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Avoid in-band error indicators while designing interfaces. This practice is commonly used by C library functions but is not recommended. One example from the C Standard of a troublesome in-band error indicator is EOF.  (See see FIO34-C. Use int to capture the return value of character IO functions and FIO35-C. Use feof() and ferror() to detect end-of-file and file errors when sizeof(int) == sizeof(char).) Distinguish between characters read from a file and EOF or WEOF). Another problematic use of in-band error indicators from the C Standard involving the size_t and time_t types is described by MSC31by 

• INT31-C. Ensure that

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• integer conversions do not result in lost or misinterpreted data
• FLP07-C. Cast the return value of a function that returns a floating-point type
• INT18-C. Evaluate integer expressions in a larger size before comparing or assigning to that size

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Noncompliant Code Example (sprintf())

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where rbytes is a pointer to a size_t. If no error occurs, and rbytes is not NULL, its value is set to the total number of bytes read, and read() returns 0. If an error occurs, read() returns a nonzero value indicating the error.

Noncompliant Code Example (

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C11, Annex K)

In this noncompliant code example, the error handler returns normally, but the strcpy_s() function's return value is not checked:

constraint_handler_t handle_errors(void) {
  constraint_handler_t data;
  /* Define what to do when error occurs */
  return data;
}

/* ... */

set_constraint_handler(handle_errors);

/* ... */

/* Returns zero on success */
errno_t function(char *dst1, size_t dst_size) {
  char src1[100] = "hello";

  strcpy_s(dst1, sizeof(dst1)dst_size, src1);
  /* 
   * At this point strcpy_s may have yielded an
   *  error, and handle_errors() might have returned.
   */

  /* ... */
  return 0;
}

Compliant Solution (

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C11, Annex K)

In this compliant solution, the error handler terminates the program, ensuring that strcpy_s() never returns unless it fully succeeds:

/*
 * The abort_handler_s() function writes 
 * a message on the
 * standard error stream and
 * then calls the abort() function.
 */
set_constraint_handler(abort_handler_s);

/* ... */

/* Returns zero on success */
errno_t function(char *dst1, size_t dst_size) {
  char src1[100] = "hello";

  strcpy_s(dst1, sizeof(dst1)dst_size, src1);
  /*
   * Because abort_handler_s() never returns,
   *  we only get here only if strcpy_s() succeeds.
   */

  /* ... */
  return 0;
}

Exceptions

ERR02-EX1: Null pointers are another example of an in-band error indicator. Use of null pointers is allowed because it is supported by the language. According to the C Standard, Section subclause 6.3.2.3 [ISO/IEC 9899:2011]:

If a null pointer constant is converted to a pointer type, the resulting pointer, called a null pointer, is guaranteed to compare unequal to a pointer to any object or function.

ERR02-EX2: You may use a function returning in-band error indicators if you can securely guarantee the program will not try to continue processing should an error occur in the function. For example, the functions defined in TR24731-1 C11 Annex K provide hooks for internal constraint violations. If a constraint violation handler is guaranteed not to return upon an error, then you may safely ignore errors returned by these functions. You might accomplish this by having the constraint-violation handler call abort() or longjmp(), for instance.

See ERR03-C. Use runtime-constraint handlers when calling the bounds-checking interfaces for more on the functions defined in TR24731-1C11 Annex K.

Risk Assessment

The risk in using in-band error indicators is difficult to quantify and is consequently given as low. However, if the use of in-band error indicators results in programmers' failing to check status codes or incorrectly checking them, the consequences can be more severe.

Automated Detection

Related Vulnerabilities

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

Related Guidelines

Bibliography

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