Noncompliant Code Example
Do not use the character sequence /* within a comment:
/* comment with end comment marker unintentionally omitted security_critical_function(); /* some other comment */
In this example, the call to the security-critical function is not executed. It is possible that a reviewer examining this page may incorrectly assume that the code is executed.
In cases where this is the result of an accidental omission, it is useful to use an editor that provides syntax highlighting or formats the code to help identify issues like missing end comment delimitors.
Because missing end delimitors are error prone and often viewed as a mistake, this approach is not recommended for commenting out code.
Compliant Solution (preprocessor)
Instead of using /* and */ to comment out blocks of code, comment out blocks of code using conditional compilation (e.g., #if, #ifdef, or #ifndef).
#if 0 /* use of critical security function no
* longer necessary */
security_critical_function();
/* some other comment */
#endif
The text inside a block of code commented out using #if, #ifdef, or #ifndef must still consist of valid preprocessing tokens. This means that the characters " and ' must each be paired just as in real C code, and the pairs must not cross line boundaries. In particular, an apostrophe within a contracted word looks like the beginning of a character constant. Consequently, natural-language comments and pseudocode should always be written between the comment delimiters /* and */ or following //.
Compliant Solution (compiler)
This compliant solution takes advantage of the compiler's ability to remove unreachable (dead) code. The code inside the if block must remain acceptable to the compiler. If other parts of the program such as macros, types, or function prototypes change later in a way that would cause syntax errors, the unexecuted code must be brought up to date to correct the problem. Then, if it is needed again in the future, all that must be done is to remove the surrounding if statement and the NOTREACHED comment.
The NOTREACHED comment tells some compilers and static analysis tools not to complain about this unreachable code. It also serves as documentation.
if (0) { /* use of critical security function no
* longer necessary, for now */
/*NOTREACHED*/
security_critical_function();
/* some other comment */
}
This is an instance of exception MSC07-EX2 to MSC07-C. Detect and remove dead code.
Noncompliant Code Example
These are some additional examples of comment styles that are confusing and should be avoided:
// */ /* comment, not syntax error */
f = g/**//h; /* equivalent to f = g / h; */
//\
i(); /* part of a two-line comment */
/\
/ j(); /* part of a two-line comment */
/*//*/ l(); /* equivalent to l(); */
m = n//**/o
+ p; /* equivalent to m = n + p; */
a = b //*divisor:*/c
+d; /* interpreted as a = b/c +d; in c90
* compiler and a = b+d; in c99 compiler */
Compliant Solution
Use a consistent style of commenting:
/* Nice simple comment */ int i; /* counter */
Risk Assessment
Confusion over which instructions are executed and which are not can lead to serious programming errors and vulnerabilities including denial of service, abnormal program termination, and data integrity violation. This problem is mitigated by the use of interactive development environments (IDEs) and editors that use fonts, colors, or other mechanisms to differentiate between comments and code. However, the problem can still manifest itself, for example, when reviewing source code printed on a black and white printer.
Recommendation |
Severity |
Likelihood |
Remediation Cost |
Priority |
Level |
|---|---|---|---|---|---|
MSC04-C |
medium |
unlikely |
medium |
P4 |
L3 |
Automated Detection
The LDRA tool suite V 7.6.0 can detect violations of this recommendation.
GCC Compiler can detect violations of this rule when the -Wcomment flag is used.
Related Vulnerabilities
Search for vulnerabilities resulting from the violation of this rule on the CERT website.
Other Languages
This rule appears in the C++ Secure Coding Standard as MSC04-CPP. Use comments consistently and in a readable fashion.
Bibliography
[ISO/IEC 9899:1999] Section 6.4.9, "Comments," and Section 6.10.1, "Conditional inclusion"
[MISRA 04] Rule 2.2, "Source code shall only use /* ... */ style comments," Rule 2.3, "The character sequence /* shall not be used within a comment," and Rule 2.4, "Sections of code should not be "commented out"
[Summit 05] Question 11.19