WebSVN – HelenOS-doc – Diff – /papers/isarcs10/isarcs10.tex

 \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 compiance
         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
 ~\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. Fortunatelly 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 enginering, 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 design. 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 component, too.
                 \item[Split of mechanism and policy] The kernel should only provide low-level mechanisms,
                       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 that do not allow to make many general assumptions about the code.
             Programming languages which target controlled environments such as Java or 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*{120mm}{!}{\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
         operating system. 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 complementing the formal guarantees 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 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 intuitivelly 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. First, we would certainly like to
+            The initial levels of abstraction do not go far from the C source code and common engineering
-            know whether our code base is compliant with the programming language specification and passes
+            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
-            only the basic semantic checks (proper number and types of arguments passed to functions, etc.).
+            and types of arguments passed to functions, etc.). It is perhaps not very surprising that
-            It is perhaps not very surprising that these decisions can be make by any plain C compiler.
+            these decisions can be make by any plain C compiler. However, with the current implementation
-            However, with the current implementation of HelenOS even this is not quite trivial.
+            of HelenOS even this is not quite trivial.
             Besides the requirement to support 7 hardware platforms, the system's compile-time configuration
-            can be also affected by approximately 65 configuration options, most of which are booleans, the rest
+            can be also affected by approximately 65 configuration options, most of which are booleans,
-            are enumerated types.
+            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. Thus, the overall number of distinct configurations in which
+            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 the relevant higher
+            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.
+            representative ones is perhaps a reasonable compromise to make the verification realistic.
         \subsection{Regression and Unit Tests}
-            Although some people would argue whether testing is a formal verification method, we still include
-            it into the big picture. Running regression and unit tests which are part of HelenOS code base
+            Running regression and unit tests which are part of HelenOS code base in the continuous
-            in the continuous integration process is fairly easy. The only complication lies in the technicalities:
+            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. They are also rarely utilized
+            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).
-            on regular basis as part of the continuous integration process. But again, it might be helpful
+            They are also rarely utilized on regular basis as part of the continuous integration process.
-            to just mention them in the context of regression and unit tests.
+            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 not considered to be formal verification tools. Many people
+            C language compilers are traditionally also not considered to be formal verification tools.
-            just say that C compilers are good at generating executable code, but do not care much about the semantics
+            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
-            of the source code (on the other hand, formal verification tools usually do not generate any executable code
+            tools usually do not generate any executable code at all). However, with recent development
-            at all). However, with recent development in the compiler domain, the old paradigms are shifting.
+            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
+            Secondly, the checks run by the verifying compilers are usually not based on abstract interpretation.
-            or exhaustive traversal of a model state space. They are mostly realized as abstract syntax tree
+            They are mostly realized as abstract syntax tree transformations much in the line with the supporting
-            transformations much in the line with the supporting routines of the compilation process (data
+            routines of the compilation process (data and control flow graph analysis, dead code elimination,
-            and control flow graph analysis, dead code elimination, register allocation, etc.) and the evaluation
+            register allocation, etc.) and the evaluation function is basically the matching of antipatterns
-            function is basically the matching of antipatterns of common programming bugs.
+            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 definition of properties is needed and false positives can be relatively easily eliminated by amending
+            No manual definition of code-specific properties is needed and false positives can be relatively easily
-            some explicit additional information to the source code within the boundaries of the programming language.
+            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
+            The authors of static analyzers claim large quantities of bugs detected or prevented~\cite{billion},
-            analyzers are still relatively limited by the kind of bugs they are designed to detect. They
+            but static analyzers are still relatively limited by the kind of bugs they are designed to detect.
-            are usually good at pointing out common issues with security implications (specific types of buffer
+            They are usually good at pointing out common issues with security implications (specific types of
-            and stack overruns, usage of well-known functions in an unsafe way, clear cases of forgotten
+            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 operating system kernel and core libraries two kinds of properties are
             common:
-            \begin{itemize}
+            \begin{description}
-                \item \emph{Consistency constrains:} These properties define the correct way how data is supposed
+                \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 \emph{Interface enforcement:} These properties define the correct semantics by which
+                \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 possible error states are detected and reported.
+                      and all reported exceptions are handled by the client code.
-            \end{itemize}
+            \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 operating system. 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
             address 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{Behavior Checker}
+        \subsection{Architecture and Behavior Checker}
-            All previously mentioned verification methods were targeting internal properties of the operating system
+            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. If we are moving to a higher-level abstraction in order to specify correct
-            components in terms of interface compatibility and communication, we can utilize \emph{Behavior Protocols}~\cite{bp}.
+            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 operating system in terms of software
+            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}.
+            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.
-            This language has the possibility to capture interface types (with method signatures), primitive
+            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 desciption 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
-            components (in terms of provided and required interfaces), composite components (an architectural
+                             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 whether
+                             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
+                             questions whether the architecture description of the system is sound with respect to the hierarchical
+                             composition of the components.
+                        \item[Compliance between the specification and the implementation] Using various means
+                             for generating artificial environments for an isolated component the 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.
-            compositions of primitive components) and the bindings between the respective interfaces of the
+                      This is similar to static verification, but on the level of component interfaces.
-            components.
+            \end{description}
-            Unfortunately, the description is usually static, which is not quite suitable for the dynamic
+            Unfortunately, most of the behavior and architecture formalisms are static, which is not quite suitable
-            nature of HelenOS and other operating systems. This limitation can be circumvented by considering a relevant
+            for the dynamic 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.
-            Behavior Protocol checkers can target three main goals:
+            \medskip
-            \begin{itemize}
-                \item \emph{Horizontal compliance:} Also called \emph{compatibility}. The goal is to check
+            The key features of software systems with respect to behavior and architecture checkers are the granularity
-                      whether the specifications of components that are bound together are semantically
+            of the individual primitive components, the level of isolation and complexity of the communication mechanism
-                      compatible. All required interfaces need to be bound to provided interfaces and
+            between them. Large monolithic OSes created in sematic-poor C present a severe challenge because the
-                      the communication between the components cannot lead to \emph{no activity} (a deadlock)
+            isolation of the individual components is vague and the communication between them is basically unlimited
-                      or a \emph{bad activity} (a livelock).
+            (function calls, shared resources, etc.).
-                \item \emph{Vertical compliance:} Also called \emph{substituability}. The goal is to check whether
-                      it is possible to replace a set of primitive components that are nested inside a composite
+            OSes with explicit component architecture and fine-grained components (such as microkernel multiserver
-                      component by the composite component itself. In other words, this compliance can answer the
-                      questions whether the architecture description of the system is sound with respect to the hierarchical
-                      composition of the components.
-                \item \emph{Compliance between the specification and the implementation:} Using various means
+            systems) make the feasibility of the verification much easier, since the degrees of freedom (and thus
-                      for generating artificial environments for an isolated component the checker is able to
-                      partially answer the question whether the implementation of the component is an instantiation
-                      of the component specification.
+            the state space) is limited.
-            \end{itemize}
             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 operating system.
+            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 operating system
+            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 operating
+            depends largely on the size and complexity of the code under verification. If the entire OS
-            system is decomposed into primitive components with a reasonable granularity, it is more likely that the
+            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.
-            In monolithic operating systems without a clear decomposition we cannot do this inference and we
-            need to verify the lower-level properties over a much larger code base (e.g. the whole monolithic kernel).
             \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 formal methods which start at the level of C programming
+            So far, we have proposed a compact combination of engineering, semi-formal and formal methods which
-            language, offer the possibility to check for the presence of various common antipatterns, to check for generic
+            start at the level of C programming language, offer the possibility to check for the presence of various
-            algorithm-related properties, consistency constrains, interface enforcements and conclude with a framework
+            common antipatterns, check for generic algorithm-related properties, consistency constrains, interface
-            to make these properties hold even in the case of a large operating system composed from many
+            enforcements and conclude with a framework to make these properties hold even in the case of a large
-            components of compliant behavior.
+            OS composed from many components of compliant behavior.
-            We are also able to fill in some of the missing pieces by other software engineering approaches
+            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
-            such as regression and unit testing and instrumentation.
+            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-functional
             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, 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{itemize}
+            \begin{description}
-                \item \texttt{-Wfloat-equal} Check for exact equality comparison between floating point values. The
+                \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
+                \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,
+                \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{itemize}
+            \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{itemize}
+            \begin{description}
-                \item \texttt{\_\_attribute\_\_((noreturn))} Functions marked in this way never finish from the point of view
+                \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
+                \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{itemize}
+            \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
+            in detecting memory and generic resource leaks. We have not tried \emph{Valgrind}~\cite{valgrind} or any similar
-            because of the estimated complexity to adopt it for the usage in HelenOS.
+            existing tool because of the estimated complexity to adopt it for the usage in HelenOS.
-            HelenOS was also scanned by \emph{Coverity}~\cite{coverity} in 2006 when no errors were detected. However, since that
+            HelenOS was also scanned by \emph{Coverity}~\cite{coverity} in 2006 when no errors were detected. However, since
-            time the code base has not been analyzed by Coverity.
+            that time the code base has not been analyzed by Coverity.
         \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.
         \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{Behavior Checker}
+        \subsection{Architecture and Behavior Checker}
-            We have created an architecture description in ADL language similar to SOFA ADL~\cite{adl} for the majority of the
+            We have created an architecture description in ADL language derived from \emph{SOFA ADL}~\cite{adl} for the
-            HelenOS components and created the Behavior Protocol specification of these components. The architecture
+            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.
-            is a snapshot of the dynamic architecture just after a successful bootstrap.
+            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
+            on Behavior Protocols (compare to~\cite{cocome}), our current work-in-progress is to optimize and fine-tune
-            checker to process the input.
+            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 have not started to generate code from the architecture description so far because of time constrains.
-            we believe that this is a very promising way to go.
+            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
+            We have not tackled the issue of behavior description generation yet, although tools such as
-            available. We do not consider it our priority at this time.
+            \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 formal verification methods and tools
+        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
+        to achieve the verification of an entire general-purpose operating system. The proposed
-        a bottom-to-top path, starting with low-level checks using state-of-the-art verifying C language compilers,
+        approach generally follows a bottom-up route, starting with low-level checks using state-of-the-art
-        following by static analyzers and static verifiers. In specific contexts regression and unit tests,
+        verifying C language compilers, following by static analyzers and static verifiers.
-        code instrumentation and model checkers for the sake of verification of key algorithms
+        In specific contexts regression and unit tests, code instrumentation and model checkers
-        are utilized.
+        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), we demonstrate
+        system design (e.g. software component encapsulation and composition, fine-grained isolated
-        that it should be feasible to successfully verify larger and more complex operating systems than
+        components), we demonstrate that it should be feasible to successfully verify larger and more
-        in the case of monolithic designs. We use formal component architecture and behavior
+        complex operating systems than in the case of monolithic designs. We use formal component
-        description for the closure. The final goal -- a formally verified operating system -- is the
+        architecture and behavior description for the closure. The final goal -- a formally verified
-        emerging property of the combination of the various methods.
+        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''
+        The contribution of this paper is the shift of focus from attempts to use a single
-        method for formal verification of an operating system to a combination of multiple methods supported
+        ``silver-bullet'' method for formal verification of an operating system to a combination
-        by a suitable architecture of the operating system.
+        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
+        \noindent\textbf{Acknowledgments.} The author would like to express his gratitude to all contributors of
-        project. Without their vision and dedication the work on this paper would be almost impossible
+        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}

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