Help crack the Java 1.6 Classfile Verifier

pdoubleya writes "As part of the development of Mustang (Java 1.6), Sun is developing a new, smaller and faster classfile verifier which they want your help in trying to break. As Sun VP Graham Hamilton puts it in his blog entry, "As part of Mustang we will be delivering a whole new classfile verifier implementation based on an entirely new verification approach. The classfile verifier is the very heart of the whole Java sandbox model, so replacing both the implementation and the basic verification model is a Really Big Deal.... The new verifier is faster and smaller than the classic verifier, but at the same time it doesn't have the ten years of reassuring shakedown history that we have with the classic verifier." You can read about the new verifier on Gilad Bracha's blog, and join the new Crack the Verifier initiative to if you can break it. Read all about the Crack the Verifier - Challenge."

Comments

Take Java seriously

[rexguo]

Before those who go on to dismiss Java for various reasons (no matter how ignorant they are), take a look at the presentation given by Google at this year's JavaZone conference on how Google is using Java internally at extreme scales. Among them are AdWords and GMail.

Re:Take Java seriously

[Naikrovek]

I don't think Java is as slow as you think it is. It is very fast lately, and it is actually giving C a run for its money in some respects. It is *definitely* not the slug everyone thinks it is.

They're probably using Java because its not as slow as its reputation, and its becoming a commodity language in the enterprise lately. My corporate overlords have dictated the use of Java (IBMs WebSphere) for all current and future enterprise development, and most of us developers couldn't be happier. Everywhere I do contracting work for lately also uses Java. Java Is A Great Language(TM), especially since 5.0.

There used to be a time when I believed that all techies had agreed that Java was slow and bloated, but once I stopped reading Slashdot comments so religiously I began to see some truth. It isn't slow, it isn't bloated, and it isn't something I expect the Slashdot crowd (that I'm a founding member of) to understand anytime soon.

Re:Take Java seriously

[Anonymous Coward]

Java may not be "slow" any more, but it's an INSANE memory hog. Part of this is because the heap NEVER shrinks - as you allocate more memory, the heap just grows and grows until it reaches the heap limit (which can be user-set). The other part is because you need to load in large numbers of "support" classes, along with the virtual machine itself, just to get to something as simple as "Hello World".

Now, I need to explain that "heap NEVER shrinks" bit, because people are going to hop in and start talking about the garbage collector. Well, yes. The garbage collector does indeed free memory. HOWEVER, it only returns the memory to the Java VM heap, NEVER to the operating system.

So, simple real-world example. Java VM starts with 8MB heap. Your application allocates 40MB worth of objects, increasing the total heap size to 64MB. Java now has 24MB "free" memory within the VM, and 40MB used. You can determine this in code using Runtime.freeMemory() to get the memory free in the heap, and Runtime.maxMemory() to get the maximum memory the heap has available.

Your program then "releases" 38MB worth of that data as unneeded. Eventually the GC collects that. Now you have only 2MB used, but the maximum heap size (as determined by Runtime.maxMemory()) REMAINS at 64MB. So now Java has 62MB "free" within the VM. Even if your program never uses more than 8MB after that, Java will ALWAYS keep the 64MB heap. It will NEVER shrink it.

This is fine in server setups, but it absolutely blows for client apps, where it's not unusual for a certain process to require a lot of memory, and then be ready to release it back to the OS.

Ultimately, on client apps, this winds up causing swapping, as chunks of unused VM heap get swapped out in favor of applications that actually need the memory they allocate. Swapping = slow.

You could call this behavior a "memory leak" but it's almost certainly by design.

Java may not be slow, but it's definitely bloated.

Re:Take Java seriously

[bbn]

You realise that the C malloc/free calls are just the same right? It never releases memory back to the OS.

The only difference in this regard, is that java might alloc extra memory from the OS, when it could have run the garbage collector and reused some memory instead.

Very few programs actually ever release memory back to the OS.

Re:Take Java seriously

[jaywee]

That depends on libc implementation. glibc for example uses mmap() for bigger allocations - therefore is able to return the freed memory to the OS.

Re:Take Java seriously

[justsomebody]

While I agree with you on all accounts I can't help but comment you.

Both Java and .Net have the same problem. Sloppy memory. As long as you don't use a lot of atomic memory blocks with higher load on machine where it runs, everything is ok and just as you said it. I tested both on the same tests and always fallen in the same problem. No direct memory control, GC waited until it was too late, and everything started to crawl. GC somehow avoids doing work if software is taking most of the CPU, it is also the same reason why Java beats C or C++ on speed tests (Every speed test where they try to proove that Java or .Net is faster than C or C++) uses allocation and freeing of memory in some loop. And while both C and C++ actualy do free memory, Java and .Net just mark that as garbage and wait for GC to clean up the mess which doesn't happen if load is too high. Just take any test where Java was faster and test loop to make 2^32 instead of 1000 or 10000 calls. This way you will actualy use more memory than you actualy have with allocations.

p.s. Not bashing, just saying results of my testing. If you can suggest some approach, do that, but so far not even one person suggested something I haven't tried yet. Both Java or .Net would be a real gift for me if only I could use them for my needs.

Re:Take Java seriously

[jiushao]

It depends on what you mean by "slow." If you're talking about long running processes, then no, it isn't slow at all; in fact, it is quite fast. If you're talking about short-running processes, then the JVM startup time overshadows any commendable performance.

Yeah, it is a shame, if only Sun would do something like writing a new faster classfile verifier.

Re:Take Java seriously

[AKAImBatman]

The reasons to use Java on the server are quite simple. The combination of factors that attracted developers to Java in the first place make them want to use it on the server. Those factors are:

1. Cross-platform capability - Many companies still prefer to deploy applications on large Sun, IBM, or Linux (name your brand) servers. However, these companies would also like to give their developers Windows desktops so they can interact with the rest of the company. (Who most likely uses MS Office/Outlook.) As long as you avoid explicit path names, it is quite easy (and common!) to develop on a Windows machines but deploy on a Unix or Unix-like machine.

2. Automatic Memory Management - So your server is running along, and suddenly someone generates an unexpected error. In Java you can sleep soundly because even the worst programmer would have a hard time doing anything to completely take down the application. If you use a language that allows direct memory management, you have a good chance of that new guy coding a General Protection Fault/Segfault. The result is that your entire system coredumps when you least expect it.

3. Security - While Java is able to control the Security of the ENTIRE JVM through its security framework, most companies are happy with the lack of buffer overruns, code injection techniques, and other common attacks. That's not to say that a poor programmer can't put a security hole in the application wide enough to drive a Mack truck through, but at least you can rely on the underlying system not to betray you.

4. Flexibility - The Java server side frameworks are exceedingly flexible in their designs. For example, the servlet framework allows you to plug in your own custom server page technology. I have seen many a programmer (including myself) implement something like Reports by simply linking the ".rpt" extension to a custom servlet. The servlet then loads the requested configuration file and executes it. Very nice.

Another example is servlet filters. Need a security framework added in a hurry? Just add a filter servlet! It will execute before the rest of the code, allowing you to check the variables and security permissions to ensure that the client isn't trying any funny business.

5. API - When Java was first introduced, it absolutely creamed all the competing languages in the richness of its bundled API. As time has worn on, this has changed. However, Java still enjoys a sizable lead over even C/C++ with features such as Type IV (tested cross-platform, pure Java) JDBC database drivers. Unlike ODBC, many of these drivers have been tuned for excellent performance. Similarly, there are free APIs for handling Office Documents, PDF Creation/Editing, SOAP/XML-RPC communications, Object-Relational mapping, Image Management/Creation/Editing, CORBA, XML Databases, XSL-T, etc. While these APIs are all available for C/C++, there are significant cross-platform issues with many of them, as well as a lack of common "pluggable" APIs that allow for one API to many implementations.

Other languages have a hit/miss score with these sorts of features, often not providing these features, providing only a small subset, or only being available in an expensive commercial package.

6. Dynamic Loading - While C/C++ can manage dynamic loading of shared objects, it's a very difficult thing to implement. Java does it out of the box, with a full reflection API and interface support, thus allowing such wonderful code as Beans, Servlets, Pluggable Drivers, self-organizing code, and a host of other features that other systems can't compete with.

(If you don't believe me, try adding support for a feature in PHP sometime. "It's so simple! Just install the SO and recompile PHP!" Meh.)

7. Performance - This may sound like an odd thing to say, but the performance of Java is a key selling feature. Java server applications may execute more slowly than one written in C/C++ (just as C/C++ may execute more slowly than

Re:Take Java seriously

[icebattle]

Have you tried running your app with -XX:+UseParNewGC -XX:+UseConcMarkSweepGC?

This will allow the vm to do small amounts of gc whenever it has a chance, as opposed to wating until an allocation request will fail and then running through the entire heap.

Our app runs 24/7 and while the markets are open and 10meg+ of raw data is coming down the line every second, we can't allow the app to take a timeout for a gc run. The app runs in 256meg, too.

Re:Take Java seriously

[swillden]

Java will ALWAYS keep the 64MB heap. It will NEVER shrink it.

Who cares? This is completely irrelevant, as long as the JVM marks the pages it's not using as discardable, which modern JVMs on modern OSes do.

You have to remember that all memory is virtual. I can allocate a 1GB array, but as long as I never actually touch the pages (making them "dirty"), no storage, whether RAM or disk, is ever used. When the JVM allocates its 64MB, that virtual memory is initially all "clean" and therefore consumes no RAM. As it's used, it gets dirtied and physical RAM gets mapped to it... but when a garbage allocation occurs, both the Sun and IBM JVMs mark the now-unused pages as clean, allowing the OS to reuse them at will. Effectively, they no longer consume any space.

Even if the JVMs didn't mark the pages as clean, the impact of the JVM holding onto the 64MB wouldn't be that significant. The allocated, dirtied but now-unused memory will simply get swapped out to disk, allowing those pages of RAM to be used by other applications.

With a decent OS, it really doesn't matter if an app holds onto some memory that it's not using, especially if it has the decency to tell the OS that it's not actually using it.

That said, there is a "problem" here, it's just not the one you're pointing out. The problem, if you want to call it that, arises from the generational, copying, compacting garbage collector used by modern VMs. Now, don't get me wrong, this GC is very cool. It's significantly more efficient than typical malloc/free memory management, *and* it eliminates some major classes of memory leaks (programmers can still leak memory with GC, but it's harder).

Without getting into the details, although GC is safer and more efficient in terms of CPU cycles, and although on average it doesn't use a great deal more memory than manual allocation would use (particularly since many performance-sensitive manual allocation apps end up managing their own memory pools in order to avoid the run-time cost of malloc() and free()), GC does tend to increase the number of memory pages that get touched over time.

Why? Two reasons. First, suppose the application allocates a small chunk of memory, uses it, frees it, allocates another small chunk (about the same size), and then uses and frees it. Most malloc()/free() implementations will tend to return the same chunk of memory for both allocations. Repeating the process a thousand times (which isn't all that uncommon) will probably only dirty a single page of memory. With the sort of garbage collector used by current JVMs, every one of those allocations will return a different chunk of memory, and many pages will therefore be dirtied. In terms of CPU cycles, GC wins big, because, rather than a thousand small frees, there is one big one. And allocation is up to two orders of magnitude faster. It may sound like the GC approach is going to use a lot more memory, on average, but it's not that bad because of the tendency of malloc/free-managed heaps to become fragmented. On average, GC implementations don't have many more pages in use than malloc/free implementations, and may actually have less, but the GC allocators tend to "roam" across the pages.

Second, the "copying" nature of the GC. The main thing that makes generational, copying GC-based memory management so fast it that it never has to deal with fragmentation. To describe it in an oversimplified way: Every now and then, the GC relocates all of the in-use objects into a nice, compact block. That makes allocation fast, and tends to reduce the number of pages in active use, but it increases the number of pages that get dirtied and therefore require actual RAM to be provided by the OS. The copying also has a cost in CPU cycles, but it's small relative to the cost of managing and searching free lists, which is what malloc/free implementations have to do.

Theoretically, as the GC copies objects and marks the pages where they used to live as discardable, the OS coul

Re:Take Java seriously

[TheRaven64]

I would refer you to some research done around a decade ago, which involved running a MIPS emulator on a MIPS machine. The emulator was doing dynamic optimisation, and got around a 10% speed increase over the same code running directly on the hardware.

A Java VM does some things that are simply not possible with C. To inline a C function, you need the source for both - this leads to some really ugly things like putting simple functions in headers, which should be reserved for interface definitions, not implementation details. The Java VM will inline functions on the fly. This can potentially give a huge performance boost - I got almost a 50% speed increase on some C code I was recently writing by shuffling things around to allow the compiler to inline some common functions.

The other advantage of higher-level languages is that they provide more semantic information to the optimiser. Consider the trivial example of autovectorisation. In C, if you want to do an operation on a vector, you will usually iterate over every element and perform the operation. The compiler then needs to check that there are no dependencies between loop iterations, which can be non-trivial. In a language like FORTRAN or Smalltalk, you can simply perform an operation on a vector type. The compiler then just needs to check if the operation you are trying to perform corresponds to one or more vector unit instructions, and substitute these in to your code. This is much easier to do.

C is a fairly easy language to write optimised code in for any CPU up to and including a 386. For anything more modern, you will find yourself fighting a language which is simply not designed to deal with parallelism - and compiler writers find themselves fighting even harder.

Re:Take Java seriously

[bbn]

You can say the same thing about the parent comment about java memory management. The J2ME for mobilephones does indead free the memory. Funny how java for embedded systems uses the same strategy eh? J2ME works with very little memory. A Nokia 3410 only has 64 KB memory for the java games.

The parent post was obviously talking about ordinary client applications running on PC under either Windows, Linux, Mac or something simelar.

On such a system, malloc cannot map directly to the OS API, because the OS will only allocate full pages at a time. So if you want 40 bytes of memory, the OS would give you 1 KB (or whatever the page size is).

This also means that if you allocate 2x40 bytes, then free the first 40 bytes block, malloc can not free the page. It is simply not possible, since it would have to free the whole page.

Some J2ME implementations can defragment the memory in this situation, to be able to release memory back to the OS. That is impossible with a C program, where direct pointers to memory is allowed.

For large blocks you can of course go directly to the OS.

Re:Take Java seriously

[Hard_Code]

And given the escape analysis that is apparently going to be implemented in the next version of Java, it looks like a large chunk of the class of problem you propose (many small allocations/deallocations causing "roaming") may be able to be eliminated. The VM will know when some references never "escape" a certain context/scope, and adjust memory management accordingly. One common scenario, and source of many "temporary" objects, will be automatic/local variables in methods, which can be stack-allocated, eliminating this problem.

Re:Take Java seriously

[Felonious Ham]

I'd also add that the pool of Java developers is pretty substantial, making it easier to staff a project than say PHP or Ruby.

Re:Only an Attaboy?

[shiznatix]

Begin able to show proof that you cracked, or helped crack, the Java verifier would be a excellent addon to your resume.

More on Gilad Bracha

[tcopeland]

If his name doesn't ring a bell, he's a Java guru who works for Sun and wrote the 2nd and 3rd editions of the Java Language Spec. A bunch of his papers are listed here.

It's a relief that JDK 1.6 won't include any language changes (as far as I know?). Updating various parsers and whatnot to work with all the JDK 1.5 language changes was a big job, although some of the new features certainly are quite handy.

Re:MS Anti spyware beta

[craigmarshall]

That's almost what I mean except everyone I know uses ad-aware and spybot S&D, so bugs in MS anti-spyware don't really have the same impact. Also - I can't see anywhere on that page inviting people to break the software, and I can't what security systems the software has that can be "broken into". I don't have lots of time at the moment, so I might have missed something.

The most damaging problems I see and hear about are related to Windows and Internet Explorer. An open beta of those (I know about the IE7 beta) with encouraged breakage and bug-reporting could do some good. I wouldn't be surprised if you're breaking some license agreement or other (nope haven't read it - just guessing) by trying to trick IE7 or Vista into buffer overflows or whatever.

Craig

dotNET is overrated

[dascandy]

It allows you to work faster and create more in a short while. It allows you to create abnormally slow programs that you can't even speed up with the willpower to do so, because of Windows internals. Those exact internals that Java won't touch with a stick.

Java doesn't look like win32 because it isn't even trying to. It's trying to look platform-independant and the same on all platforms, with the option to skin it to any GUI you want. dotNET IS windows. There's no wonder that it looks a lot more like windows.

I must strongly disagree on the OO implementation however, aside from it not supporting multiple inheritance, it's just good. Microsofts methodics are plain stupid, because for everything you want to do you have to specify it so explicitly my fingers still hurt last time I tried it.

Compare:

Java:
public class xyz {
      int function() { // some function
      }
}

public class abc extends xyz {
      int function() { // different, automatic polymorphism
      }
}

C++:
class xyz {
      public: virtual int function() { // some function
      };
      public: int functiontoo() { // some function too
      };
};

class abc : xyz {
      public: virtual int function() { // some other function! again automatic!
      };
      public: int functiontoo() { // some function too, not polymorphic!
      }:
};

C#: (might contain errors, been a while)
public class xyz {
      public virtual int function() { // function
      }

      public int functiontoo() { // functiontoo
      }
}

public class abc : xyz {
      public override int function() { // override, kind of pointless...
      }

      public new int functiontoo() { // ... why new? that's reserved for memory allocation...
      }
}

My point is, .NET (in C#) requires you to make everything you want so explicit that I'm inclined to say that you're wasting time doing that more than you're gaining time due to other factors.

Plus, I just don't like their idea of a good library. Rape the C++ STL, why don't ya. Either support c++ (and the STL), or don't support it at all.

Re:Aren't QA people supposed to get paid?

[Naikrovek]

the onus isn't on the community, the onus is on the developers and their QA team. This is just an attempt to get a few more eyeballs on the verifier in case something falls through. There's nothing wrong with that.

Also it is an opportunity for someone to get recognition for breaking a new peice of software.

It is important to get extra scrutiny on newly designed peices of software, for it is the new designs that usually break in the least expected ways.

Re:Don't believe the hype!

[craigmarshall]

> C is portable, fast, very complex and since 35+ years the leading standard for professional OS and APP development.

I agree that C is portable and fast, however I don't it can be called very complex.

The smallest programming language manual I have ever owned (and I've owned quite a number), has to be "The C Programming Language", often hailed as the One True Reference to the language. How can it be that complex if the manual is less than half the size of most of my other manuals? I think languages (in general) have got more complex since then. The size of the .Net Framework is huge, there's no way that's simpler than the C standard library. Then you've got to think about reflection, inheritance, dozens of things that C just doesn't have.

If what you mean is that C programs end up looking more complex, that's probably because C is used for systems programming. If you mean that you have to write more code to do it in C, then you may have a point, but I think C is actually one of the simpler languages. The closer to assembly you get, the simpler the language has to be.

Craig

Optimization and late binding

[jfengel]

Another nice thing about the new classfile specification is that it's going to make certain new kinds of optimization possible. The more you can prove about what's on the stack at any given point, the more you can inline.

Not only does inlining eliminate method call overhead, but it allows you to re-run the peephole optimizer, which can eliminate range checks, reduce redudant type checks, etc.

The ultimate performance promise of Java is that it can do optimization very, very late in the process. Native libraries are basically black boxes in C/C++, and it's very hard to do that sort of inlining because most of the type information has been lost. Java may, someday, with sufficient ingenuity, rival or even beat C++ in performance, and it already does in certain limited areas.

Of course C# has all of the same advantages, and even though it's more recent there are some areas where its performance beats Java. I'd love to see all the Microsoft reasearchers vs. all the Sun researchers coming up with increasingly brilliant ways to take advantage of the late binding to turn a performance hindrance into a benefit.

Re:Why not prove it?

[TrappedByMyself]

Why not do what it takes: Prove that it will work, and prove that it cannot be broken!

Did you just walk out of an undergrad Computer Science class? ;)
Popping in pre/postconditions and doing line-by-line proofs doesn't cut it for an application of this complexity. While that is an important part of a real process, it doesn't guarentee coverage. You still have to make assumptions about the environment, which is the gotcha. Testing and QA is all about the assumptions you make and the boundaries you set. With a complex application the number of factors grows so large, that you cannot have the resources to cover every possible test. You can grab the most common stuff, but really need to dump it to the community to get the real 'out of the box' thinking hitting it.

Re:Why not prove it?

[fuzzy12345]

Here's a flame to all the respondents to the parent post. They say (if I may paraphrase) "Code verification is hard. I want my MOOMMMMMY!" Well, it's certainly more difficult, in the short run, than the "throw it against the wall and see if it sticks" approach. But it has been done, it isn't as hard as the naysayers are making out, and it's one of those things that you don't improve at unless you try. Google VLISP for an example of a provably correct compiler.

One thing's for sure: Improvements in software quality will be harder to come by if everybody's attitude continues to be "Bugs are inevitable. Formal proofs are beyond us. Let's keep doing it the old way."

Not execution speed, but predictability

[awol]

Look, I wish people who keep banging on about how Java is nearly as fast as C would get their heads out of their asses and realise that the biggest defect in Java is not raw execution speed but the "business processing" holiday that the system takes every "completely unpredictible once in a while". If I have a throughput capacity system, I can control the rate of throughput in a number of ways (eg selling less than my total capacity and then throttling at times of peak use) but when the system goes and does something like a garbage collection and the whole pipe goes "fnark" for a some seconds I am quite pissed since my users who want the service level in their SLA aren't getting it.

Predictability and execution control are why I use C (and to a lesser extent c++) not Java. That cannot change for languages that give me no control over the raw execution path.

you have no clue

[RelliK]

You obviously have never worked on large server-side applications. Other posters have already listed some reasons for java's popularity. I'll add some more:

* java is nearly as fast as C++ according to all the benchmarks I've seen. Yes, really. The perception of java as being "slow" is simply the legacy of the old awt apps. Yes, the awt gui was (and is) slow. Server-side java applications are not. The "much better performance" is simply not there, particularly for typical enterprise apps.

* *All* the enterprise apps (which is the area where java is particularly successful) store stuff in a database and/or talk to remote apps. Newsflash: a database query or a remote procedure call is *orders of magnitude slower* than an in-process procedure call. Once you include DB/RPC into the equation, whatever little speed advantage C++ has is wiped out completely.

* This is CS 101: performance of a program is largely determined by the algorithm used. You can write a linear search in assembly, and it will be very fast for small lists. But for large lists, a binary search written in shell script will beat it.

* In an enterprise application scalability is much more important than raw speed. So what if I can write a C++ app that's 20% faster than an equivalent java app? Java has frameworks that make it easy to write an app that you can scale horizontally (i.e. by adding more boxes). Easy being the keyword.

* Developer's time is much more expensive than runtime. It is *much faster* to write an app in java than in C++. And for all but the smallest/simplest apps it is faster to write the app in java than in PHP/perl/whatever.

If it's a safety/security issue then again you could build the same thing in a native compiled language, sandbox and all.

Uhhhm, yes. Safety and security are *big* issues in enterprise apps. Show me *one* native language and platform that does it. You are saying it like one can just wave a magic wand and have it built in no time. "You could build the same thing" is not "it's already built".

I mean really, is it just because Java provides a lot of easy to use API's?

Yes. among all the other things I've mentioned.

These are just a few reasons why java is so popular in enterprise apps. Sure, I wouldn't write a game in java, but for enterprise apps, it's perfect. Why java and not PHP/perl/? Simply because java is better. It has all the advantages of compiled laguages (type safety, variable declaration checking, syntax checking, etc.) without some of the disadvantages (manual memory management). Think of java compiler as a sanity checker for your code. It will catch common mistakes like typos, missing return statements, invalid function parameters, etc. A scripting language will not complain about that, but force you to spend hours tracking down the bug. That's why java is faster to develop in than any scripting language for large apps.