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The three types char, signed char, and unsigned char are collectively called the character types. Compilers have the latitude to define char to have the same range, representation, and behavior as either signed char or unsigned char. Irrespective of the choice made, char is a separate type from the other two and is not compatible with either.

Non-Compliant Code Example

This non-compliant code example is taken from an actual vulnerability in bash versions 1.14.6 and earlier that resulted in the release of CERT Advisory CA-1996-22. This vulnerability resulted from the declaration of the string variable in the yy_string_get() function as char * in the parse.y module of the bash source code:

static int yy_string_get() {
  register char *string;
  register int c;

  string = bash_input.location.string;
  c = EOF;

  /* If the string doesn't exist, or is empty, EOF found. */
  if (string && *string) {
      c = *string++;
      bash_input.location.string = string;
    }
  return (c);
}

The string variable is used to traverse the character string containing the command line to be parsed. As characters are retrieved from this pointer, they are stored in a variable of type int. For compilers in which the char type defaults to signed char, this value is sign-extended when assigned to the int variable. For character code 255 decimal (-1 in two's complement form), this sign extension results in the value -1 being assigned to the integer (which is indistinguishable from the EOF integer constant expression.

Compliant Solution

This problem is easily repaired by explicitly declaring the string variable as unsigned char.

static int yy_string_get() {
  register unsigned char *string;
  register int c;

  string = bash_input.location.string;
  c = EOF;

  /* If the string doesn't exist, or is empty, EOF found. */
  if (string && *string) {
      c = *string++;
      bash_input.location.string = string;
    }
  return (c);
}

Use only signed char and unsigned char types for the storage and use of numeric values because it is the only portable way to guarantee the signedness of the character types (see STR00-C. Represent characters using an appropriate type for more information on representing characters).

Noncompliant Code Example

In this noncompliant code example, the char-type variable c may be signed or unsigned. Assuming 8-bit, two's complement character types, this code may print out either i/c = 5 (unsigned) or i/c = -17 (signed). It is much more difficult to reason about the correctness of a program without knowing if these integers are signed or unsigned.

char c = 200;
int i = 1000;
printf("i/c = %d\n", i/c);

Compliant Solution

In this compliant solution, the variable c is declared as unsigned char. The subsequent division operation is now independent of the signedness of char and consequently has a predictable result.

unsigned char c = 200;
int i = 1000;
printf("i/c = %d\n", i/c);

Exceptions

INT07-C-EX1: void FIO34-C. Use int to capture the return value of character IO functions that might be used to check for end of file mentions that certain character IO functions return a value of type int. Despite being returned in an arithmetic type, the value is not actually numeric in nature, so it is acceptable to later store the result into a variable of type char.

Risk Assessment

This is a subtle error that result results in a disturbingly broad range of potentially severe vulnerabilitiles. vulnerabilities. At the very least, this error can lead to unexpected numerical results on different platforms. Unexpected arithmetic values when applied to arrays or pointers can yield buffer overflows or other invalid memory access.

References

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Automated Detection

Related Vulnerabilities

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

Related Guidelines

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