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

Variadic functions 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 enforce this contract may result in undefined behavior. See undefined behavior 141 of Appendix J of the C Standard.

Argument Processing

In the following code example, the variadic function average() calculates the average value of the positive integer arguments passed to the function [Seacord 2013]. The function processes arguments until it encounters an argument with the value of va_eol (-1).

enum { va_eol = -1 };

unsigned int average(int first, ...) {
  unsigned int count = 0;
  unsigned int sum = 0;
  int i = first;
  va_list args;

  va_start(args, first);

  while (i != va_eol) {
    sum += i;
    count++;
    i = va_arg(args, int);
  }

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

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

Noncompliant Code Example

In this noncompliant code example, the average() function is called as follows:

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

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

Compliant Solution

This compliant solution enforces the contract by adding va_eol as the final argument:

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

Noncompliant Code Example

Another common mistake is to use more conversion specifiers than supplied arguments, as shown in this noncompliant code example:

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

This code results in nonexistent arguments being processed by the function, potentially leaking information about the process.

Compliant Solution

This compliant solution 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

C 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.

Automated Detection

Related Guidelines

Bibliography