Question

Secure Coding Lab
Zune Failure Part 2

Purpose: Explore the Zune Failure from December 2008.

Tools: Students can use any tool of their choice

Assignment: Students will do individual work. This is a follow-on to the previous lab.
The deliverable is an explanation of the code that resulted in the Microsoft Zune failure of December 31, 2008.
Enhanced Code is attached.

Deliverable: Analyzing the code, explain why it failed, show the specific lines of code with an explanation of why. And how to fix it.

So what’s the purpose of the while loop? To put it simply, it’s designed to get the number of years from the number of days since 1980 as well as a reminder of days out of the current year.
Keep in mind that you’re never supposed to retry the loop without committing some sort of action which will ultimately exit the loop:

1. year is set to ORIGINYEAR. Since ORIGINYEAR is 1980, any addition to year is added on top of 1980. This is fine because the hardware is passing the number of days since January 1, 1980.
2. Is the number of days greater than 365? If so, proceed. Otherwise, skip to number 6.
3. Is the current year a leap year? If so, proceed. Otherwise, subtract 365 from the number of days, add 1 to the number of years, and skip to number 5.
4. Is the number of days greater than 366? If so, subtract 366 from the number of days, add 1 to the number of years, and proceed. 5. retry number 2.
6. This is the loop exit point – success if you get here  

Code: #define ORIGINYEAR 1980
BOOL ConvertDays(UINT32 days, SYSTEMTIME* lpTime)
{
int dayofweek, month, year;
UINT8 *month_tab;

//Calculate current day of the week
dayofweek = GetDayOfWeek(days);

year = ORIGINYEAR;

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

// Determine whether it is a leap year
month_tab = (UINT8 *)((IsLeapYear(year))? monthtable_leap : monthtable);
for (month=0; month<12; month++)
{
if (days <= month_tab[month])
break;
days -= month_tab[month];
}
month += 1;
lpTime->wDay = days;
lpTime->wDayOfWeek = dayofweek;
lpTime->wMonth = month;
lpTime->wYear = year; return TRUE;
}

Answer 1

From the given code,

The outcome is processed within the while loop. Initially, the while loop checks for the days are greater than 365, and it is true, then checking for leap year, and it is also true. But, in the following condition at if statement, if we have exactly 366 days, then it will not satisfy the condition and makes an infinite loop. So, the code failed to produce the required outcome.

We can modify the code inside the while to fix the issue,

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

The solution is, we can simple break the loop in such condition.