Integer values used in any of the the following ways must be guaranteed correct:
- as an array index
- in any pointer arithmetic
- as a length or size of an object
- as the bound of an array (for example, a loop counter)
- in security-critical code
Most integer operations can result in overflow if the resulting value cannot be represented by the underlying representation of the integer. The following table indicates which operators can result in overflow:
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The following sections examine specific operations that are susceptible to integer overflow. When operating on small types (smaller than int), integer conversion rules apply. The usual arithmetic conversions may also be applied to (implicitly) convert operands to equivalent types before arithmetic operations are performed. Make sure you understand implicit conversion rules before trying to implement secure arithmetic operations (see INT02-A. Understand integer conversion rules).
Integer overflow is undefined behavior. This means that implementations have a great deal of latitude in how they deal with signed integer overflow. An implementation that defines signed integer types as being modulo, for example, does 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 (see MSC15-A. Do not depend on undefined behavior). For these reasons, it is important to ensure that operations on signed integers do no result in signed overflow. Of particular importance, however, are operations on signed integer values that originate from untrusted sources and are used in any of the following ways:
- as an array index
- in any pointer arithmetic
- as a length or size of an object
- as the bound of an array (for example, a loop counter)
- as an argument to a memory allocation function
- in security critical code
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