INT13-C. Use bitwise operators only on unsigned operands

Bitwise operators include the complement operator ~, bitwise shift operators >> and <<, bitwise AND operator &, bitwise exclusive OR operator ^, and bitwise inclusive OR operator |. Bitwise operators should be used only with unsigned integer operands, as the results of some bitwise operations on signed integers are implementation-defined.

Noncompliant Code Example (Right Shift)

The right-shift operation may be implemented as either an arithmetic (signed) shift or a logical (unsigned) shift. If E1 in the expression E1 >> E2 has a signed type and a negative value, the resulting value is implementation-defined. Also, a bitwise shift can result in undefined behavior. (See INT34-C. Do not shift a negative number of bits or more bits than exist in the operand.)

This noncompliant code example can result in an error condition on implementations in which an arithmetic shift is performed, and the sign bit is propagated as the number is shifted [Dowd 2006].

int rc = 0;
int stringify = 0x80000000;
char buf[sizeof("256")];
rc = snprintf(buf, sizeof(buf), "%u", stringify >> 24);
if (rc == -1 || rc >= sizeof(buf)) {
  /* handle error */
}

In this example, stringify >> 24 evaluates to 0xFFFFFF80, or 4,294,967,168. When converted to a string, the resulting value "4294967168" is too large to store in buf and is truncated by snprintf().

If this code had been implemented using sprintf() instead of snprintf(), this noncompliant code example would have resulted in a buffer overflow.

Compliant Solution (Right Shift)

In this compliant solution, stringify is declared as an unsigned integer. The value of the result of the right-shift operation is the integral part of the quotient of stringify / 2^24.

int rc = 0;
unsigned int stringify = 0x80000000;
char buf[sizeof("256")];
rc = snprintf(buf, sizeof(buf), "%u", stringify >> 24);
if (rc == -1 || rc >= sizeof(buf)) {
  /* handle error */
}

Also, consider using the sprintf_s() function, defined in ISO/IEC TR 24731-1, instead of snprintf() to provide some additional checks. (See STR07-C. Use the bounds-checking interfaces for remediation of existing string manipulation code.)

Exceptions

INT13-EX1: When used as bit flags, it is acceptable to use preprocessor macros as arguments to the & and | operators even if the value is not explicitly declared as unsigned.

fd = open(file_name, UO_WRONLY | UO_CREAT | UO_EXCL | UO_TRUNC, 0600);

INT13-EX2: If the right-side operand to a shift operator is known at compile time, it is acceptable for the value to be represented with a signed type provided it is positive.

#define SHIFT 24
foo = 15u >> SHIFT;

Risk Assessment

Performing bitwise operations on signed numbers can lead to buffer overflows and the execution of arbitrary code by an attacker in some cases, unexpected or implementation-defined behavior in others.

Automated Detection

Related Vulnerabilities

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

Related Guidelines

Bibliography