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Bitwise shifts include left-shift operations of the form shift-expression << additive-expression and right-shift operations of the form shift-expression >> additive-expression. The standard integer promotions are first performed on the operands, each of which has an integer type. The type of the result is that of the promoted left operand. If the value of the right operand is negative or is greater than or equal to the width of the promoted left operand, the behavior is undefined. (See undefined behavior 51.)

Do not shift an expression by a negative number of bits or by a number greater than or equal to the precision of the promoted left operand. The precision of an integer type is the number of bits it uses to represent values, excluding any sign and padding bits. For unsigned integer types, the width and the precision are the same; whereas for signed integer types, the width is one greater than the precision. This rule uses precision instead of width because, in almost every case, an attempt to shift by a number of bits greater than or equal to the precision of the operand indicates a bug (logic error). A logic error is different from overflow, in which there is simply a representational deficiency.  In general, shifts should be performed only on unsigned operands. (See INT13-C. Use bitwise operators only on unsigned operands.)

Noncompliant Code Example (Left Shift, Unsigned Type)

The result of E1 << E2 is E1 left-shifted E2 bit positions; vacated bits are filled with zeros. The following diagram illustrates the left-shift operation.

According to the C Standard, if E1 has an unsigned type, the value of the result is E1 * 2E2, reduced modulo 1 more than the maximum value representable in the result type. 

This noncompliant code example fails to ensure that the right operand is less than the precision of the promoted left operand:

Compliant Solution (Left Shift, Unsigned Type)

This compliant solution eliminates the possibility of shifting by greater than or equal to the number of bits that exist in the precision of the left operand:

The PRECISION() macro and popcount() function provide the correct precision for any integer type. (See INT35-C. Use correct integer precisions.)

Modulo behavior resulting from left-shifting an unsigned integer type is permitted by exception INT30-EX3 to INT30-C. Ensure that unsigned integer operations do not wrap.

Noncompliant Code Example (Left Shift, Signed Type)

The result of E1 << E2 is E1 left-shifted E2 bit positions; vacated bits are filled with zeros. If E1 has a signed type and nonnegative value, and E1 * 2E2 is representable in the result type, then that is the resulting value; otherwise, the behavior is undefined.

This noncompliant code example fails to ensure that left and right operands have nonnegative values and that the right operand is less than the precision of the promoted left operand. This example does check for signed integer overflow in compliance with INT32-C. Ensure that operations on signed integers do not result in overflow.

Shift operators and other bitwise operators should be used only with unsigned integer operands in accordance with INT13-C. Use bitwise operators only on unsigned operands.

Compliant Solution (Left Shift, Signed Type)

In addition to the check for overflow, this compliant solution ensures that both the left and right operands have nonnegative values and that the right operand is less than the precision of the promoted left operand:

 

Noncompliant Code Example (Right Shift)

The result of E1 >> E2 is E1 right-shifted E2 bit positions. If E1 has an unsigned type or if E1 has a signed type and a nonnegative value, the value of the result is the integral part of the quotient of E1 / 2E2. If E1 has a signed type and a negative value, the resulting value is implementation-defined and can be either an arithmetic (signed) shift

or a logical (unsigned) shift

This noncompliant code example fails to test whether the right operand is greater than or equal to the precision of the promoted left operand, allowing undefined behavior:

When working with signed operands, making assumptions about whether a right shift is implemented as an arithmetic (signed) shift or a logical (unsigned) shift can also lead to vulnerabilities. (See INT13-C. Use bitwise operators only on unsigned operands.)

Compliant Solution (Right Shift)

This compliant solution eliminates the possibility of shifting by greater than or equal to the number of bits that exist in the precision of the left operand:

Implementation Details

GCC has no options to handle shifts by negative amounts or by amounts outside the width of the type predictably or to trap on them; they are always treated as undefined. Processors may reduce the shift amount modulo the width of the type. For example, 32-bit right shifts are implemented using the following instruction on x86-32:

The sarl instruction takes a bit mask of the least significant 5 bits from %cl to produce a value in the range [0, 31] and then shift %eax that many bits:

where %eax stores the least significant bits in the doubleword to be shifted, and %edx stores the most significant bits.

Risk Assessment

Although shifting a negative number of bits or shifting a number of bits greater than or equal to the width of the promoted left operand is undefined behavior in C, the risk is generally low because processors frequently reduce the shift amount modulo the width of the type.

Rule

Severity

Likelihood

Remediation Cost

Priority

Level

INT34-C

Low

Unlikely

Medium

P2

L3

Automated Detection

Tool

Version

Checker

Description

Astrée17.04i Supported, but no explicit checker
CodeSonar4.4

LANG.ARITH.BIGSHIFT
LANG.ARITH.NEGSHIFT

Shift amount exceeds bit width
Negative shift amount

Compass/ROSE

 

 

Can detect violations of this rule. Unsigned operands are detected when checking for INT13-C. Use bitwise operators only on unsigned operands

Coverity2017.07

BAD_SHIFT

Implemented
ECLAIR1.2CC2.INT34Partially implemented
LDRA tool suite9.5.6

51 S, 403 S, 479 S

Partially implemented

Parasoft C/C++test9.5MISRA-038 
Polyspace Bug FinderR2016a

Shift of a negative value, Shift operation overflow

Shift operator on negative value

Overflow from shifting operation

PRQA QA-C9.3

0499
2790
2791 (D)
2792 (A)
2793 (S)

Partially implemented
Cppcheck1.66shiftNegative, shiftTooManyBits

Context sensitive analysis
Warns whenever Cppcheck sees a negative shift for a POD expression
(The warning for shifting too many bits is written only if Cppcheck has sufficient type information and you use --platform to specify the sizes of the standard types.)

Related Vulnerabilities

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

Related Guidelines

Bibliography

[C99 Rationale 2003]6.5.7, "Bitwise Shift Operators"
[Dowd 2006]Chapter 6, "C Language Issues"
[Seacord 2013b]Chapter 5, "Integer Security"
[Viega 2005]Section 5.2.7, "Integer Overflow"

 


21 Comments

  1. What's up with the test program in the references?

  2. In the code for the compliant solution (left shift, unsigned type) it appears that mod2 and mod1 are never assigned a value. Is there something in the solution that sets mod1 and mod2 that I am missing?

    1. Yeah, that's true. Also, this code snippets are non-compliant with DCL04-C. Do not declare more than one variable per declaration

      Also, the last compliant solution didn't even compile because the variables had different names (result vs uresult.

      I've fixed up some of these examples. I'll finish the rest later, unless someone beats me to it.

      1. I'm also confused as to why the Non-Compliant Code Example (Right Shift) has signed and unsigned examples, but the compliant solution only uses unsigned.

        I think we have a recommendation somewhere that INT13-C. Use bitwise operators only on unsigned operands. This should probably be referenced above.

        It may still be OK to show compliant solutions for signed types, so that if someone tries to do this we have an example of how to do it. However, they should reference INT13-C. Use bitwise operators only on unsigned operands to indicate that this practice is not recommended.

        1. This problem still (urgently) needs to be addressed.

          1. I think I addressed this problem by removing the signed types from the non-compliant solution. I also think we should not introduce a compliant solution that would violate one of our own recommendations.

  3. In the first NCCE, the diagram shows a '1' in the leftmost box, which disappears when the byte is shifted. Doesn't this represent an int that is 'too large' to be shifted properly? (either it's unsigned and > UINT_MAX/2 or it's signed & <0.

    1. Yes that is true, but perhaps OK in this context. The example is only meant to show how this works, and it is in the NCE section so is actually an example of the problem.

  4. uhhh... doesn't this line in the first CCE violate the rule?

    Note how UINT_MAX is being shifted by more bits than exist in the operand...

    1. robert sez:

      I'm thinking that because of short circuiting rules, the second operand to || is not executed if the first one is true

      1. In that case I guess this is a false positive that we can't really avoid without dynamic analysis

        1. Why? By the logic of his if statement, if the 2nd part of the if expression executes then {{ui2} < sizeof(CHAR_BIT). And since we know it is unsigned, we can therefore validate the shift operation at compile time (or with ROSE).

          1. And how do you propose we match the condition ui2 < sizeof(CHAR_BIT) using ROSE? Especially knowing that there are at least 100 different ways to structure that conditional statement?

            1. Right, there are 100 different ways to structure the cond. For now just worry about the two that are demonstrated by the code examples here.

  5. Sorry to come on such an old article.

    First of all : thank you very much because I was becoming creasy.

    Secondly : using a negative parameter should not be a bug. In terms of algorithmic, it is usefull. At least for me and my algorithm. To escape from undefined, I have to write two lines instead of one with an if. 

    I understand that in assembly language, the result is undefined. But in my opinion, C++ standard should accept it and manage it because writting code with a parameter that can be negative is natural, while shaking one's mind to anticipate what the processor will do is not. Or suppress simply every languages except assembly ! natural here  means that I do expect it to work and I don't ask myself the question.

    Moreover, the compiler does not issue any warning, even with all the warnings enabled. That's another argument to make it work.

    One may argue against me that if the compiler adds a test in the assembly translation of << or >>, it will be less efficient. My answer is C and C++ are (also) high level languages and the first priority is robustness. And one can include assembly code easily. Or we may have variants : >>> and <<<.

    1. Shifting by a negative number is explicitly mentioned as undefined behavior by the C and C++ standards. This gives different platform vendors the latitude to have the system do whatever they want (usually whatever is fastest) when they encounter it. I believe i86 ignores all but the least significant 5 bits of the right operand to a shift operation, so if you know your code only runs on those platforms you could rely on that behavior. But we don't recommend it, as your code is inherently non-portable.

      Given that other vendors most likely do something different when the right operand is negative, I doubt the standards committees are inclined to change this behavior.

      The compiler is not officiially required to issue a warning, although some try to. Perhaps your compiler simply cannot determine that shifting by a negative number is occurring. Alas, that means it is your responsibility to make sure your right operand is positive (and not more than the number of bits in your left operand.)

      1. So the C and C++ standards are bad ! Just my opinion.

        1. Only if you think that this feature (shifting a negative number of bits) outweighs the portability and speed that platforms gain by leaving this behavior undefined. Clearly the C committee believed otherwise (and still does).

  6. // 64-bit shifts on IA-32 platforms become
    sh[rl]dl  %eax, %edx
    sa[rl]l   %cl, %eax

     

    where %eax stores the least significant bits in the doubleword to be shifted, and %edx stores the most significant bits.

    about implementation details.

    I think 64bit data shifting cannot be written in a single pattern.
    LEFT shifing 64bit data should be
      shldl %cl, %eax, %edx
      s[ah]ll  %cl, %eax
    and RIGHT shifing 64bit data should be
      shrdl  %cl, %edx, %eax
      s[ah]rl %cl, %edx
    anyone please confirm this?
    1. GCC 4.9.0 produces (using AT&T syntax):

      So you are correct, two different patterns are needed. I have rectified this now, thank you for pointing it out!

      You can play with this yourself using http://gcc.godbolt.org if you don't have a GCC implementation handy.

      1. thanks, Aaron.

        I fixed the register operands for correct right shifting (-:

        and thanks too for the information on gcc.godbolt.org!