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CS485G Spring 2015 3

(d) Operating systems need to deal with the intricacies of machine

code as they manipulate processes (keeping track of ﬂoating

point registers, the program counter, memory mapping tables)

(e) Malware is often written in x86 assembler.

4 Memory-referencing bugs

1. C (and C++) are subject to memory-referencing bugs.

(a) Array references out of bounds. Example (the actual result is

architecture-speciﬁc; the results shown are on x86 64)

1 void fun(int index)

2 {

3 volatile double d[1] = {3.14};

4 volatile long int a[2];

5 a[index] = 9223372036854775803L; // index out of bounds?

6 printf("%lf\n", d[0]);

7 }

8 fun(0); // 3.14

9 fun(1); // 3.14

10 fun(-1); // 3.14

11 fun(-2); // 0.0

12 fun(2); // nan (not a number)

13 fun(-3); // 3.14

14 fun(3); // 3.14 followed by fault during the return

Reason: a and d are adjacent on the stack. When you go past the

end of a, you might modify d, or you might access saved state

on the stack, ruining the return address.

(b) Lecture 2, 1/16/2015 : lab 1

(d) Dereferencing invalid pointers

(e) Improper allocating and deallocating memory regions

(f) Unfortunately, the symptoms may be unrelated to the causes,

and the effect might be visible only long after it is generated.

(g) Some languages prevent such errors: Java, Ruby, Haskell, ML,

but they are not usually used for systems programming.

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