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- 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:
Operator | Overflow |
| Operator | Overflow |
| Operator | Overflow |
| Operator | Overflow |
|---|---|---|---|---|---|---|---|---|---|---|
yes |
| yes |
| yes |
| < | no | |||
yes |
| yes |
| >> | yes |
| > | no | ||
yes |
| yes |
| & | no |
| >= | no | ||
yes |
| yes |
| | | no |
| <= | no | ||
yes |
| yes |
| ^ | no |
| == | no | ||
++ | yes |
| >>= | yes |
| ~ | no |
| != | no |
-- | yes |
| &= | no |
| ! | no |
| && | no |
= | no |
| |= | no |
| un + | no |
| || | no |
yes |
| ^= | no |
| yes |
| ?: | no |
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The following sections examine specific operations that are susceptible to integer overflow. The specific tests that are required to guarantee that the operation does not result in an integer overflow depend on the signedness of the integer types. 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 \[[INT02-A. Understand integer conversion rules]\]).
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INT32-EX1. Unsigned integers can exhibit modulo behavior only when this behavior is necessary for the proper execution of the program. It is recommended that the variable declaration is be clearly commented as supporting modulo behavior and that each operation on that integer is also be clearly commented as supporting modulo behavior.
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A Linux kernel vmsplice exploit, described at http://www.avertlabs.com/research/blog/index.php/2008/02/13/analyzing-the-linux-kernel-vmsplice-exploit/,
documents a vulerability vulnerability and exploit arising directly out of integer
overflow.
References
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\[[Dowd 06|AA. C References#Dowd 06]\] Chapter 6, "C Language Issues" (Arithmetic Boundary Conditions, pp. 211-223) \[[ISO/IEC 9899-1999|AA. C References#ISO/IEC 9899-1999]\] Section 6.5, "Expressions," and Section 7.10, "Sizes of integer types <limits.h>" \[[Seacord 05|AA. C References#Seacord 05]\] Chapter 5, "Integers" \[[Viega 05|AA. C References#Viega 05]\] Section 5.2.7, "Integer overflow" \[[VU#551436|AA. C References#VU551436]\] \[[Warren 02|AA. C References#Warren 02]\] Chapter 2, "Basics" |
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