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Software vulnerabilities can result when a programmer fails to consider all possible data states.

Noncompliant Code Example (if Chain)

This noncompliant code example fails to test for conditions where a is neither b nor c. This behavior may be the correct behavior in this case, but failure to account for all the values of a may can result in logic errors if a unexpectedly assumes a different value.

if (a == b) {
  /* ... */
}
else if (a == c) {
  /* ... */
}

Compliant Solution (if Chain)

This compliant solution explicitly checks for the unexpected condition and handles it appropriately.:

if (a == b) {
  /* ... */
}
else if (a == c) {
  /* ... */
}
else {
  /* handleHandle error condition */
}

Noncompliant Code Example (switch)

This noncompliant code example fails to consider all possible cases. This may be the correct behavior in this case, but failure Failure to account for all the valid values of widget_ type may Color will result in logic errors if widget_type unexpectedly assumes a different value or if its valid range is expanded during code maintenance and the programmer overlooks the need to add a case to the switch.This is particularly problematic in C, because an identifier declared as an enumeration constant has type int. As a result, a programmer can accidentally assign an arbitrary integer value to an enum type, as shown in this examplea logic error. Because valid values of an enumerated type include all those of its underlying integer type, unless enumeration constants have been provided for all those values, the default label is appropriate and necessary.

typedef enum
enum WidgetEnum { WE_WRed, WE_XGreen, WE_Y, WE_Z Blue } widget_typeColor;

widget_type = 45;

switch (widget_typeconst char* f(Color c) {
  switch (c) {
    case WE_XRed:
 return "Red";
  /* ... */
    break case Green: return "Green";
    case WE_YBlue:
 return   /* ... */
    break;
  case WE_Z:
    /* ... */
    break"Blue";
  }
}

void g() {
  Color unknown = (Color)123;
  puts(f(unknown));
}

Implementation Details

Microsoft Visual C++ .NET with /W4 does not warn when assigning an integer value to an enum type , or when the switch statement does not contain all possible values of the enumeration.

Compliant Solution (switch)

This compliant solution explicitly checks for the unexpected condition by adding a default clause to the switch statement.takes care to provide the default label to handle all valid values of type Color:

typedef enum
enum WidgetEnum { WE_WRed, WE_XGreen, WE_Y, WE_Z Blue } widget_typeColor;

widget_type = WE_X;

switch (widget_typeconst char* f(Color c) {
  case WE_W:
    /* ... */
    break;
switch (c) {
    case WE_XRed:
    /* ... */return "Red";
    break;
  case WE_YGreen:
    /* ... */return "Green";
    break;
  case WE_ZBlue:
    /* ... */
    break;
return "Blue";
    default: return /* can't happen */
    /* handle error condition"Unknown color";   /* Necessary */
    break;}
}

Adding Note that adding a default case to a switch statement, even when all possible switch labels are specified, is an allowable exception an exception (MSC07-C-EX1) to MSC07-C. Detect and remove dead code, as the unreachable code is added as a precautionary measure..

An alternative compliant solution to the noncompliant code example is to provide a return statement after the switch statement. Note, however, that this solution may not be appropriate in all situations.

typedef enum { Red, Green, Blue } Color;
const char* f(Color c) {
  switch (c) {
    case Red: return "Red";
    case Green: return "Green";
    case Blue: return "Blue";
  }
  return "Unknown color";   /* Necessary */
}

Historical Discussion

This practice has been a subject of debate for some time, but a clear direction has emerged.

Originally, the consensus among those writing best practices was simply that each switch statement should have a default label. Eventually there emerged , emerging compilers and static analysis tools that could verify that a switch on an enum type contained a case label for each enumeration value, but only if no default label existed. This led to a shift toward purposely leaving out the default label to allow static analysis. However, the resulting code was then vulnerable to enum variables being assigned int values outside the set of enum values.

These two practices have now been merged. A switch on an enum type should now contain a case label for each enum value , but should also contain a default label for safety. This is not more difficult to analyze staticallypractice does not add difficulty to static analysis.

Existing implementations are in transition, with some not yet analyzing switch statements with default labels. Developers must take extra care to check their own switch statements until the new practice becomes universal.

Risk Assessment

Noncompliant Code Example (Zune 30)

This noncompliant code example shows incomplete logic when converting dates. The code appeared in the Zune 30 media player, causing many players to lock up on December 30, 2008, at midnight PST. This noncompliant code example comes from the ConvertDays function in the real-time clock (RTC) routines for the MC13783 PMIC RTC. It takes the number of days since January 1, 1980, and computes the correct year and number of days since January 1 of the correct year.

The flaw in the code occurs when days has the value 366 because the loop never terminates. This bug manifested itself on the 366th day of 2008, which was the first leap year in which this code was active.

#define ORIGINYEAR 1980
UINT32 days = /* Number of days since January 1, 1980 */
int year = ORIGINYEAR;
/* ... */

while (days > 365) {
  if (IsLeapYear(year)) {
    if (days > 366) {
      days -= 366;
      year += 1;
    }
  }
  else {
    days -= 365;
    year += 1;
  }
}

Compliant Solution (Zune 30)

The following proposed rewrite is provided at http://winjade.net/2009/01/lesson-on-infinite-loops. The loop is guaranteed to exit, because days decreases for each iteration of the loop, unless the while condition fails and the loop terminates.

#define ORIGINYEAR 1980
UINT32 days = /* Input parameter */
int year = ORIGINYEAR;
/* ... */

int daysThisYear = (IsLeapYear(year) ? 366 : 365);
while (days > daysThisYear) {
  days -= daysThisYear;
  year += 1;
  daysThisYear = (IsLeapYear(year) ? 366 : 365);
}

This compliant solution is for illustrative purposes and is not necessarily the solution implemented by Microsoft.

Risk Assessment

Failing to account for all Failing to take into account all possibilities within a logic statement can lead to a corrupted running state, potentially resulting in unintentional information disclosure or abnormal termination.

Automated Detection

The LDRA tool suite Version 7.6.0 can detect violations of this recommendation.

GCC Compiler Version can detect some violations of this recommendation when the -Wswitch and -Wswitch-default flags are used.

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  if (x > 0) {
	  /* ... */
  } else if (x < 0) {
    /* ... */
  } else if (x == 0) {
    /* ... */
  }

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Klocwork Version 8.0.4.16 can detect violations of this rule with the LA_UNUSED checker.

Related Vulnerabilities

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

References

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Related Guidelines

Bibliography

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Image Added Image Added Image Added account for default case in switch"Image Removed      13. Miscellaneous (MSC)       MSC02-C. Avoid errors of omission