Integer Overflow - Building Secure Software - Lecture Slides, Slides of Software Engineering

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Integer Overflow

Overflow example

• byte b = 120; //0x
• for(int i = 0; i < 140; i++)
• System. out.println(b++);
• Where b = 0x80, b becomes

negative

**-

0 1 2 3**

Overflow Due to Downcasting

• int test = 128; //0x
• byte c = (byte) test;//should be 128
• System.out.printf("byte c should be %d but is %d%n",test, c);
• if(c == -128)
• System.out.println("error");
• The 8-th bit is the sign bit for bytes.
• Code Output:
• byte c should be 128 but is -
• error

Avoiding Overflow

• Use unsigned types when possible

• Check inputs and outputs

– I.e. Adding two positives should not result in

negative

• Make tests as simple as possible, do not try to

be clever

• Set limits on method parameters

• Use David LeBlanc’s SafeInt class (C++)