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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, C11 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 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 zero on success.

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Users of the C11 Annex K functions are less likely to introduce a security flaw because the size of the destination buffer and the maximum number of characters to append must be specified. ISO/IEC TR 24731 Part II [ISO/IEC TR 24731-2:2010] offers another approach, supplying functions that allocate enough memory for their results. ISO/IEC TR 24731 functions also ensure null termination of the destination string.

ISO/IEC TR 24731-1 [ISO/IEC TR 24731-1:2007] functions are still capable of overflowing a buffer if the maximum length of the destination buffer and number of characters to copy are incorrectly specified. ISO/IEC TR 24731-2 functions can make it more difficult to keep track of memory that must be freed, leading to memory leaks. As a result, the ISO/IEC TR 24731 functions are not particularly secure but may be useful in preventive maintenance to reduce the likelihood of vulnerabilities in an existing legacy code base.

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The following compliant solution performs some of the checking at compile time using a static assertion. (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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Related Vulnerabilities

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ISO/IEC 9899:2011 Section 7.24, "String handling <string.h>"

ISO/IEC TR 24731-1:2007

ISO/IEC TR 24731-2:2010

ISO/IEC TR 24772 "TRJ Use of libraries"

Bibliography

[Seacord 2005a] Chapter 2, "Strings"
[Seacord 2005b]

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