CWE-1335: Incorrect Bitwise Shift of Integer
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Edit Custom FilterAn integer value is specified to be shifted by a negative amount or an amount greater than or equal to the number of bits contained in the value causing an unexpected or indeterminate result.
Specifying a value to be shifted by a negative amount is undefined in various languages. Various computer architectures implement this action in different ways. The compilers and interpreters when generating code to accomplish a shift generally do not do a check for this issue. Specifying an over-shift, a shift greater than or equal to the number of bits contained in a value to be shifted, produces a result which varies by architecture and compiler. In some languages, this action is specifically listed as producing an undefined result. ![]()
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![]() Languages C (Undetermined Prevalence) C++ (Undetermined Prevalence) C# (Undetermined Prevalence) Java (Undetermined Prevalence) JavaScript (Undetermined Prevalence) Operating Systems Class: Not OS-Specific (Undetermined Prevalence) Technologies Class: Not Technology-Specific (Undetermined Prevalence) Example 1 A negative shift amount for an x86 or x86_64 shift instruction will produce the number of bits to be shifted by taking a 2's-complement of the shift amount and effectively masking that amount to the lowest 6 bits for a 64 bit shift instruction. (bad code)
Example Language: C
unsigned int r = 1 << -5;
The example above ends up with a shift amount of -5. The hexadecimal value is FFFFFFFFFFFFFFFD which, when bits above the 6th bit are masked off, the shift amount becomes a binary shift value of 111101 which is 61 decimal. A shift of 61 produces a very different result than -5. The previous example is a very simple version of the following code which is probably more realistic of what happens in a real system. (bad code)
Example Language: C
int choose_bit(int reg_bit, int bit_number_from_elsewhere)
{
if (NEED_TO_SHIFT)
}{
reg_bit -= bit_number_from_elsewhere;
}return reg_bit; unsigned int handle_io_register(unsigned int *r) {
unsigned int the_bit = 1 << choose_bit(5, 10);
}
*r |= the_bit; return the_bit; (good code)
Example Language: C
int choose_bit(int reg_bit, int bit_number_from_elsewhere)
{
if (NEED_TO_SHIFT)
}{
reg_bit -= bit_number_from_elsewhere;
}return reg_bit; unsigned int handle_io_register(unsigned int *r) {
int the_bit_number = choose_bit(5, 10);
}
if ((the_bit_number > 0) && (the_bit_number < 63)) {
unsigned int the_bit = 1 << the_bit_number;
}*r |= the_bit; return the_bit; Note that the good example not only checks for negative shifts and disallows them, but it also checks for over-shifts. No bit operation is done if the shift is out of bounds. Depending on the program, perhaps an error message should be logged.
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