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Most functions defined by the C Standard, Annex K Bounds-checking interfaces, include, as part of their specification, a list of runtime constraints, violations of which can be consistently handled at runtime. Library implementations must verify that the runtime constraints for a function are not violated by the program. If a runtime constraint is violated, the runtime-constraint handler currently registered with set_constraint_handler_s() is called.

Section Annex K.3.6.61.1 [ISO/IEC TR 24731-1:20079899:2011] states:

When the handler is called, it is passed the following arguments in the following order:

1. A pointer to a character string describing the runtime-constraint violation.
2. A null pointer or a pointer to an implementation-defined object.
3. If the function calling the handler has a return type declared as errno_t, the return value of the function is passed. Otherwise, a positive value of type errno_t is passed.

The implementation has a default constraint handler that is used if no calls to the set_constraint_handler_s() function have been made or the handler argument to set_constraint_handler_s() is a null pointer. The behavior of the default handler is implementation-defined, and it may cause the program to exit or abort.

And Section 6Annex K.3.1.4 [ISO/IEC TR 24731-1:20079899:2011] states:

The runtime-constraint handler might not return. If the handler does return, the library function whose runtime-constraint was violated shall return some indication of failure as given by the returns section in the function's specification.

These runtime-constraint handlers mitigate some of the potential insecurity caused by in-band error indicators. (See ERR02-C. Avoid in-band error indicators.)

Noncompliant Code Example (

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C11 Annex K)

In this noncompliant code example, the strcpy_s() function is called, but no runtime-constraint handler has been explicitly registered. As a result, the implementation-defined default handler is called on a runtime error.

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It is therefore prudent to explicitly install a runtime-constraint handler to ensure consistent behavior across implementations.

Compliant Code Example (

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C11 Annex K)

This compliant solution explicitly installs a runtime-constraint handler by invoking the set_constraint_handler_s() function. It would typically be performed during system initialization and before any functions that used the mechanism were invoked.

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Compliant Code Example (Visual Studio 2008 and later)

Although the ISO/IEC TR 24731-1 C11 Annex K functions were created by Microsoft, Microsoft Visual Studio does not support the same interface defined by the technical report for installing runtime-constraint handlers. Visual Studio calls these functions "invalid parameter handlers," and they are installed by calling the _set_invalid_parameter_handler() function. The signature of the handler is also significantly different [MSDN].

_invalid_parameter_handler handle_errors(
   const wchar_t* expression,
   const wchar_t* function,
   const wchar_t* file,
   unsigned int line,
   uintptr_t pReserved
) {
  /* Handle invalid parameter */
}

/*...*/

_set_invalid_parameter_handler(handle_errors)

/*...*/

errno_t function(char *dst1, size_t size) {
  char src1[100] = "hello";

  if (strcpy_s(dst1, size, src1) != 0) {
    return -1;
  }
  /* ...  */
  return 0;
}

Risk Assessment

The TR24731-1 standard C11 Annex K indicates that if no constraint handler is set, a default one executes when errors arise. The default handler is implementation-defined and "may cause the program to exit or abort." It is important to understand the behavior of the default handler for all implementations being used and replace it if the behavior is inappropriate for the application.

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