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    Background:
    The type , precision, and range of both time_t and clock_t are implementation defined. Local time zone and daylight savings time are also implementation defined. The Unix time standard can also vary slightly.. IE, the type of time_t and clock_t are precisely "It's a number guys!". It is therefore important to be very careful when using time_t and clock_t in C because assumptions can lead to problems ranging from errors in program timing to possible overflow from invalid type conversions. What follows are some recommendations that help one to avoid common pitfalls that cause security vulnerabilities.

    Recommendation #1:
    Use difftime when subtracting time, and avoid other arithmetic operations when possible.

   The result of performing arithmetic operations on time_t are undefined. Even if your system's time_t is an integer, adding and subtracting times may or may not produce a meaningful result.
    However, situations do arise in which it is necessary to add and subtract time. In general, it should be avoided. If you must do so, below is described the "best guess" at how to add and subtract time.   

    Subtraction:

    C99 defines difftime(), which allows for the subtraction of two time types and returns a double representing the number of seconds between them. Always use difftime() and do not use arithmetic subtraction.

    Addition:

   C99 does not define any addition functions for time; the best thing to do when you need to add time is to not. However, if you must do so, it is best to write a custom function for the platform you are on (a good place to start on ideas for structure is the difftime() function for your platform).

    Recommendation #2:
    Use proper form when subtracting clock_t's for processing time.

    Often times you will wish to clock the amount of time it takes your processor to run a given process. The C99 standard specifies the clock() function for this particular purpose. "In order to measure the time spent in a program, the clock function should be called at the start of the program and its return value subtracted from the value returned by subsequent calls..." It is further specified "To determine the time in seconds, the value returned by the clock function should be divided by the value of the macro CLOCKS_PER_SEC. If the processor time used is not available or its value cannot be represented, the function returns the value (clock_t)(-1)."

    Two common errors are made when performing this operation. They are illustrated below.

    Non Compliant Code:

int run_big_program() {
    clock_t start, finish;
    int seconds;
    start = clock();
    run_long_program()
    finish = clock();
    seconds = ((finish-start)/CLOCKS_PER_SEC);
    return seconds;
}

    The problem here is: 1) the return values of clock() are not checked (this occurs in many implementations of this functionality) and 2) the return value seconds in inappropriate.

    The way to remedy this is to check for return values properly and use an appropriate data structure for the subtraction for the number of seconds. It is implied by the C standard that finish-start will return the number of clock cycles necessary for run_long_program(), so you can be fairly sure of that.

C99 Section 7.23.1 states that CLOCKS_PER_SEC expands to a constant expression with type clock_t that is the number per second of the value returned by the clock() function, so dividing by CLOCKS_PER_SEC will net you the number of seconds. What is not specified, however, is the type that will be yielded by dividing CLOCKS_PER_SEC. The best recommendation is to look at the types defined in time.h and use a compatible (and large, to prevent overflow) type. The convention for linux and most x86 architectures seems to be to use a double.

    Compliant Code:

is specified as an "arithmetic type capable of representing times." However, the way time is encoded within this arithmetic type by the function time() is unspecified. See unspecified behavior 48 in Annex J of the C Standard. Because the encoding is unspecified, there is no safe way to manually perform arithmetic on the type, and as a result, the values should not be modified directly.

Note that POSIX specifies that the time() function must return a value of type time_t, representing time in seconds since the Epoch. POSIX-conforming applications that are not intended to be portable to other environments therefore may safely perform arithmetic operations on time_t objects.

Noncompliant Code Example

This noncompliant code example attempts to execute do_work() multiple times until at least seconds_to_work has passed. However, because the encoding is not defined, there is no guarantee that adding start to seconds_to_work will result in adding seconds_to_work seconds.

int do_work(int seconds_to_work) {
  time_t start = time(NULL);

  if (start == (time_t)(-1)) {
    /* Handle error */
  }
  while (time(NULL) < start + seconds_to_work) {
    /* ... */
  }
  return 0;
}

Compliant Solution

This compliant solution uses difftime() to determine the difference between two time_t values. The difftime() function returns the number of seconds, from the second parameter until the first parameter and result, as a double.

int do_work(int seconds_to_work) {
  time_t start = time(NULL);
  time_t current = start;

double run_big_program ()
{
    clock_t start, finish;
    double seconds;
    start = clock();
    if (start == (clocktime_t)(-1)) {
    /* Handle error */
 return -1.0;}
  while  }
    run_long_program();(difftime(current, start) < seconds_to_work) {
    finishcurrent = clocktime(NULL);
    if (finishcurrent == (clocktime_t)(-1)) {
       /* Handle return -1.0;error */
    }
    seconds = (double) (finish-start)/CLOCKS_PER_SEC;/* ... */
  }
  return seconds0;
}

   When appropriate one should also check for overflow in seconds. The key, though, is that because the proper function is being called and a reasonable value will result. This function will correctly return the time in seconds.
 

Credits/Interesting Links:

- The original idea for this came from the C Language Gotchas site, accessible here

- The wikipedia article on Unix Time is quite enlightening. Read it here

- An article about a denial-of-service in 64bit microsoft time code. Read it here 

Note that this loop still might not exit because the range of time_t might not be able to represent two times seconds_to_work apart.

Risk Assessment

Using time_t incorrectly can lead to broken logic that can place a program in an infinite loop or cause an expected logic branch to not execute.

Automated Detection

Related Vulnerabilities

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

Related Guidelines

Bibliography

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Image Added Image Added Image Added- Interesting time_t discussion from which I pulled my example code. Read it here