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 removed from a program.

Non-Compliant Code Example 1

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

Compliant Solution 1

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 = malloc(10);
        if (s == NULL) {
           /* Handle Error */
        }
        /* Process s */
    }
    /* ... */
    if (s) {
        /* This code is now reachable */
    }
    return 0;
}

Non-Compliant Code Example 2

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

Compliant Solution 2

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

Exceptions

MSC00-EX1 In some situations, dead code may make software more robust against 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).

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 in to a program and the effort required to remove it can be complex. Given this, resolving dead code can be an in-depth process requiring significant changes.

Related Vulnerabilities

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

References

[[Fortify 06]] Code Quality, "Dead Code"

MSC06-A. Be aware of insecure compiler optimization when dealing with sensitive data      14. Miscellaneous (MSC)       MSC08-A. Library functions should validate their parameters