This document summarizes an overview of the MISRA C coding standard presented at a conference. It provides a brief history of MISRA C, describing the 1998, 2004, and 2012 editions. It discusses some misunderstandings about MISRA C, explaining that it applies to safety-critical systems in any industry, not just automotive, and that it addresses both safety and security. The document also compares MISRA C to other standards like ISO/IEC TS 17961 and CERT-C, finding a high degree of coverage between the standards while identifying some gaps. Proposed next steps include updating MISRA C:2012 to address the identified gaps.

1. MISRA C - Safety v Security
Fifth VDA Automotive SYS Conference
Andrew Banks
BSc IEng MIET FBCS CITP
Frazer-Nash Research Limited, and
Chairman, MISRA C Working Group

2. MISRA C
A Quick History
July 16, 20152

3. MISRA-C – The Rationale
Despite its popularity, there are several drawbacks with the C language, eg:
• The ISO Standard language definition is incomplete
• Behaviour that is Undefined
• Behaviour that is Unspecified
• Behaviour that is Implementation Defined
• Language misuse and obfuscation
• Language misunderstanding
• Run-time error checking
MISRA C is one solution...
July 16, 20153

4. MISRA-C – A Quick History
MISRA-C:1998 (aka MISRA-C1)
- “Guidelines for the use of the C language in vehicle based software”
- Compatible with ISO/IEC 9899:1990 (aka C90)
MISRA-C:2004 (aka MISRA-C2)
- “Guidelines for the use of the C language in critical systems”
- Remains compatible with ISO/IEC 9899:1990 (aka C90)
MISRA C:2012 (aka MISRA-C3)
- “Guidelines for the use of the C language in critical systems”
- Adds compatibility with ISO/IEC 9899:1999 (aka C99)
July 16, 20154

5. MISRA-C – The 2012 Edition
Published early 2013
159 Guidelines in total
- 16 Directives
o 9 Required
o 7 Advisory
- 143 Rules
o 10 Mandatory
o 101 Required
o 32 Advisory
A compliance and deviation policy
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6. MISRA-C – The Vision
The vision of MISRA C is set out in the opening paragraph of the
Guidelines:
The MISRA C Guidelines define a subset of the C language in which
the opportunity to make mistakes is either removed or reduced.
Many standards for the development of safety-related software
require, or recommend, the use of a language subset, and this can
also be used to develop any application with high integrity or high
reliability requirements.
July 16, 20156

7. MISRA-C – Work In Progress
MISRA C:2012 Technical Corrigendum 1
- Address typographical and clarification
MISRA Compliance
- Enhances guidance for compliance guidance
- Clarifies/tightens the Deviation process
- Standalone document
o Compatible with MISRA C:2012 (and any future versions)
o Compatible with MISRA C++:20xx
o No reason it cannot be applied to earlier versions of either document!
And a few other things...
- Looking at C11 amongst them
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8. MISRA C
Misunderstandings
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9. Myth Busting #1
The Misunderstanding
- MISRA C is only applicable to the automotive industry
The History
- MISRA C was originated by the automotive industry, for the automotive
industry... and we are proud of our automotive heritage.
The Reality
- MISRA C is applicable to any industry that requires high-integrity software
- MISRA C has been adopted by many industries, including medical, rail,
aerospace, space and defence. eg:
• http://lars-lab.jpl.nasa.gov/JPL_Coding_Standard_C.pdf
• http://www.stroustrup.com/JSF-AV-rules.pdf
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10. Myth Busting #2
The Misunderstanding
- MISRA C is only a safety coding standard, not a secure/security one
The History
- MISRA C suggests (in its vision) its use in safety-related software
The Reality
- MISRA C also suggests (in its vision) its applicability to any application
with high integrity or high reliability requirements
- The difference between safety and security are largely semantic
- Unfortunately, a perception remains...
July 16, 201510

11. ESCAR 2014, Hamburg
July 16, 201511

12. MISRA C Directives
Guidelines that are not Rules!
July 16, 201512

13. MISRA C Directives – What is a Directive?
From the MISRA C:2012
- A directive is a guideline for which it is not possible to provide the full
description necessary to perform a check for compliance.
- Additional information, such as might be provided in design documents or
requirement specifications, is required in order to be able to perform the
check.
- Static analysis tools may be able to assist in checking compliance with
directives, but different tools may place different interpretations on what
constitutes non-compliance.
Note: Compliance is still required – just as for the Rules!
July 16, 201513

14. MISRA C Directives – Directive 4.1
From the MISRA C:2012 headline
- Run-time failures shall be minimized
Rationale
- The C language was designed to provide very limited built-in run-time
checking. This places the burden on the programmer...
What does this mean?
- Techniques to avoid run-time failures should be planned and documented,
for example in design standards, test plans and code review checklists.
- Dynamic checks should be added where-ever there is a potential for
errors to occur
Problem areas
- arithmetic errors, array bound errors, function parameters, pointer
arithmetic/de-referencing
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15. MISRA C Directives – Directive 4.11
From the MISRA C:2012 headline
- The validity of values passed to library functions shall be checked
Rationale
- The C standard does not require the standard library to check the validity
of parameters passed to them.
What does this mean?
- Dynamic checks should be added where-ever there is a potential for
errors to occur
Problem areas
- Libraries ctype.h math.h and string.h (and others!)
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16. Safety v Security
Comparison with other guidelines
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17. ISO/IEC TS 17961
C Secure Coding Rules
July 16, 201517

18. ISO/IEC TS 17961 – C Secure Coding Rules
Produced by ISO/IEC JTC 1/SC 22/WG 14 – the same people responsible for
the C standard itself
Originally proposed to be based on CERT-C (see later) but significantly
rationalised
From the document’s Background:
- “In practice, security-critical and safety-critical code have the same
requirements”
- “The purpose of this Technical Specification is to specify analyzable
secure coding rules that can be automatically enforced to detect security
flaws in C-conforming applications”
July 16, 201518

19. ISO/IEC TS 17961 – C Secure Coding Coverage
Coverage Method # Comments
MISRA covers fully – explicitly 22 Some rules are stricter than SecureC
MISRA covers fully – broad 11 Eg: bans dynamic memory, signal.h
MISRA covers fully – implicitly
6 Undefined/unspecified behaviour
3 Standard library
MISRA covers partially – broad 2 getenv() and related functions
MISRA does not cover directly 2 sizeof(pointer), padding
46
July 16, 201519

20. ISO/IEC TS 17961 – The Gaps
The gaps (partial or not covered) can be grouped as follows:
- Taintedness as a concept
- The use of getenv(), localeconv(), setlocale() and strerror() 2 rules
[or indeed other library functions relating to a hosted environment]
- Use of sizeof() on a pointer function parameter 1 rule
- Comparisons of padding data 1 rule
Proposal
- MISRA C:2012 be enhanced to address these gaps
July 16, 201520

21. The Gaps – Taintedness
C Secure
- Many!
MISRA C:2012
- No explicit coverage of taintedness
- Directives D4.1 and D4.11 cover many of the consequences.
- The undefined behaviours are also trapped by R1.3
- Some unwanted behaviour also trapped by broad rules
o General prohibition in the use of stdio.h, signal.h etc
Proposed way ahead
- Add a new MISRA C directive to require validation of externally sourced
data to protect against taintedness.
- Additional explicit rules may be added as required.
July 16, 201521

22. The Gaps – Use of stdlib.h environment functions
C Secure
- Rule 5.29 and Rule 5.42
MISRA C:2012
- Rule R21.8 prohibits the use of getenv() but does not mention the use of
localeconv(), setlocale() and strerror()
Ideal Solution
- Ideally, the C Standard should defines these functions as returning
const char * rather than straight char *
- Note: additional thread-safe functions added in C11
Proposed way ahead
- Permit use of getenv(); Add MISRA C rule(s) to enforce read-only nature,
and to prevent wrong data being used after multiple calls.
- Also applies to asctime() and ctime()
July 16, 201522

23. The Gaps – Use of sizeof() on a pointer parameter
C Secure
- Rule 5.38
MISRA C:2012
- No explicit coverage
- Could tenuously claim D4.1 and D4.11 covers, but...
Proposed way ahead
- Add an appropriate MISRA C rule to detect this.
July 16, 201523

24. The Gaps – Comparison of padding data
C Secure
- Rule 5.9
MISRA C:2012
- No explicit coverage
- Could tenuously claim D4.1 and D4.11 covers, but...
Proposed way ahead
- Add appropriate MISRA C rule(s) to prevent use of memcmp() with
structures or unions.
- Add appropriate MISRA C rule(s) to prevent use of memcmp() with
character strings – use strcmp() or strncmp() instead.
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25. ISO/IEC TS 17961 – The Broad Approaches
Some C Secure rules are implicitly fully covered by broad approaches
- MISRA C:2012 prohibits the use of the restrict keyword 1 rule
- MISRA C:2012 prohibits the use of dynamic memory allocation 3 rules
- MISRA C:2012 prohibits the use of the features in <signal.h> 3 rules
- MISRA C:2012 prohibits the use of the features in <stdio.h> 4 rules
o The use of string formatting functions
o The use of EOF
Proposal
- Keep these broad approaches under review
- Establish more targeted rules where appropriate
July 16, 201525

26. The Broad – string formatting functions
C Secure
- Rule 5.24 and Rule 5.45
MISRA C:2012
- Use of <stdio.h> generally prohibited by Advisory R21.6
- Some undefined behaviour generally trapped by R1.3
- Directives D4.1 and D4.11 also apply
Possible way ahead
- No change – exiting undefined behaviour is caught
- Add catchall taint directive?
- Add explicit MISRA C rule(s)
- Avoid interaction by existing Rule R21.6
July 16, 201526

27. The Broad – The use of EOF
C Secure
- Rule 5.16 and Rule 5.43
MISRA C:2012
- Use of <stdio.h> generally prohibited by Advisory R21.6
- Directives D4.1 and D4.11 apply
Ideal Solution
- Ideally, the C Standard should be fixed. But given the response, when this
was raised at the C99 CD2 ballot, that is not likely to happen!
“Has been like this for at least 10 years, no need to change. Already
known problem with too much existing practice.”
Possible way ahead
- Add appropriate MISRA C rule(s) to protect against tainted values around
EOF
July 16, 201527

28. ISO/IEC TS 17961 – The Implicit?
Many of the Secure C rules are implicitly covered by Directives
- D4.1 Run-time failures shall be minimised
- D4.11 The validity of values passed to library functions shall be checked
Some of these may benefit from additional, focussed, rules
- The use of errno 1 rule
- The use of character handling functions 1 rules
- Use of string copying functions 1 rule
July 16, 201528

29. The Implicit – Use of errno
C Secure
- Rule 5.25
MISRA C:2012
- No explicit coverage... permitted without restrictions
- Directive D1.1 mentions errno in passing
- Directives D4.1, D4.11 and D4.7 all apply
- Note: MISRA C:2004 and earlier simply banned the use of errno
Possible way ahead
- Add appropriate MISRA C rules to protect against tainted values and
inappropriate use.
July 16, 201529

30. The Implicit – string copying functions
C Secure
- Rule 5.37
MISRA C:2012
- No explicit coverage...
- Directives D4.1 and D4.11 do apply
Possible way ahead
- Add explicit MISRA C rule(s)
- Also applies to strncpy and strncat()
July 16, 201530

31. ISO/IEC TS 17961 – Revised C Secure Coverage
Coverage Method # Comments
MISRA covers fully – explicitly 31
MISRA covers fully – broad approach 7 Eg: bans dynamic memory, signals
MISRA covers fully – implicitly
3 Taint
5 Undefined/unspecified behaviour
MISRA covers partially or not at all 0
46
July 16, 201531
# = Coverage assuming adoption of all proposals listed.

32. ISO/IEC TS 17961 – The Next Steps
Proposed Next Steps
- MISRA C Working Group will finalise deliberations
- Update to existing MISRA C:2012 document to be issued
o Target late 2015 or early 2016 (TBC)
o Timeline to coincide with release of Technical Corrigendum 1
o Propose to be issued as Amendment 1
- Update will include a coverage matrix against ISO/IEC TS 19761
July 16, 201532

33. CERT-C
C Secure Coding Rules
July 16, 201533

34. CERT-C – Secure Coding Standard
What is CERT-C
- Produced by the Software Engineering Institute (SEI) at Carnegie Mellon
University.
- Sponsored by the U.S. Department of Defense
- Originally proposed to be adopted as an ISO standard, but this was not
progressed by WG14, who progressed ISO/IEC TS 17961 instead.
The MISRA C Position
- We view CERT-C as complementary to MISRA C
o Most rules align with the MISRA C rules
o Some small variance due to difference of focus (not just safety v security)
- We are reviewing CERT-C’s rules and recommendations
July 16, 201534

35. CERT-C (April 2014) – MISRA C:2012 Coverage
Coverage Method #1 #2 Comments
MISRA covers – fully 36 42
MISRA covers – partially 18 22
MISRA does not cover explicitly 41 33 But many are covered by directives
Possible Contradictions! 1 1? But see following slides...
96 98
#1 Assessment presented at escrypt.
#2 MISRA C Working Group assessment
(MISRA C:2012 against CERT-C:Apr14)
July 16, 201535

36. CERT-C v MISRA C:2012 Contradiction?
Array bound specification on initialization with string literals
- CERT-C Rule STR11-C
o Do not specify the bound of a character array initialised with a string literal
- MISRA C:2012 rule R9.5 (Required)
o Where designated initializers are used to initialise an array object, the size of
the array shall be specified explicitly.
Example Cited
- char test[] = { [0]=“abc” }; // Compliant to CERT-C but not MISRA C
// ... null-terminated string of three characters
- char test[3] = { [0]=“abc” }; // Compliant to MISRA C but not CERT-C
// ... probably wrong size of array length
Note: MISRA C rule R9.5 only applies to Designated Initializers
July 16, 201536

37. CERT-C v MISRA C:2012 Contradiction?
Alternative example #1
- char test[] = { [0]=“a” }; // Compliant to CERT-C but not MISRA C
// ... but really only a single character array?
- char test[10] = { [0]=“a” }; // Compliant to MISRA C but not CERT-C
// ... we really wanted 10 characters
Alternative example #2
- char test[] = { [0]=“abc” }; // Compliant to CERT-C but not MISRA C
// ... how big should that array be?
- char test[4] = { [0]=“abc” }; // Compliant to MISRA C but not CERT-C
// ... three characters plus null-terminator
- char test[3] = { [0]=“abc” }; // Compliant to MISRA C but not CERT-CC
// ... three characters without null-terminator
- char test[3] = { [0]=“abcd” }; // Constraint error
July 16, 201537

38. CERT-C v MISRA C:2012 Contradiction?
Let me repeat: MISRA C:2012 rule R9.5 only applies to Designated Initializers
Compare with advisory MISRA C:2012 rule R8.11
- The rule Headline seems to maintain the contradiction:
o When an array with external linkage is declared, its size should be explicitly
specified.
- And the rule Rationale explains why
o Providing size information for each declaration permits them to be checked for
consistency. It may also permit a static checker to perform some array bounds
analysis without needing to analyse more than one translation unit.
- But the rule Amplification contains the following clarification:
o It is possible to define an array and specify its size implicitly by means of
initialization.
No other MISRA C:2012 rule requires the array size to be explicitly specified.
July 16, 201538

39. In Summary
July 16, 201539

40. MISRA C – In Summary
MISRA C is
- widely respected as a safety-related coding standard
- equally applicable as a security-related coding standard
Proposed Way Ahead
- Address the gaps between MISRA C:2012 and ISO/IEC TS 17961:2013
- The MISRA C Working Group will continue the review activity against
o CERT-C
o Common Weakness Enumeration
o ... and any other sources that may become known
- The MISRA C Working Group welcomes feedback from users
July 16, 201540

41. Any Questions?
July 16, 201541

42. Danke Schön
Thank You!
I would like to acknowledge the support of the members of the MISRA C Working Group for their
assistance in preparing this presentation.
July 16, 201542

43. References
MISRA C:2012
http://misra.org.uk/
Embedded Security in Cars (November 2014, Hamburg)
https://www.escar.info/history/escar-europe/escar-europe-2014-lectures-and-program-committee.html
ISO/IEC TS 17961:2013 – C secure coding rules
http://www.iso.org/iso/catalogue_detail.htm?csnumber=61134
CERT-C
https://www.securecoding.cert.org
ISO/IEC 9899 CD2 comments and decisions
http://www.open-std.org/jtc1/sc22/wg14/www/docs/n847.htm
http://www.open-std.org/jtc1/sc22/wg14/www/docs/n872.htm
July 16, 201543

44. About the speaker
Biography
- Chairman of MISRA-C since June 2013
- Over 25 years experience in developing real-time
embedded software systems, across a number of
industries
- Chartered Fellow of the British Computer Society
- Member of the Institution of Engineering & Technology
Social Media
AndrewBanks.com
@AndrewBanks
https://linkedin.com/in/AndrewBanks
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