DCL10-A. Maintain the contract between the writer and caller of variadic functions

Variadic functions can accept a variable number of arguments, but are problematic. Variadic functions define an implicit contract between the function writer and the function user that allows the function to determine the number of arguments passed in any particular invocation. Failure to exercise care when invoking a variadic function to ensure that it knows when to stop processing arguments may result in undefined behavior.

Argument Processing

In the following code example, the variadic function average() is used to determine the average value of its passed integer arguments [[Seacord 05c]]. The function processes arguments until it finds one with a value of -1.

int average(int first, ...) {
  size_t count = 0;
  int sum = 0;
  int i = first;
  va_list marker;

  va_start(marker, first);

  while (i != -1) {
    sum += i;
    count++;
    i = va_arg(marker, int);
  }

  va_end(marker);
  return(count ? (sum / count) : 0);
}

Note that va_start() must be called to initialize the argument list and va_end() must always be called when finished with a variable argument list.

Non-Compliant Code Example

If the function above is called as follows:

int avg = average(1, 4, 6, 4, 1);

The omission of the -1 terminating value means that the function will continue to process values from the stack until it encounters a -1 by coincidence or an error occurs.

Compliant Solution

The following call maintains the contract by adding a -1 as the last argument.

int avg = average(1, 4, 6, 4, 1, -1);

Non-Compliant Code Example 

Another common mistake is to use more format specifiers than supplied arguments. This results in undefined behavior, which could end up pulling extra values off the stack and unintentionally exposing data. The following example illustrates a case of this:

const char *error_msg = "Resource not available to user.";
/* ... */
printf("Error (%s): %s", error_msg);

Compliant Solution

The following code matches the number of format specifiers with the number of variable arguments.

const char *error_msg = "Resource not available to user.";
/* ... */
printf("Error: %s", error_msg);

Argument List Caveats

C99 functions that accept the variadic primitive va_list as an argument pose an additional risk. Calls to vfprintf(), vfscanf(), vprintf(), vscanf(), vsnprintf(), vsprintf(), and vsscanf() use the va_arg() macro, invalidating the parameterized va_list. Consequently, once a va_list is passed as an argument to any of these functions, it cannot be used again without a call to va_end() followed by a call to va_start().

Risk Assessment

Incorrectly using a variadic function can result in abnormal program termination or unintended information disclosure.

Related Vulnerabilities

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

References

[[ISO/IEC 9899-1999]] Section 7.15, "Variable arguments"; 7.19.6.8 "The vfprintf function"
[[Seacord 05c]]

DCL09-A. Declare functions that return an errno with a return type of errno_t       02. Declarations and Initialization (DCL)       DCL11-A. Understand the type issues associated with variadic functions