The size_t type is the unsigned integer type of the result of the sizeof operator. Variables of type size_t are guaranteed to be of sufficient precision to represent the size of an object. The limit of size_t is specified by the SIZE_MAX macro.

The type size_t generally covers the entire address space. The C Standard, Annex K (normative), "Bounds-checking interfaces," introduces a new type, rsize_t, defined to be size_t but explicitly used to hold the size of a single object [Meyers 2004]. In code that documents this purpose by using the type rsize_t, the size of an object can be checked to verify that it is no larger than RSIZE_MAX, the maximum size of a normal single object, which provides additional input validation for library functions. See STR07-C. Use the bounds-checking interfaces for string manipulation for additional discussion of C11 Annex K.

Any variable that is used to represent the size of an object, including integer values used as sizes, indices, loop counters, and lengths, should be declared rsize_t, if available. Otherwise, it should be declared size_t.

Noncompliant Code Example

In this noncompliant code example, the dynamically allocated buffer referenced by p overflows for values of n > INT_MAX:

char *copy(size_t n, const char *c_str) {
  int i;
  char *p;

  if (n == 0) {
    /* Handle unreasonable object size error */
  }
  p = (char *)malloc(n);
  if (p == NULL) {
    return NULL; /* Indicate malloc failure */
  }
  for ( i = 0; i < n; ++i ) {
    p[i] = *c_str++;
  }
  return p;
}

/* ... */

char c_str[] = "hi there";
char *p = copy(sizeof(c_str), c_str);

Signed integer overflow causes undefined behavior. The following are two possible conditions under which this code constitutes a serious vulnerability:

sizeof(size_t) == sizeof(int)

The unsigned n may contain a value greater than INT_MAX. Assuming quiet wraparound on signed overflow, the loop executes n times because the comparison i < n is an unsigned comparison. Once i is incremented beyond INT_MAX, i takes on negative values starting with (INT_MIN). Consequently, the memory locations referenced by p[i] precede the memory referenced by p, and a write outside array bounds occurs.

sizeof(size_t) > sizeof(int)

For values of n where 0 < n <= INT_MAX, the loop executes n times, as expected.

For values of n where INT_MAX < n <= (size_t)INT_MIN, the loop executes INT_MAX times. Once i becomes negative, the loop stops, and i remains in the range 0 through INT_MAX.

For values of n where (size_t)INT_MIN < n <= SIZE_MAX, i wraps and takes the values INT_MIN to INT_MIN + (n - (size_t)INT_MIN - 1). Execution of the loop overwrites memory from p[INT_MIN] through p[INT_MIN + (n - (size_t)INT_MIN - 1)].

Compliant Solution (C11, Annex K)

Declaring i to be of type rsize_t eliminates the possible integer overflow condition (in this example). Also, the argument n is changed to be of type rsize_t to document additional validation in the form of a check against RSIZE_MAX:

char *copy(rsize_t n, const char *c_str) {
  rsize_t i;
  char *p;

  if (n == 0 || n > RSIZE_MAX) {
    /* Handle unreasonable object size error */
  }
  p = (char *)malloc(n);
  if (p == NULL) {
    return NULL;  /* Indicate malloc failure */
  }
  for (i = 0; i < n; ++i) {
    p[i] = *c_str++;
  }
  return p;
}

/* ... */

char c_str[] = "hi there";
char *p = copy(sizeof(c_str), c_str);

Noncompliant Code Example

In this noncompliant code example, the value of length is read from a network connection and passed as an argument to a wrapper to malloc() to allocate the appropriate data block. Provided that the size of an unsigned long is equal to the size of an unsigned int, and both sizes are equal to or smaller than the size of size_t, this code runs as expected. However, if the size of an unsigned long is greater than the size of an unsigned int, the value stored in length may be truncated when passed as an argument to alloc().  Both read functions return zero on success and nonzero on failure.

void *alloc(unsigned int blocksize) {
  return malloc(blocksize);
}

int read_counted_string(int fd) {
  unsigned long length;
  unsigned char *data;

  if (read_integer_from_network(fd, &length)) {
    return -1;
  }

  data = (unsigned char*)alloc(length+1);
  if (data == NULL) {
    return -1;  /* Indicate failure */
  }

  if (read_network_data(fd, data, length)) {
    free(data);
    return -1;
  }
  data[length] = '\0';

  /* ... */
  free( data);
  return 0;
}

Compliant Solution (C11, Annex K)

Declaring both length and the blocksize argument to alloc() as rsize_t eliminates the possibility of truncation. This compliant solution assumes that read_integer_from_network() and read_network_data() can also be modified to accept a length argument of type pointer to rsize_t and rsize_t, respectively. If these functions are part of an external library that cannot be updated, care must be taken when casting length into an unsigned long to ensure that integer truncation does not occur.

void *alloc(rsize_t blocksize) {
  if (blocksize == 0 || blocksize > RSIZE_MAX) {
    return NULL;  /* Indicate failure */
  }
  return malloc(blocksize);
}

int read_counted_string(int fd) {
  rsize_t length;
  unsigned char *data;

  if (read_integer_from_network(fd, &length)) {
    return -1;
  }

  data = (unsigned char*)alloc(length+1);
  if (data == NULL) {
    return -1; /* Indicate failure */
  }

  if (read_network_data(fd, data, length)) {
    free(data);
    return -1;
  }
  data[length] = '\0';

  /* ... */
  free( data);
  return 0;
}

Risk Assessment

The improper calculation or manipulation of an object's size can result in exploitable vulnerabilities.

Automated Detection

Tool	Version	Checker	Description
Axivion Bauhaus Suite		CertC-INT01
CodeSonar		LANG.TYPE.BASIC	Basic numerical type used
Compass/ROSE			Can detect violations of this recommendation. In particular, it catches comparisons and operations where one operand is of type size_t or rsize_t and the other is not
Splint

Related Vulnerabilities

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

Related Guidelines

Bibliography