CertC-INT32¶

Ensure that operations on signed integers do not result in overflow

Required inputs: IR, StaticSemanticAnalysis

Signed integer overflow is undefined behavior 36. Consequently, implementations have considerable latitude in how they deal with signed integer overflow. (See MSC15-C. Do not depend on undefined behavior.) An implementation that defines signed integer types as being modulo, for example, need not detect integer overflow. Implementations may also trap on signed arithmetic overflows, or simply assume that overflows will never happen and generate object code accordingly. It is also possible for the same conforming implementation to emit code that exhibits different behavior in different contexts. For example, an implementation may determine that a signed integer loop control variable declared in a local scope cannot overflow and may emit efficient code on the basis of that determination, while the same implementation may determine that a global variable used in a similar context will wrap.

For these reasons, it is important to ensure that operations on signed integers do not result in overflow. Of particular importance are operations on signed integer values that originate from a tainted source and are used as

Integer operands of any pointer arithmetic, including array indexing
The assignment expression for the declaration of a variable length array
The postfix expression preceding square brackets [] or the expression in square brackets [] of a subscripted designation of an element of an array object
Function arguments of type size_t or rsize_t (for example, an argument to a memory allocation function)

Integer operations will overflow if the resulting value cannot be represented by the underlying representation of the integer. The following table indicates which operations can result in overflow.

Operator	Overflow	Operator	Overflow	Operator	Overflow	Operator	Overflow
`+`	Yes	`-=`	Yes	`<<`	Yes	`<`	No
`-`	Yes	`*=`	Yes	`>>`	No	`>`	No
`*`	Yes	`/=`	Yes	`&`	No	`>=`	No
`/`	Yes	`%=`	Yes	`\|`	No	`<=`	No
`%`	Yes	`<<=`	Yes	`^`	No	`==`	No
`++`	Yes	`>>=`	No	`~`	No	`!=`	No
`--`	Yes	`&=`	No	`!`	No	`&&`	No
`=`	No	`\|=`	No	`unary +`	No	`\|\|`	No
`+=`	Yes	`^=`	No	`unary -`	Yes	`?:`	No

The following sections examine specific operations that are susceptible to integer overflow. When operating on integer types with less precision than int, integer promotions are applied. The usual arithmetic conversions may also be applied to (implicitly) convert operands to equivalent types before arithmetic operations are performed. Programmers should understand integer conversion rules before trying to implement secure arithmetic operations. (See INT02-C. Understand integer conversion rules.)

Implementation Details

GNU GCC invoked with the -fwrapv command-line option defines the same modulo arithmetic for both unsigned and signed integers.

GNU GCC invoked with the -ftrapv command-line option causes a trap to be generated when a signed integer overflows, which will most likely abnormally exit. On a UNIX system, the result of such an event may be a signal sent to the process.

GNU GCC invoked without either the -fwrapv or the -ftrapv option may simply assume that signed integers never overflow and may generate object code accordingly.

Atomic Integers

The C Standard defines the behavior of arithmetic on atomic signed integer types to use two's complement representation with silent wraparound on overflow; there are no undefined results. Although defined, these results may be unexpected and therefore carry similar risks to unsigned integer wrapping. (See INT30-C. Ensure that unsigned integer operations do not wrap.) Consequently, signed integer overflow of atomic integer types should also be prevented or detected.

Addition

Addition is between two operands of arithmetic type or between a pointer to an object type and an integer type. This rule applies only to addition between two operands of arithmetic type. (See ARR37-C. Do not add or subtract an integer to a pointer to a non-array object and ARR30-C. Do not form or use out-of-bounds pointers or array subscripts.)

Incrementing is equivalent to adding 1.

Noncompliant Code Example

This noncompliant code example can result in a signed integer overflow during the addition of the signed operands si_a and si_b:

void func(signed int si_a, signed int si_b) {
  signed int sum = si_a + si_b;
  /* ... */
}

Compliant Solution

This compliant solution ensures that the addition operation cannot overflow, regardless of representation:

#include <limits.h>
 
void f(signed int si_a, signed int si_b) {
  signed int sum;
  if (((si_b > 0) && (si_a > (INT_MAX - si_b))) ||
      ((si_b < 0) && (si_a < (INT_MIN - si_b)))) {
    /* Handle error */
  } else {
    sum = si_a + si_b;
  }
  /* ... */
}

Compliant Solution (GNU)

This compliant solution uses the GNU extension __builtin_sadd_overflow, available with GCC, Clang, and ICC:

void f(signed int si_a, signed int si_b) {
  signed int sum;
  if (__builtin_sadd_overflow(si_a, si_b, &sum)) {
    /* Handle error */
  }
  /* ... */
}

Subtraction

Subtraction is between two operands of arithmetic type, two pointers to qualified or unqualified versions of compatible object types, or a pointer to an object type and an integer type. This rule applies only to subtraction between two operands of arithmetic type. (See ARR36-C. Do not subtract or compare two pointers that do not refer to the same array, ARR37-C. Do not add or subtract an integer to a pointer to a non-array object, and ARR30-C. Do not form or use out-of-bounds pointers or array subscripts for information about pointer subtraction.)

Decrementing is equivalent to subtracting 1.

Noncompliant Code Example

This noncompliant code example can result in a signed integer overflow during the subtraction of the signed operands si_a and si_b:

void func(signed int si_a, signed int si_b) {
  signed int diff = si_a - si_b;
  /* ... */
}

Compliant Solution

This compliant solution tests the operands of the subtraction to guarantee there is no possibility of signed overflow, regardless of representation:

#include <limits.h>
 
void func(signed int si_a, signed int si_b) {
  signed int diff;
  if ((si_b > 0 && si_a < INT_MIN + si_b) ||
      (si_b < 0 && si_a > INT_MAX + si_b)) {
    /* Handle error */
  } else {
    diff = si_a - si_b;
  }

  /* ... */
}

Compliant Solution (GNU)

This compliant solution uses the GNU extension __builtin_ssub_overflow, available with GCC, Clang, and ICC:

void func(signed int si_a, signed int si_b) {
  signed int diff;
  if (__builtin_ssub_overflow(si_a, si_b, &diff)) {
    /* Handle error */
  }

  /* ... */
}

Multiplication

Multiplication is between two operands of arithmetic type.

Noncompliant Code Example

This noncompliant code example can result in a signed integer overflow during the multiplication of the signed operands si_a and si_b:

void func(signed int si_a, signed int si_b) {
  signed int result = si_a * si_b;
  /* ... */
}

Compliant Solution

The product of two operands can always be represented using twice the number of bits than exist in the precision of the larger of the two operands. This compliant solution eliminates signed overflow on systems where long long is at least twice the precision of int:

#include <stddef.h>
#include <assert.h>
#include <limits.h>
#include <inttypes.h>
 
extern size_t popcount(uintmax_t);
#define PRECISION(umax_value) popcount(umax_value)
 
void func(signed int si_a, signed int si_b) {
  signed int result;
  signed long long tmp;
  assert(PRECISION(ULLONG_MAX) >= 2 * PRECISION(UINT_MAX));
  tmp = (signed long long)si_a * (signed long long)si_b;
 
  /*
   * If the product cannot be represented as a 32-bit integer,
   * handle as an error condition.
   */
  if ((tmp > INT_MAX) || (tmp < INT_MIN)) {
    /* Handle error */
  } else {
    result = (int)tmp;
  }
  /* ... */
}

The assertion fails if long long has less than twice the precision of int. The PRECISION() macro and popcount() function provide the correct precision for any integer type. (See INT35-C. Use correct integer precisions.)

Compliant Solution

The following portable compliant solution can be used with any conforming implementation, including those that do not have an integer type that is at least twice the precision of int:

#include <limits.h>
 
void func(signed int si_a, signed int si_b) {
  signed int result;
  if (si_a > 0) {  /* si_a is positive */
    if (si_b > 0) {  /* si_a and si_b are positive */
      if (si_a > (INT_MAX / si_b)) {
        /* Handle error */
      }
    } else { /* si_a positive, si_b nonpositive */
      if (si_b < (INT_MIN / si_a)) {
        /* Handle error */
      }
    } /* si_a positive, si_b nonpositive */
  } else { /* si_a is nonpositive */
    if (si_b > 0) { /* si_a is nonpositive, si_b is positive */
      if (si_a < (INT_MIN / si_b)) {
        /* Handle error */
      }
    } else { /* si_a and si_b are nonpositive */
      if ( (si_a != 0) && (si_b < (INT_MAX / si_a))) {
        /* Handle error */
      }
    } /* End if si_a and si_b are nonpositive */
  } /* End if si_a is nonpositive */

  result = si_a * si_b;
}

Compliant Solution (GNU)

This compliant solution uses the GNU extension __builtin_smul_overflow, available with GCC, Clang, and ICC:

void func(signed int si_a, signed int si_b) {
  signed int result;
  if (__builtin_smul_overflow(si_a, si_b, &result)) {
    /* Handle error */
  }
}

Division

Division is between two operands of arithmetic type. Overflow can occur during two's complement signed integer division when the dividend is equal to the minimum (negative) value for the signed integer type and the divisor is equal to -1. Division operations are also susceptible to divide-by-zero errors. (See INT33-C. Ensure that division and remainder operations do not result in divide-by-zero errors.)

Noncompliant Code Example

This noncompliant code example prevents divide-by-zero errors in compliance with INT33-C. Ensure that division and remainder operations do not result in divide-by-zero errors but does not prevent a signed integer overflow error in two's-complement.

void func(signed long s_a, signed long s_b) {
  signed long result;
  if (s_b == 0) {
    /* Handle error */
  } else {
    result = s_a / s_b;
  }
  /* ... */
}

Implementation Details

On the x86-32 architecture, overflow results in a fault, which can be exploited as a denial-of-service attack.

Compliant Solution

This compliant solution eliminates the possibility of divide-by-zero errors or signed overflow:

#include <limits.h>
 
void func(signed long s_a, signed long s_b) {
  signed long result;
  if ((s_b == 0) || ((s_a == LONG_MIN) && (s_b == -1))) {
    /* Handle error */
  } else {
    result = s_a / s_b;
  }
  /* ... */
}

Remainder

The remainder operator provides the remainder when two operands of integer type are divided. Because many platforms implement remainder and division in the same instruction, the remainder operator is also susceptible to arithmetic overflow and division by zero. (See INT33-C. Ensure that division and remainder operations do not result in divide-by-zero errors.)

Noncompliant Code Example

Many hardware architectures implement remainder as part of the division operator, which can overflow. Overflow can occur during a remainder operation when the dividend is equal to the minimum (negative) value for the signed integer type and the divisor is equal to -1. It occurs even though the result of such a remainder operation is mathematically 0. This noncompliant code example prevents divide-by-zero errors in compliance with INT33-C. Ensure that division and remainder operations do not result in divide-by-zero errors but does not prevent integer overflow:

void func(signed long s_a, signed long s_b) {
  signed long result;
  if (s_b == 0) {
    /* Handle error */
  } else {
    result = s_a % s_b;
  }
  /* ... */
}

Implementation Details

On x86-32 platforms, the remainder operator for signed integers is implemented by the idiv instruction code, along with the divide operator. Because LONG_MIN / -1 overflows, it results in a software exception with LONG_MIN % -1 as well.

Compliant Solution

This compliant solution also tests the remainder operands to guarantee there is no possibility of an overflow:

#include <limits.h>
 
void func(signed long s_a, signed long s_b) {
  signed long result;
  if ((s_b == 0 ) || ((s_a == LONG_MIN) && (s_b == -1))) {
    /* Handle error */
  } else {
    result = s_a % s_b;
  }
  /* ... */
}

Left-Shift Operator

The left-shift operator takes two integer operands. The result of E1 << E2 is E1 left-shifted E2 bit positions; vacated bits are filled with zeros.

The C Standard, 6.5.7, paragraph 4 [ ISO/IEC 9899:2011], states

If E1 has a signed type and nonnegative value, and E1 × 2^E2 is representable in the result type, then that is the resulting value; otherwise, the behavior is undefined.

In almost every case, an attempt to shift by a negative number of bits or by more bits than exist in the operand indicates a logic error. These issues are covered by INT34-C. Do not shift an expression by a negative number of bits or by greater than or equal to the number of bits that exist in the operand.

Noncompliant Code Example

This noncompliant code example performs a left shift, after verifying that the number being shifted is not negative, and the number of bits to shift is valid. The PRECISION() macro and popcount() function provide the correct precision for any integer type. (See INT35-C. Use correct integer precisions.) However, because this code does no overflow check, it can result in an unrepresentable value.

#include <limits.h>
#include <stddef.h>
#include <inttypes.h>

extern size_t popcount(uintmax_t);
#define PRECISION(umax_value) popcount(umax_value)

void func(signed long si_a, signed long si_b) {
  signed long result;
  if ((si_a < 0) || (si_b < 0) ||
      (si_b >= PRECISION(ULONG_MAX)) {
    /* Handle error */
  } else {
    result = si_a << si_b;
  }
  /* ... */
}

Compliant Solution

This compliant solution eliminates the possibility of overflow resulting from a left-shift operation:

#include <limits.h>
#include <stddef.h>
#include <inttypes.h>

extern size_t popcount(uintmax_t);
#define PRECISION(umax_value) popcount(umax_value)

void func(signed long si_a, signed long si_b) {
  signed long result;
  if ((si_a < 0) || (si_b < 0) ||
      (si_b >= PRECISION(ULONG_MAX)) ||
      (si_a > (LONG_MAX >> si_b))) {
    /* Handle error */
  } else {
    result = si_a << si_b;
  }
  /* ... */
}

Unary Negation

The unary negation operator takes an operand of arithmetic type. Overflow can occur during two's complement unary negation when the operand is equal to the minimum (negative) value for the signed integer type.

Noncompliant Code Example

This noncompliant code example can result in a signed integer overflow during the unary negation of the signed operand s_a:

void func(signed long s_a) {
  signed long result = -s_a;
  /* ... */
}

Compliant Solution

This compliant solution tests the negation operation to guarantee there is no possibility of signed overflow:

#include <limits.h>
 
void func(signed long s_a) {
  signed long result;
  if (s_a == LONG_MIN) {
    /* Handle error */
  } else {
    result = -s_a;
  }
  /* ... */
}

Risk Assessment

Integer overflow can lead to buffer overflows and the execution of arbitrary code by an attacker.

Rule	Severity	Likelihood	Remediation Cost	Priority	Level
INT32-C	High	Likely	High	P9	L2

Related Guidelines

Taxonomy	Taxonomy item	Relationship
CERT C	INT02-C. Understand integer conversion rules	Prior to 2018-01-12: CERT: Unspecified Relationship
CERT C	INT35-C. Use correct integer precisions	Prior to 2018-01-12: CERT: Unspecified Relationship
CERT C	INT33-C. Ensure that division and remainder operations do not result in divide-by-zero errors	Prior to 2018-01-12: CERT: Unspecified Relationship
CERT C	INT34-C. Do not shift an expression by a negative number of bits or by greater than or equal to the number of bits that exist in the operand	Prior to 2018-01-12: CERT: Unspecified Relationship
CERT C	ARR30-C. Do not form or use out-of-bounds pointers or array subscripts	Prior to 2018-01-12: CERT: Unspecified Relationship
CERT C	ARR36-C. Do not subtract or compare two pointers that do not refer to the same array	Prior to 2018-01-12: CERT: Unspecified Relationship
CERT C	ARR37-C. Do not add or subtract an integer to a pointer to a non-array object	Prior to 2018-01-12: CERT: Unspecified Relationship
CERT C	MSC15-C. Do not depend on undefined behavior	Prior to 2018-01-12: CERT: Unspecified Relationship
CERT C	CON08-C. Do not assume that a group of calls to independently atomic methods is atomic	Prior to 2018-01-12: CERT: Unspecified Relationship
CERT Oracle Secure Coding Standard for Java	INT00-J. Perform explicit range checking to avoid integer overflow	Prior to 2018-01-12: CERT: Unspecified Relationship
ISO/IEC TR 24772:2013	Arithmetic Wrap-Around Error [FIF]	Prior to 2018-01-12: CERT: Unspecified Relationship
ISO/IEC TS 17961	Overflowing signed integers [intoflow]	Prior to 2018-01-12: CERT: Unspecified Relationship
CWE 2.11	CWE-190, Integer Overflow or Wraparound	2017-05-18: CERT: Partial overlap
CWE 2.11	CWE-191	2017-05-18: CERT: Partial overlap
CWE 2.11	CWE-680	2017-05-18: CERT: Partial overlap

Bibliography

[ Dowd 2006]	Chapter 6, "C Language Issues" ("Arithmetic Boundary Conditions," pp. 211-223)
[ ISO/IEC 9899:2011]	Subclause 6.5.5, "Multiplicative Operators"
[ Seacord 2013b]	Chapter 5, "Integer Security"
[ Viega 2005]	Section 5.2.7, "Integer Overflow"
[ Warren 2002]	Chapter 2, "Basics"

Excerpt from SEI CERT C Coding Standard: Rules for Developing Safe, Reliable, and Secure Systems (2016 Edition) and SEI CERT C Coding Standard [https://cmu-sei.github.io/secure-coding-standards/sei-cert-c-coding-standard/rules/integers-int/int32-c], Copyright (C) 1995-2026 Carnegie Mellon University. See section 9.4. "3rd-Party Licenses" in the documentation for full details.

Possible Messages

Key	Text	Severity	Disabled
certain_shift_amount_negative	Shift by a negative bit count (undefined behavior)	None	False
certain_shift_amount_too_large	Shift by the integer width or more (undefined behavior)	None	False
certain_shift_right_negative	Right shift with negative left-hand-side	None	False
overflow	Arithmetic computation may cause overflow	None	False
underflow	Arithmetic computation may cause overflow	None	False

Options¶

This rule shares the following common options: exclude_in_macros, exclude_messages_in_system_headers, excludes, extend_exclude_to_macro_invocations, includes, justification_checker, languages, post_processing, provider, report_at, severity
The following places define options that affect this rule: Stylechecks, Analysis-GlobalOptions

abstract_interpretation_maximal_tracked_array_index¶

abstract_interpretation_maximal_tracked_array_index : int = 10

The number of explicit indices in array expressions per routine tracked by the "symbolic expression analysis". For example, consider the following program.

extern signed char a[6];
int main()
{
    if (a[2] < 0)
    {
        a[2]++;
    }
    if (a[3] < 0)
    {
        a[3]++;
    }
    if (a[4] < 0)
    {
        a[4]++;
    }
    return 0;
}

If the value of this option is set to 2, the first two array index expressions encountered in the routine are tracked. Hence, the analysis can use the facts a[2] < 0 and a[3] < 0 to infer that a[2]++ and a[3]++ do not overflow, but it will not track the third array access in this routine.

A higher value of the option can cause more consumption of memory and time for the analysis.

abstract_interpretation_overflow¶

abstract_interpretation_overflow : bool = True

Use abstract-interpretation-based "symbolic expression analysis" as additional postprocessing step.

abstract_interpretation_overflow_unrolling_level¶

abstract_interpretation_overflow_unrolling_level : int = 0

How many levels of conditions are traversed to compute additional constraints for the "symbolic expression analysis".

check_signed¶

check_signed : bool = True

Whether issues for signed integer operations should be reported. For casts including implicit conversions, the target type of the cast is used.

check_unsigned¶

check_unsigned : bool = False

Whether wrap-around for unsigned integer operations should be reported. For casts including implicit conversions, the target type of the cast is used.

suppress_well_defined_findings¶

suppress_well_defined_findings : SuppressionMode = 'NONE'

Some overflows have well-defined semantics in all C/C++ standard versions. The typical example is UINT_MAX+1 which is well-defined as 0 via wraparound. This differs from INT_MAX+1 which is either undefined or implementation-defined depending on the considered standard version. Most CPUs will compute INT_MIN but this wraparound is not guaranteed by any C/C++ standard.

Both cases are overflows and are reported by this rule. However, one might want to suppress messages for the well-defined cases. To suppress these activate this option.

Different C and C++ standard versions differ in what is well-defined, implementation-defined, or undefined. Luckily, if we only consider well-defined and do not discern between implementation-defined and undefined, we end up with only two groups: pre-C++20 and since-C++20.

Option Types¶

These types are used by options listed above:

SuppressionMode¶

An enumeration.

NONE

Suppress nothing.

PRE_CPP2020

Suppress findings that are well-defined before C++20. These are:

Over- and underflows of unsigned integers during addition, subtraction, and multiplication
Conversions from unsigned to unsigned integers
Wrap-around caused by left-shifting of unsigned integer

CPP2020

Suppress findings that are well-defined since C++20. These are:

Over- and underflows of unsigned integers during addition, subtraction, and multiplication
Conversions between signed and unsigned integers
Wrap-around caused by left-shifting
Shifting negative integers

Surprising mechanics of C++20 signed narrow integers

Since C++20, casts between signed and unsigned are defined as two-complement wrap-around. Overflows of signed integers are still undefined behavior and are reported by this rule. But, due to integer promotion rules, certain expressions are computed using wider integer types, which can lead to the false impression that this is no longer the case, because no overflow findings are reported there.

Suppose, that the code is compiled on a platform where short is smaller than or equal to half the size of an int. Very commonly the sizes are 2 and 4. This assumption is thus true for many platforms.

In this case, narrow signed integer types such as short or signed char are first implicitly promoted to int before the arithmetic operation is executed. Because of this promotion, the actual operation does not overflow and is thus well-defined. After the operation, an implicit cast is performed to the narrower type. This cast is well-defined in C++20 as wrapping around.

Consider the following snippet:

    static_assert(sizeof(short) == 2);
    static_assert(sizeof(int) == 4);
    short a = 0x1000;
    short b = 0x1001;
    short c = a*b;

C++20 defines c as 0x1000. The reason is that a*b is implicitly promoted to

static_cast<int>
(a)*static_cast<int>(b)

. After the promotion, the multiplication does not overflow and yields a well-defined 0x1001000. This number is then implicitly cast to 0x1000 which is also a well-defined operation.

An analogous effect can be observed for signed short addition and multiplication. Another effect is that it is well-defined to shift by up to as many bits as int has even if the shifted integer has fewer bits.

DERIVE_FROM_IR

Derive the language version from the IR compilation flags and suppress findings accordingly.