MSC07-A. Detect and remove dead code

Code that is never executed is known as dead code. Typically, the presence of dead code indicates that a logic error has occurred as a result of changes to a program or the program's environment. Dead code is usually optimized out of a program during compilation. However, to improve readability and ensure that logic errors are resolved, dead code should be identified, understood, and eliminated.

This recommendation is related to MSC12-A. Detect and remove code that has no effect.

Non-Compliant Code Example

This non-compliant code example demonstrates how dead code can be introduced into a program [[Fortify 06]]. The second conditional statement, if (s), will never evaluate true because it requires that s not be assigned NULL, and the only path where s can be assigned a non-NULL value ends with a return statement.

int func(int condition) {
    char *s = NULL;
    if (condition) {
        s = (char *)malloc(10);
        if (s == NULL) {
           /* Handle Error */
        }
        /* Process s */
        return 0;
    }
    /* ... */
    if (s) {
        /* This code is never reached */
    }
    return 0;
}

Compliant Solution

Remediation of dead code requires the programmer to determine why the code is never executed and then resolve that situation appropriately. To correct the example above, the return is removed from the body of the first conditional statement.

int func(int condition) {
    char *s = NULL;
    if (condition) {
        s = (char *)malloc(10);
        if (s == NULL) {
           /* Handle error */
        }
        /* Process s */
    }
    /* ... */
    if (s) {
        /* This code is now reachable */
    }
    return 0;
}

Non-Compliant Code Example

In this example, the strlen() function is used to limit the number of times the function string_loop() will iterate. The conditional statement inside the loop evaluates to true when the current character in the string is the null terminator. However, because strlen() returns the number of characters that precede the null terminator, the conditional statement never evaluates true.

int string_loop(char *str) {
    size_t i;
    size_t len = strlen(str);
    for (i=0; i < len; i++) {
        /* ... */
	if (str[i] == '\0')
	    /* This code is never reached */
    }
    return 0;
}

Compliant Solution

Removing the dead code depends on the intent of the programmer. Assuming the intent is to flag and process the last character before the null terminator, the conditional is adjusted to correctly determine if the i refers to the index of the last character before the null terminator.

int string_loop(char *str) {
    size_t i;
    size_t len = strlen(str);
    for (i=0; i < len; i++) {
        /* ... */
	if (str[i+1] == '\0')
	    /* This code is now reached */
    }
    return 0;
}

Exceptions

MSC07-EX1: In some situations, dead code may make software resilient to future changes. An example of this is adding a default case to a switch statement even when all possible switch labels are specified (see MSC01-A. Strive for logical completeness for an illustration of this example). 

MSC07-EX2: It is also permissible to temporarily remove code that may be needed later (see MSC04-A. Use comments consistently and in a readable fashion for an illustration).

Risk Assessment

The presence of dead code may indicate logic errors that can lead to unintended program behavior. The ways in which dead code can be introduced into a program and the effort required to remove it can be complex. As a result, resolving dead code can be an in-depth process requiring significant analysis.

Automated Detection

The LDRA tool suite V 7.6.0 is able to detect violations of this recommendation.

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

References

[[Fortify 06]] Code Quality, "Dead Code"
[[ISO/IEC PDTR 24772]] "BRS Leveraging human experience," "BVQ Unspecified Functionality," and "XYQ Dead and Deactivated Code"
[[MISRA 04]] Rule 2.4

MSC06-A. Be aware of compiler optimization when dealing with sensitive data      13. Miscellaneous (MSC)       MSC08-A. Library functions should validate their parameters