EXP58-CPP. Pass an object of the correct type to va_start

While it is generally prohibited to define a C-style variadic function, such a function may still be defined when that function has external, C language linkage (see DCL50-CPP. Do not define a C-style variadic function for details). Under these circumstances, care must be taken when invoking the va_start() macro. The C standard library macro va_start() defines several semantic restrictions on the type of the value of its second parameter. The C Standard, subclause 7.16.1.4, paragraph 4 [ISO/IEC 9899:2011], states:

The parameter parmN is the identifier of the rightmost parameter in the variable parameter list in the function definition (the one just before the ...). If the parameter parmN is declared with the register storage class, with a function or array type, or with a type that is not compatible with the type that results after application of the default argument promotions, the behavior is undefined.

These restrictions are superseded by the C++ Standard, [support.runtime], paragraph 3 [ISO/IEC 14882-2014], which states:

The restrictions that ISO C places on the second parameter to the va_start() macro in header <stdarg.h> are different in this International Standard. The parameter parmN is the identifier of the rightmost parameter in the variable parameter list of the function definition (the one just before the ...). If the parameter parmN is of a reference type, or of a type that is not compatible with the type that results when passing an argument for which there is no parameter, the behavior is undefined.

The primary differences between the semantic requirements are as follows:

• You must not pass a reference as the second argument to va_start().
• Passing an object of a class type which has a nontrivial copy constructor, nontrivial move constructor, or nontrivial destructor as the second argument to va_start is conditionally supported with implementation-defined semantics ([expr.call] paragraph 7).
• You may pass a parameter declared with the register keyword ([dcl.stc] paragraph 3) or a parameter with a function type.
  

Passing an object of array type still produces undefined behavior in C++ because an array type as a function parameter requires use of a reference, which is prohibited. Additionally, passing an object of a type that undergoes default argument promotions still produces undefined behavior in C++ as well.

Noncompliant Code Example

In this noncompliant code example, the object passed to va_start() will undergo a default argument promotion, which results in undefined behavior:

#include <cstdarg>
 
extern "C" void f(float a, ...) {
  va_list list;
  va_start(list, a);
  // ...
  va_end(list);
}

Compliant Solution

In this compliant solution, f() accepts a double instead of a float:

#include <cstdarg>
 
extern "C" void f(double a, ...) {
  va_list list;
  va_start(list, a);
  // ...
  va_end(list);
}

Noncompliant Code Example

In this noncompliant code example, a reference type is passed as the second argument to va_start():

#include <cstdarg>
#include <iostream>
 
extern "C" void f(int &a, ...) {
  va_list list;
  va_start(list, a);
  if (a) {
    std::cout << a << ", " << va_arg(list, int);
    a = 100; // Assign something to a for the caller
  }
  va_end(list);
}

Compliant Solution

Instead of passing a reference type to f(), this compliant solution passes a pointer type:

#include <cstdarg>
#include <iostream>
 
extern "C" void f(int *a, ...) {
  va_list list;
  va_start(list, a);
  if (a && *a) {
    std::cout << a << ", " << va_arg(list, int);
    *a = 100; // Assign something to *a for the caller
  }
  va_end(list);
}

Noncompliant Code Example

In this noncompliant code example, a nontrivially copyable type is passed as the second argument to va_start(), which is conditionally supported depending on the implementation:

#include <cstdarg>
#include <iostream>
#include <string>
 
extern "C" void f(std::string s, ...) {
  va_list list;
  va_start(list, s);
  std::cout << s << ", " << va_arg(list, int);
  va_end(list);
}

Compliant Solution

This compliant solution passes a const char * instead of a std::string, which has well-defined behavior on all implementations:

#include <cstdarg>
#include <iostream>
 
extern "C" void f(const char *s, ...) {
  va_list list;
  va_start(list, a);
  std::cout << (s ? s : "") << ", " << va_arg(list, int);
  va_end(list);
}

Risk Assessment

Passing an object of an unsupported type as the second argument to va_start() can result in undefined behavior that might be exploited to cause data integrity violations.

Automated Detection

Related Vulnerabilities

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

Related Guidelines

Bibliography