DCL50-CPP. Do not define a C-style variadic function

Functions can be defined to accept more formal arguments at the call site than are specified by the parameter declaration clause. Such functions are called variadic functions because they can accept a variable number of arguments from a caller. C++ provides two mechanisms by which a variadic function can be defined: function parameter packs and use of a C-style ellipsis as the final parameter declaration. Variadic functions are flexible in that they accept a varying number of arguments of differing types. However, they can also be hazardous. A variadic function using a C-style ellipsis (hereafter called a C-style variadic function) has no mechanisms to check the type safety of arguments being passed to the function or to check that the number of arguments being passed matches the semantics of the function definition. Consequently, a runtime call to a C-style variadic function that passes inappropriate arguments yields undefined behavior. Such undefined behavior could be exploited to run arbitrary code.

Do not define C-style variadic functions. Issues with C-style variadic functions can be avoided by using variadic functions defined with function parameter packs for situations in which a variable number of arguments should be passed to a function. Additionally, function currying can be used to build object state piecemeal, such as the standard output stream does with its std::cout::operator<<() overloads.

Note that the declaration of C-style variadic functions is not harmful and can be useful in unevaluated contexts. When a function call expression appears in an unevaluated context, such as the argument in a sizeof expression, overload resolution is performed to determine the result type of the call but does not require a function definition. Some template metaprogramming techniques that employ "substitution failure is not an error" (SFINAE) use variadic functions to implement compile-time type queries, as in the following:

template <typename Ty>
class has_foo_function {
  typedef char yes[1];
  typedef char no[2];

  template <typename Inner>
  static yes& test(Inner *I, decltype(I->foo()) * = nullptr);

  template <typename>
  static no& test(...);

public:
  static const bool value = sizeof(test<Ty>(nullptr)) == sizeof(yes);
};

In this example, the value of value is determined on the basis of which overload of test() is selected. The declaration of Inner *I allows use of the variable I within the decltype specifier, which results in a pointer of some (possibly void) type, with a default value of nullptr. However, if there is no declaration of Inner::foo(), the decltype specifier will be ill-formed, and that variant of test() will not be a candidate function for overload resolution due to SFINAE. The result is that the C-style variadic function variant of test() will be the only function in the candidate set. Both test() functions are declared, but never defined, because their definitions are not required for use within an unevaluated expression context.

Noncompliant Code Example

This noncompliant code example uses a C-style variadic function to add a series of integers together until the value 0 is found. Calling this function without passing the value 0 as an argument results in undefined behavior. Further, passing any type other than an int also results in undefined behavior.

#include <cstdarg>

int Add(int First, int Second, ...) {
  int R = First + Second;  
  va_list va;
  va_start(va, First);
  while (int V = va_arg(va, int)) {
    R += V;   
  }
  va_end(va);
  return R;
}

Compliant Solution (Recursive Pack Expansion)

In this compliant solution, a variadic function using a function parameter pack is used to implement the Add() function, allowing identical behavior for call sites. Unlike the C-style variadic function used in the noncompliant code example, this compliant solution does not result in undefined behavior if the list of parameters is not terminated with 0. Additionally, if any of the values passed to the function are not an integer, it results in the code being ill-formed instead of in undefined behavior.

#include <type_traits>
 
template <typename Arg, typename std::enable_if<std::is_integral<Arg>::value>::type * = nullptr>
int Add(Arg F, Arg S) { return F + S; }
 
template <typename Arg, typename... Ts, typename std::enable_if<std::is_integral<Arg>::value>::type * = nullptr>
int Add(Arg F, Ts... Rest) {
  return F + Add(Rest...);
}

Note, this compliant solution makes use of std::enable_if to ensure that any nonintegral argument values result in an ill-formed program.

Compliant Solution (Braced Initializer List Expansion)

An alternative compliant solution that does not require recursive expansion of the function parameter pack instead expands the function parameter pack into a list of values as part of a braced-init-list. Since narrowing conversions are not allowed in a braced-init-list, the type safety is preserved despite the std::enable_if not involving any of the variadic arguments.

#include <type_traits>
 
template <typename Arg, typename... Ts, typename std::enable_if<std::is_integral<Arg>::value>::type * = nullptr>
int Add(Arg I, Arg J, Ts... All) {
  int Values[] = { J, All... };
  int R = I;
  for (auto V : Values) {
    R += V;
  }
  return R;
}

Risk Assessment

Incorrectly using a variadic function can result in abnormal program termination, unintended information disclosure, or execution of arbitrary code.

Automated Detection

Related Vulnerabilities

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

Bibliography