Skip to end of metadata
Go to start of metadata

While rule DCL50-CPP. Do not define a C-style variadic function forbids creation of such functions, they may still be defined when that function has external, C language linkage. Under these circumstances, care must be taken when invoking the va_start() macro. The C-standard library macro va_start() imposes 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 following:

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These restrictions are superseded by the C++ Standard, [support.runtime], paragraph 3 [ISO/IEC 14882-2014], which states the following:

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 that 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 the 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++.

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 class with a nontrivial copy constructor (std::string) 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, s);
  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.

Rule

Severity

Likelihood

Remediation Cost

Priority

Level

EXP58-CPP

Medium

Unlikely

Medium

P4

L3

Automated Detection

Tool

Version

Checker

Description

Clang
3.9
-WvarargsDoes not catch the violation in the third noncompliant code example (it is conditionally supported by Clang)
Parasoft C/C++test

10.4.2

CERT_CPP-EXP58-a

Use macros for variable arguments correctly

Polyspace Bug Finder

R2019a

CERT C++: EXP58-CPPChecks for incorrect data types for second argument of va_start (rule fully covered)

Related Vulnerabilities

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

Related Guidelines

Bibliography

[ISO/IEC 9899:2011]Subclause 7.16.1.4, "The va_start Macro"
[ISO/IEC 14882-2014]Subclause 18.10, "Other Runtime Support"



1 Comment

  1. This rule does not have an analogy in C, but it should have one since most of the UB here also applies in C. However, the C++ rule is still required because there is a lot of C++-specific behavioral differences.

    Also, for either the C or the C++ rules, should we add the following as an NCCE?

    enum E {
      one
    };
     
    void f(enum E, ...) {
      va_list list;
      va_start(list, E);
      va_end(list);
    }

    If the compiler elects to represent enum E as a char rather than an int (per 6.7.2.2p4), the call to va_start() will have undefined behavior. However, most compilers require you to pass a flag (like -fshort-enums) or use an attribute (like __attribute__((packed))) on the enumeration declaration to force the enumeration to be treated as a char for efficiency reasons. I bring it up as an example of a way to violate the "or with a type that is not compatible with the type that results after application of the default argument promotions" clause from 7.16.1.4p4.