Positioning an object in memory using Java?

Question

I used simple test code and created an object in Java.

I used the System.identityHashCode function to get a roughly equivalent address. Please review this question. I asked for more context: Click here

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 End 

Now the question is how to find out that the numbers above are caused by ASLR. I am using Windows 7 and cannot disable ASLR.

In other words, if ASLR is disabled, do you think that I will get the same number generated every time I run the code?

Sample code that I used:

 // public class HashCodeTest { public int hashCode() { return 0xDEADBEEF; } public static void main(String[] argv) { HashCodeTest o1 = new HashCodeTest(); System.out.println("First: " + o1); System.out.println("Using System.identityHashCode():"); System.out.println("First: " + System.identityHashCode(o1)); } } 

EDIT:

What am I trying to do? I want to see if address layout randomization (ASLR) is effective in Windows 7 than in Windows Vista. I want to plot the frequency distribution or use some other statistical tools for the data to make sure that it is more predictable in one OS than others for ALL applications in general.

Answer 1

I was the person who suggested that the OP should use identityHashcode() values ​​as ersatz memory addresses. My idea is that although they are not real memory addresses, they are based on the memory address of the object in question, when identityHashcode() first called ... on a typical JVM implementation. This makes them a reasonable proxy for real memory addresses for the purposes of this experiment.

My answer to OP is that these results strongly indicate that ASLR is either not enabled or not efficient at randomizing the actual addresses of Java objects.

I would expect some natural unpredictability of the object address due to non-deterministic factors in the JVM bootstrap sequence. That is why you see 3 or 4 different hash values ​​for this object in different runs. However, the fact that you see certain values ​​multiple times indicates that systematic randomization does not occur. In other words, there is no ASLR.

Strictly speaking, we need to study the JVM source code to verify that identityHashcode does communicate something that would change if ASLR were effective ... but I don't have time right now.

---

(FWIW: I agree with other people who answered / commented that ASLR is not required for Java anyway. Modify some unsupported error in the JVM native code, Java JVM just does not allow an isolated executable program as an isolated Java program (for example, native) code. And even a privileged Java program could only do this through its own custom library.)

Answer 2

In general, in Java, the identityHashCode() function does not indicate anything about memory addresses. The only thing indicated is that these numbers are constant throughout the life of the object and that they are different in different ways for different objects.

• In 64-bit virtual machines with sufficient memory, you can create more than 2 ^ 32 objects at a time, so even the second condition cannot mean "always different for different objects."

• Modern Sun VMs use a collective garbage collector, which means objects move around their lives. Then the memory address is changed, and the identification hash code must be the same - therefore, it can not always be the memory address.

Therefore, you cannot rely on anything other than the above condition. On some virtual machines (especially old ones with non-moving garbage collectors), the number may be something that could be converted in C to a memory address (or the starting address when creating the object), but another valid implementation would be the pseudo-random number specified at creation object, or simple counter.

Answer 3

Address Space Allocation Randomization (ASLR):

• not required for Java applications
• unlikely to be useful for Java applications
• actively played out by memory management methods of many JVM implementations

The Sun JVM, for example, will allocate a large section of virtual memory at startup (presumably using the heap functions of the OS to capture one large block), and then proceed to manage this memory as its own heap in very good condition - a known method. That is, the spaces Eden, Survivor, Tenured heap and PermGen are all laid out equally, and I am sure that ASLR can not do anything about it.

However, hacker methods that ASLR is trying to prevent are simply impossible in any case in Java for several reasons:

• Most JVM implementations often copy objects from one place to another in memory. Therefore, it is impossible to predict with any certainty where any object will be at any given time.

• Java code cannot allocate objects of arbitrary size on the Java stack, so stack overflows are not possible.

• It is not possible to undermine the Java memory model from Java code; the only code for which this is possible is platform-based code called through JNI. (Or sun.misc.Unsafe , which is an insecure shell, mostly for the same thing.) Therefore, if you use the Java SecurityManager functions correctly, none of the rogue processes can write to any dangerous memory location.

Perhaps you should describe what you are trying to accomplish and why you think ASLR will help you with this. I am sure that (a) ASLR has no effect on your Java programs, and (b) that OK, because you really do not want this.