Variable argument functions

A variable argument function (variadic function) is a function that can accept an undefined number of arguments. In many programming languages, formatted output functions are defined as variadic functions.

In C++, variable argument functions are declared with the ellipsis (...) in the argument list field. The compiler checks types only for explicitly defined arguments. To get access to the variable argument list macros va_arg, va_end and va_start from the STDARG.H header file are used. The printf function family is one of the well-known examples of variable argument functions.

In some languages using variable argument functions might cause type safety troubles. For example, C/C++ functions of the printf family may create uncontrolled string format vulnerabilities when used carelessly. Mismatch of the format string and passed arguments may cause unpredictable behavior, buffer overflows, stack smashing and execution of unsafe code and lead to destruction of dynamic memory areas.

Although these troubles related to the use of variadic functions are known for a long time, they were identified as serious security vulnerabilities only in 1999. Considering that such vulnerabilities were thought to be rather harmless for many years, this is one of the most widespread classes of errors today.

To fix and avoid errors related to using variadic functions you can rely on compiler-generated warnings or third-party static code analyzers. Keep in mind that static analysis allows you to detect incorrect format strings only if the analyzer knows of them before the start of compilation. That's why you should be especially careful when the program is dynamically forming a format string during execution or receives it from the user.

Below are some examples of errors related to using variable argument functions found in popular open-source projects with the PVS-Studio static analyzer.

The ReactOS project. Incorrect printing of a WCHAR character.

static void REGPROC_unescape_string(WCHAR* str)
{
  ...
  default:
    fprintf(stderr,
      "Warning! Unrecognized escape sequence: \\%c'\n",
      str[str_idx]);
  ...
}

The fprinf() function should print a character of the char type. But the third argument is a character of the WCHAR type. The user will see an incorrectly formed message. To make the code correct we should replace '%c' with '%C' in the format string.

The Intel AMT SDK project. Unused argument.

bool GetUserValues(...)
{
  ...
  printf("Error: illegal value. Aborting.\n", tmp);
  return false;
}

The error is this: the 'tmp' variable is not used in any way when printing the information message.

References

• Wikipedia. Variadic function.
• Wikipedia. Format string attack.
• MSDN Library. Functions with Variable Argument Lists.
• MSDN Library. va_arg, va_end, va_start.
• Andrey Karpov. Wade not in unknown waters. Part two.

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