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The signature is similar to strcpy() but takes an extra argument of type rsize_t that specifies the maximum length of the destination buffer. Functions that accept parameters of type rsize_t diagnose a constraint violation if the values of those parameters are greater than RSIZE_MAX. Extremely large object sizes are frequently a sign that an object's size was calculated incorrectly. For example, negative numbers appear as very large positive numbers when converted to an unsigned type like size_t. For those reasons, it is sometimes beneficial to restrict the range of object sizes to detect errors. For machines with large address spaces, the C Standard, Annex K, recommends that RSIZE_MAX be defined as the smaller of the size of the largest object supported or (SIZE_MAX >> 1), even if this limit is smaller than the size of some legitimate, but very large, objects . See (see also INT01-C. Use rsize_t or size_t for all integer values representing the size of an object).

The semantics of strcpy_s() are similar to the semantics of strcpy(). When there are no input validation errors, the strcpy_s() function copies characters from a source string to a destination character array up to and including the terminating null character. The function returns 0 on success.

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This compliant solution performs some of the checking at compile time using a static assertion . (See see DCL03-C. Use a static assertion to test the value of a constant expression).)

void complain(const char *msg) {
  errno_t err;
  static const char prefix[] = "Error: ";
  static const char suffix[] = "\n";
  char buf[BUFSIZ];

  /* 
   * Ensure that more than one character
   * is available for msg
   */
  static_assert(sizeof(buf) > sizeof(prefix) + sizeof(suffix),
                "Buffer for complain() is too small");
  strcpy(buf, prefix);

  err = strcat_s(buf, sizeof(buf), msg);
  if (err != 0) {
    /* Handle error */
  }

  err = strcat_s(buf, sizeof(buf), suffix);
  if (err != 0) {
    /* Handle error */
  }
  fputs(buf, stderr);
}

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String-handling functions defined in the C Standard, subclause 7.24, and elsewhere are susceptible to common programming errors that can lead to serious, exploitable vulnerabilities. Proper use of the C11 Annex K functions can eliminate most of these issues.

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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