Shift operation overflow

Overflow from shifting operation

Description

This defect occurs when a shift operation can result in values that cannot be represented by the result data type. The data type of a variable determines the number of bytes allocated for the variable storage and constrains the range of allowed values.

The exact storage allocation for different data types depends on your processor. See Target processor type (-target).

Risk

Shift operation overflows can result in undefined behavior.

Fix

The fix depends on the root cause of the defect. Often the result details show a sequence of events that led to the defect. Use this event list to determine how the variables in the shift operation acquire their current values. You can implement the fix on any event in the sequence. If the result details do not show the event history, you can trace back using right-click options in the source code and see previous related events. See also Interpret Bug Finder Results in Polyspace Desktop User Interface.

You can fix the defect by:

• Using a bigger data type for the result of the shift operation so that all values can be accommodated.

• Checking for values that lead to the overflow and performing appropriate error handling.

See examples of fixes below.

If you do not want to fix the issue, add comments to your result or code to avoid another review. See Address Polyspace Results Through Bug Fixes or Justifications.

Examples

Left Shift of Integer

int left_shift(void) {

    int foo = 33;
    return 1 << foo; 
}

In the return statement of this function, bit-wise shift operation is performed shifting 1 foo bits to the left. However, an int has only 32 bits, so the range of the shift must be between 0 and 31. Therefore, this shift operation causes an overflow.

Correction — Different storage type

One possible correction is to store the shift operation result in a larger data type. In this example, by returning a long long instead of an int, the overflow defect is fixed.

long long left_shift(void) {

    int foo = 33;
    return 1LL << foo; 
}

Result Information

Group: Numerical

Language: C | C++

Default: Off

Command-Line Syntax: SHIFT_OVFL

Impact: Low

CWE ID: 189, 190