INT13-A. 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.

Non-Compliant 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 [INT36-C. Do not shift a negative number of bits or more bits than exist in the operand]). This non-compliant 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 non-compliant 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-A. Use TR 24731 for remediation of existing string manipulation code).

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 is able to detect violations of this recommendation.

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

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]] "XYY Wrap-around Error"

INT12-A. Do not make assumptions about the type of a plain int bit-field when used in an expression      04. Integers (INT)       INT14-A. Avoid performing bitwise and arithmetic operations on the same data