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 only be used with unsigned integer operands, as the results of some bitwise operations on signed integers is 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, be careful to avoid undefined behavior while performing a bitwise shift (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 can be propagated as the number is shifted [[Dowd 06]].

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 TR 24731 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 hand side operand to a shift operator is known at compile time, it is acceptable for the value to be represented with a signed type in the compiler so long as the it is positive.

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

Risk Assessment

Improper range checking can lead to buffer overflows and the execution of arbitrary code by an attacker.

Automated Detection

The LDRA tool suite V 7.6.0 can detect violations of this recommendation.

Fortify SCA Version 5.0 with the CERT C Rule Pack can detect violations of this recommendation.

Splint Version 3.1.1 can detect violations of this recommendation.

Compass/ROSE can detect violations of this rule. In particular, it flags bitwise operations that involved variables not declared with unsigned type.

Related Vulnerabilities

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

References

[[Dowd 06]] Chapter 6, "C Language Issues"
[[ISO/IEC 03]] Section 6.5.7, "Bitwise shift operators"
[[ISO/IEC 9899:1999]] Section 6.5.7, "Bitwise shift operators"
[[ISO/IEC PDTR 24772]] "STR Bit Representations," "XYY Wrap-around Error," and "XZI Sign Extension Error"
[[MITRE 07]] CWE ID 682, "Incorrect Calculation"

      04. Integers (INT)