finish from the point of view
				      of the current sequential execution flow. The most common case are the routines which restore previously saved
				      execution context.
				\item[\texttt{\_\_attribute\_\_((returns\_twice))}] Functions marked in this way may return multiple times from
				      the point of view of the current sequential execution flow. This is the case of routines which save the current
				      execution context (first the function returns as usual, but the function can eventually ``return again''
				      when the context is being restored).
			\end{description}
			
			The use of these extensions has pointed out to several hard-to-debug bugs on the IA-64 platform.
			
			\medskip
			
			The automated continuous integration building system is currently work-in-progress. Thus, we do not
			test all possible configurations of HelenOS with each changeset yet. Currently only
			a representative set of 14 configurations (at least one for each supported platform) is tested by hand
			by the developers before committing any non-trivial changeset.
			
			From occasional tests of other configurations by hand and the frequency of compilation, linkage and
			even run-time problems we conclude that the automated testing of all feasible configurations will
			be very beneficial.
			
		\subsection{Regression and Unit Tests}
			As already stated in the previous section, the continuous integration building system has not been finished
			yet. Therefore regression and unit tests are executed occasionally by hand, which is time consuming
			and prone to human omissions. An automated approach is definitively going to be very helpful.
		
		\subsection{Instrumentation}
			We are in the process of implementing our own code instrumentation framework which is motivated mainly
			by the need to support MMU-less architectures in the future. But this framework might be also very helpful
			in detecting memory and generic resource leaks. We have not tried \emph{Valgrind}~\cite{valgrind} or any similar
			existing tool because of the estimated complexity to adopt it for the usage in HelenOS.
		
		\subsection{Static Analyzer}
			The integration of various static analyzers into the HelenOS continuous integration process is underway.
			For the initial evaluation we have used \emph{Stanse}~\cite{stanse} and \emph{Clang Analyzer}~\cite{clanganalyzer}.
			Both of them showed to be moderately helpful to point out instances of unreachable dead code, use of language
			constructs which have ambiguous semantics in C and one case of possible NULL-pointer dereference.
			
			The open framework of Clang seems to be very promising for implementing domain-specific checks (and at
			the same time it is also a very promising compiler framework). Our mid-term goal is to incorporate some of the features
			of Stanse and VCC (see Section \ref{staticverifier2}) into Clang Analyzer.
			
			HelenOS was also scanned by \emph{Coverity}~\cite{coverity} in 2006 when no errors were detected. However, since
			that time the code base has not been analyzed by Coverity.
		
		\subsection{Static Verifier}
			\label{staticverifier2}
			We have started to extend the source code of HelenOS kernel with annotations understood
			by \emph{Frama-C}~\cite{framac} and \emph{VCC}~\cite{vcc}. Initially we have targeted simple kernel data structures
			(doubly-linked circular lists) and basic locking operations. Currently we are evaluating the initial experiences
			and we are trying to identify the most suitable methodology, but we expect quite promising results.
			
			As the VCC is based on the Microsoft C++ Compiler, which does not support many GCC extensions, we have been
			faced with the requirement to preprocess the source code to be syntactically accepted by VCC. This turned out
			to be feasible.
		
		\subsection{Model Checker}
			We are in the process of creating models of kernel wait queues (basic HelenOS kernel synchronization
			primitive) and futexes (basic user space thread synchronization primitive) using \emph{Promela} and
			verify several formal properties (deadlock freedom, fairness) in \emph{Spin}~\cite{spin}. As both the Promela language
			and the Spin model checker are mature and commonly used tools for such purposes, we expect no major problems
			with this approach. Because both synchronization primitives are relatively complex, utilizing a model checker
			should provide a much more trustworthy proof of the required properties than ``paper and pencil''.
			
			The initial choice of Spin is motivated by its suitability to model threads, their interaction and verify
			properties related to race conditions and deadlocks (which is the common sources of OS-related bugs). Other
			modeling formalisms might be more suitable for different goals.
		
		\subsection{Architecture and Behavior Checker}
			We have created an architecture description in ADL language derived from \emph{SOFA ADL}~\cite{adl} for the
			majority of the HelenOS components and created the Behavior Protocol specification of these components.
			Both descriptions were created independently, not by reverse-engineering the existing source code.
			The architecture is a snapshot of the dynamic architecture just after a successful bootstrap of HelenOS.
			
			Both the architecture and behavior description is readily available as part of the source code repository
			of HelenOS, including tools which can preprocess the Behavior Protocols according to the architecture description
			and create an output suitable for \emph{bp2promela} checker~\cite{bp}.
			
			As the resulting complexity of the description is larger than any of the previously published case studies
			on Behavior Protocols (compare to~\cite{cocome}), our current work-in-progress is to optimize and fine-tune
			the bp2promela checker to process the input.
			
			\medskip
			
			We have not started to generate code from the architecture description so far because of time constrains.
			However, we believe that this is a very promising way to go and provide reasonable guarantees about
			the compliance between the specification and the implementation.
		
		\subsection{Behavior Description Generator}
			We have not tackled the issue of behavior description generation yet, although tools such as
			\emph{Procasor}~\cite{procasor} are readily available. We do not consider it our priority at this time.
	
	\section{Conclusion}
		\label{conclusion}
		In this paper we propose a complex combination of various verification methods and tools
		to achieve the verification of an entire general-purpose operating system. The proposed
		approach generally follows a bottom-up route, starting with low-level checks using state-of-the-art
		verifying C language compilers, following by static analyzers and static verifiers.
		In specific contexts regression and unit tests, code instrumentation and model checkers
		for the sake of verification of key algorithms are utilized.
		
		Thanks to the properties of state-of-the-art microkernel multiserver operating
		system design (e.g. software component encapsulation and composition, fine-grained isolated
		components), we demonstrate that it should be feasible to successfully verify larger and more
		complex operating systems than in the case of monolithic designs. We use formal component
		architecture and behavior description for the closure. The final goal -- a formally verified
		operating system -- is the emerging property of the combination of the various methods.
		
		\medskip
		
		The contribution of this paper is the shift of focus from attempts to use a single
		``silver-bullet'' method for formal verification of an operating system to a combination
		of multiple methods supported by a suitable architecture of the operating system.
		The main benefit is a much larger coverage of the set of all hypothetical properties.
		
		We also argue that the approach should be suitable for the mainstream
		general-purpose operating systems in the near future, because they are gradually
		incorporating more microkernel-based features and fine-grained software components.
		
		Although the evaluation of the proposed approach on HelenOS is still work-in-progress, the
		preliminary results and estimates are promising.
		
		\medskip
		
		\noindent\textbf{Acknowledgments.} The author would like to express his gratitude to all contributors of
		the HelenOS project. Without their vision and dedication the work on this paper would be almost impossible
		
		This work was partially supported by the Ministry of Education of the Czech Republic
		(grant MSM\-0021620838).
	
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\end{document}