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Janvier 2010
Software aspects of aeronautical certification and static analysis
tools
Gérard LADIER
Airbus / Aerospace Valley
[email protected] 09/2010
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Equipment Rules (JAR/FAR 25-1309)
• “Essential” equipment must be designed to perform its intended functions
• The airplane systems and associated components, considered separately and in relation to other systems, must be designed so that :
– The occurrence of any failure condition which would prevent the continued safe flight and landing of the airplane is extremely improbable, and
– The occurrence of any other failure conditions which would reduce the capability of the airplane or the ability of the crew to cope with adverse operating conditions is improbable
• ...
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Means of conformance
Software
It is in general not feasible to assess the number or kinds of software errors, if any, that may remain after the completion of system design, development, and test. DO-178B/ED-12B, provides acceptable means for assessing and controlling the software used to program digital -computer-based systems ”
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Define HLR's
Process T(q)
Define HLR(1)
Process T(q+1)
Define HLR(2)
Process T(q+2)
Produce Object Code
Process T(x)
HLR
HLR(1)+ Implementation detail
HLR(2)+ Implementation detail
Object Code
First principle
We can’t get clean water from a dirty pipe
Evidences are requested on the pipe …
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DO-178/ED-12 : evidences on the pipe …
• “DO-178B/ED-12B is primarily a process-oriented document” => Set of requirements on the processes (dev., verif, etc.) and
their outputs
• “The occurrence of any failure condition which would prevent the continued safe flight and landing of the airplane is extremely improbable”
=> Evidences on the fulfilment of these requirements vary depending on the software “criticality” level
ObjectifApplicabilité par niveau
logicielProduit
Catégorie de contrôle par
niveau logicieldescription Réf A B C D Description Réf. A B C D
3 La couverture de test des exigences de haut niveau est assurée
6.4.4.1 Résultats de Vérification du Logiciel
11.14
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Life cycle and processes
• Definition of separate processes that will be combined for a given project to describe its life cycle:
– Planning process (organization/plans rather than scheduling)
– Development process (specification, design, coding, integration)
– Integral processes (verification, configuration management, quality assurance, certification liaison process).
• Define for each process:– The Assurance objectives
– The means of achieving those objectives
– The process input data
– The process activities
– The process products
– The transition criteria, which must be met in order to proceed
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Main common requirements on the developement processes• Standards must be written and evidences of compliance with the rules should be
provided• Rules ?
Dozen of documentsDefine precisely how to perform an activity (methods, means, constraints,
expected outputs, etc.
Each requirement
or design item should be
precise
verifiabledeveloped
tracable
consistent
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Configuration management & Quality Assurance
To assist in satisfying general objectives to:
Control configuration of the software throughout the software life cycle
Be able to replicate the executable object code
Control process inputs and outputs during the software life cycle
Provide baselines for review, assessment and change control
Ensure problems management and change control
Ensure archiving and recovery.
To provide evidences that:What’s done is what’s described in plansTransition criteria are reachedA conformity review of the software is conducted
Main characteristics :IndependenceActive role during the life cycle process
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Tools qualification -1
Necessary when processes required by the rest of
the document are eliminated, reduced
or automated by the use of
a deterministic software
tool whose outputs are not verified.
3 qualification criteria depending on the
risk associated to the tool usage :
Criteria 1 : Development Tool
Criteria 3 : verification tool
Criteria 2 : Verification tool which could have an impact on the resulting software :
used to justify the elimination or reduction of:Verification process other than that automated by the tool,or Development process which could have an impact on the
resulting software
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Tools qualification - 2
– Combination of the qualification criteria with the software level to give the Tool Qualification Level:
Two distinct roles are defined :– The user
– The developper
• He defines his needs (Tool Operational Requirements-TOR) and validate the tool in its usage context
• He defines his development specification (Tool Requirement-TR), develops the tool and provides the life cycle data
Sharing of activities between these two roles are defined for COTS tools(11.3)
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TQL 1, 2 et 3 :– TQL 4 ones + « implementation » requirements :
• Depending on the level of the final product software developed with the tool
TQL 4 : – TQL 5 ones + «project management» requirements :
• TOR reviews (complete, accurate, and consistent)
• Definition of processes in plans
• Definition of the TR and verification / TOR
• Verification of the tool / TR et requirements coverage
• Configuration and change managementsTQL 5 :
– «contracting authority» requirements, mainly concentrated in the TOR :• Table T-0 defining all the “user” requirements including the validation of the
tool versus the need expressed in the “avionics” PSAC
• Plus one requirement : «Impact of known problems on the TOR »
Tools qualification - 3
Qualification requirements :
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Second principe
A clean pipe may not deliver clean water
Filters are piled to detect and eliminate impurities
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Verification
• The most important section of DO-178/ED-12, in term of : volume : 13 pages of description ( ~ 5 pages for other processes )Workload incurred (A380 : 4 lines of test for line of embedded code …)
• Basic principles:
– “Integral” process => applies to all the development processes
– Combination of :
• Reviews ,
• Analysis
• Tests
to detect and identify errors introduced during development
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Reviews ? Analysis ? Tests ?
Review : inspection of a product by an independant (level A) person; qualitative evaluation
Analysis : detailed examination of a process, potentially done by a tool quantitative evaluation
Test : running the software and comparison of actual outputs to expected ones
– Functional test
– NO TEST BASED ON CODE STRUCTURE
– Functional & Structural coverage analysis
• Three major tools for bug-busters :
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DO-178/ED-12 - The verification process
Compliance: with requirements Conformance: with standards
A-3.2 Accuracy & Consistency A-3.3 HW Compatibility A-3.4 VerifiabilityA-3.5 Conformance A-3.7 Algorithm Accuracy
A-5.3 Verifiability A-5.4 ConformanceA-5.6 Accuracy & Consistency
A-5. 7 Complete & Correct
A-3.1 Compliance A-3.6 Traceability
A-4.1 Compliance A-4.6 Traceability
A-4. 8 Architecture Compatibility
SystemRequirements
High-LevelRequirements
SoftwareArchitecture
Source Code
ExecutableObject Code
(A-2: 3, 4, 5)
(A-2: 7)
(A-2: 6)
(A-2: 1, 2)
Low-LevelRequirements
A-5.1 Compliance A-5.5 Traceability
A-5.2 Compliance
A-6.5 Compatible With Target
A-6.3 Compliance A-6.4 Robustness
A-6.1 Compliance A-6.2 Robustness
A7 Verification of verification (Functional & Structural coverage)
A-4.9 ConsistencyA-4.10 HW Compatibility A-4.11 Verifiability A-4.12 Conformance A-4.13 Partition Integrity
A-4.2 Accuracy & ConsistencyA-4.3 HW Compatibility A-4.4 Verifiability A-4.5 Conformance A-4.7 Algorithm Accuracy
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Third principe
Potentially opposite interests are at stake
Independant authorities assess the process and product
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The ED-12B/DO-178B - certification liaison
• Objective: ensure effective communication/understanding between the applicant and the certification authorities
• Means:– The Plan for Software Aspects of Certification, given to the
Authorities as early as possible– Reviews carried out by the certification authorities
“software” specialists as much as they want
– Software Accomplishment Summary and Software Configuration Index.
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Fourth principe
All the interests but must be taken into account to build a recognized set of requirements
DO-178/ED-12 is built and updated by all the concerned specialists and actors
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Consensus on requirements
• Joint meetings between the RTCA SC-205 EUROCAE WG-71• Openness, consensus :
– More than 1200 people registered on the WEB site– about 120 attendees in each meeting : aircraft manufacturers, engine
makers, equipment suppliers, authorities, scientists and consultants – The final text has to be agreed by each of the attendees
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DO-178C/ED-12C
DO-178C
ED-12C
Supplement A
Supplement B
Supplement …
...Supplement N
Interface Spec
DO-248C/ED-94C FAQ/DP/RATIONALE
DO-278A
/
ED-109A
• The « core document » will be completed by supplements :
Supplement –guidance used in conjunction with DO-178C/ED-12C that addresses the unique nature of a specific technology or a specific method. A supplement adds, deletes or otherwise modifies: objectives, activities, explanatory text, and software life cycle data in DO-178C/ED-12C.
• This structuring principle enables incremental future development of software requirements
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The DO-178C/ED-12C : evolution of the content
• Major improvements are in the supplements : – Object Oriented technology – Model Based Development– Formal Method Technology– Tools Qualification
• In the core document : – Basically clarification and improvement in consistency and
accuracy. – Except for the tool qualification part :
• Separation of the «why ?» part (>in the core doc) from the «how ?» part (>supplement)
• Significant evolution of the « why ? »
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Formal Methods Technology supplement
• The FM introductory section insists on the interest of using FM for verification
• The FA (Formal Analysis) may completely replace :
– Reviews & analysis (except for validation of « derived requirements »)
– Conformance test versus /HLR et /LLR
– Software integration tests
– Robustness tests
• FA may help for the verification of compatibility with the hardware
• FA cannot replace HW/SW integration test
• The structural coverage objectives are achieved if it can be demonstrated that :
– Each requirement is completely covered
– The set of requirements is complete in regards of the attended function
– There is no non expected dependences between output and input data
– There is no dead code
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Fitfh principe
Only requirements are mandatory, not the means
Building the « pipe » is to be dealt with by the suppliers
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Which industrial use for Static Analysis tools ?
Today
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Not so frequent …
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How to convince certification Authorities ?
Example of the “unit level proof of LLR”• Discussion with Cert. Auth. software specialists much before the
actual use of the tool, to get a general feedback (go/no go)
• Demonstration of the soundness of the approach
• Definition of specific rules and standards
• Demonstration of the completeness of the properties
• Definition of LLR as pseudo-code + properties
• Smooth integration in the overall verif./traceability processes
• Detect and eliminate dead code
• Verify the executable code
• Qualify the tool (verification ++ tool)
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Which industrial use for Static Analysis tools ?
Tomorrow …
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Tomorrow … 1
• Proof of absence of Real Time Execution error :– ASTREE (ENS/ABSINT)
• For effective certification credit
• Precision of Floating-point calculus : – Fluctuat (CEA) :
• Abstract Interpretation based; analysis of C or assembly code
• Safe computation of the numerical (rounding) errors introduced by basic operators or input filtering code
• In use by Airbus for evaluation :– Research prototype for the C language– Pre-industrial prototype for the TMS320C33
assembly language
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Tomorrow … 2
• Validation of the compilation – Lcertify (ENS) :
• Research activity in Airbus
– Compcert (INRIA) : • Industrial prototype available• Efficient compilation of a complete subset of the A380 fly-by-
wire software
• Other “translation validation tools” would be highly desirable (e.g. Scade -> C)
• Various analysis of C code : – FRAMA-C (CEA) :
• Plugin WP to succeed CAVEAT • Plugin TASTER for syntaxic control (coding rules enforcement)• Plugin for data flow/control flow verification, coming soon
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Special thanks …
• To the co-chairs of the fantastic formal methods SG of the DO-178C joint committee: – Kelly Hayhurst (NASA) – Duncan Brown (Rolls Royce)
• To the Airbus Formal Methods dream team : – Famantanantsoa Randimbivololona– Jean Souyris– David Delmas
• And to Hervé Delseny from Airbus, who liaised both …
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