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The typedef name uintN_t designates an unsigned integer type with width N. Consequently, uint32_t denotes an unsigned integer type with a width of exactly 32 bits. Bitmaps are normally declared as unsigned.

Left and right shift operators are often employed to multiply or divide a number by a power of 2. However, using shift operators to represent multiplication or division is an optimization that renders the code less portable and less readable. Furthermore, most compilers routinely will optimize multiplications and divisions by constant powers of 2 with bit shift operations, and they are more familiar with the implementation details of the current platform.

Non-Compliant Code Example (Left Shift)

In this non-compliant code example, both bit manipulation and arithmetic manipulation are performed on the integer type x. The result is a (prematurely) optimized statement that assigns 5x + 1 to x for implementations where integers are represented as two's complement values.

unsigned int x = 50;
x += (x << 2) + 1;

Although this is a valid manipulation, the result of the shift depends on the underlying representation of the integer type and is consequently implementation-defined. Additionally, the readability of the code is reduced.

Compliant Solution (Left Shift)

In this compliant solution, the assignment statement is modified to reflect the arithmetic nature of x, resulting in a clearer indication of the programmer's intentions.

unsigned int x = 50;
x = 5 * x + 1;

A reviewer may now recognize that the operation should also be checked for integer overflow. This might not have been apparent in the original, non-compliant code example.

Non-Compliant Code Example (Right Shift)

In this non-compliant code example, the programmer prematurely optimizes code by replacing a division with a right shift.

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Although this code is likely to perform the division correctly, it is not guaranteed to. If x has a signed type and a negative value, the operation is implementation-defined and could be implemented as either an arithmetic shift or a logical shift. In the event of a logical shift, if the integer is represented in either one's complement or two's complement form, the most significant bit (which controls the sign in a different way for both representations) will be set to zero. This will cause a once negative number to become a possibly very large positive number. For more details, see INT13-A. Do not assume that a right shift operation is implemented as a logical or an arithmetic shift.

For example, if the internal representation of x is 0xFFFF FFCE (two's complement), an arithmetic shift results in 0xFFFF FFF3 (-13 in two's complement), while a logical shift results in 0x3FFF FFF3 (1 073 741 811 in two's complement).

Compliant Solution (Right Shift)

In this compliant solution, the right shift is replaced by division.

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The resulting value is now more likely to be consistent with the programmer's expectations.

Risk Assessment

Performing bit manipulation and arithmetic operations on the same variable obscures the programmer's intentions and reduces readability. This in turn makes it more difficult for a security auditor or maintainer to determine which checks must be performed to eliminate security flaws and ensure data integrity.

Automated Detection

Fortify SCA Version 5.0 with the CERT C Rule Pack can detect violations of this recommendation that perform arithmetic and bit operations are performed on same line but cannot distinguish between operations of positive and negative numbers.

Related Vulnerabilities

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

References

\[[ISO/IEC 9899-1999|AA. C References#ISO/IEC 9899-1999]\] Section 6.2.6.2, "Integer types"
\[[ISO/IEC PDTR 24772|AA. C References#ISO/IEC PDTR 24772]\] "STR Bit Representations"
\[[MISRA 04|AA. C References#MISRA 04]\] Rules 6.4 and 6.5
\[[Steele 77|AA. C References#Steele 77]\]

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