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Enabling jit debugging - solutions to the problems

What does JIT debugging mean?

Just-In-Time debugging is a feature that launches the Visual Studio debugger automatically when a program, running outside Visual Studio, encounters a fatal error. Just-In-Time debugging allows you to examine the error before the application is terminated by the operating system.

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In the last article, we examined how JavaScriptCore, the Webkit's JavaScript engine, stores objects and values ​​in memory.

In this article I would like to learn a little more about JIT, the just-in-time compiler. This browser exploitation series is supported by SSD Secure Disclosure. For more information, see the description Simply put, the just-in-time compiler compiles JavaScript bytecode executed by the JavaScript virtual machine into native machine code.

Would compile code. But JIT in JavaScriptCore is a bit more nifty. So how can we learn about it? Got a great tip from Linus for checking out some official Webkit resources like this article on the Webkit blog.

JavaScriptCore CSI: A Crash Site Investigation Story. Today I'm going to describe some of these tools that WebKit engineers use by telling the story of how we diagnosed a real bug in the JSC Virtual Machine (VM). This is a blog for people interested in contributing to Webkit, and the author shares a lot of very valuable insights on how to debug a crash to find the cause.

And that's exactly what we want to know as security researchers. For example, it describes how to create an address sanitizer build of Webkit to detect possible errors such as heap overflow or use-after-free problems, which often do not crash immediately with normal programs. But we're interested in the JIT stuff right now.

And further on in the article we find the following. JSC comes with several tiers of execution engines. You may have read about it here, this is an article introducing the FTL JIT.

There are 4 tiers: Tier 1: the LLInt interpreter Tier 2: the Baseline JIT Compiler Tier 3: the DFG JIT and Tier 4: the FTL JITS So Tier 1 is the regular interpreter. This is the basic JavaScript virtual machine. We can take a quick look at the source file LowLevelInterpreter.cpp, which contains the interpreter loop.

So it just goes through the Javascript bytecode and then executes each statement. So ... if a function is called often now, it can get 'hot'.

This is a term that describes that there is a lot going on. And then JavaScriptCore could decide to JIT the function with the first stage. The basic JIT.

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And in the corresponding JIT.cpp file we can again get some additional information. If the LLInt detects that there is an OSR entry in the baseline JIT in a loop, it is passed the bytecode offset with which it was executed when it started our compilation.

We just need to compile code for everything that is reachable from that bytecode offset. OSR, On Stack Replacement, is basically a kind of JIT that allows you to switch to the compiled code on the fly. It is still very compatible with the bytecode, so to speak, since there have not yet been any further optimizations.

From the “Introducing the Webkit FTL JIT:” we also read: The The first execution of a function always begins in the interpreter layer. Once a statement in the function is executed more than 100 times or the function is called more than 6 times (whichever comes first), execution is redirected to code compiled by the baseline JIT which eliminates some of the overhead of the Interpreters, but there is a lack of serious compiler optimizations. As soon as an instruction is executed more than 1000 times in the baseline code or the baseline function is called more than 66 times, we direct the execution back to the DFG JIT.DFG stands for Data Flow Graph, so that does tell a bit about what this step is about.

The article also includes a nice picture that describes the DFG pipeline. The DFG starts converting bytecode into the DFG-CPS format. CPS stands for Continuation-Passing Style, which means that your code doesn't use returns, but instead continues and moves on to the next function.

If you've ever done a NodeJS Express development, you can think of it like the next () function flow relationships between variables and temporary ones. Profiling information is then used to infer types of guesswork, and those guesses are used to introduce a minimal set of type tests. Traditional compiler optimizations follow.

The compiler ends up generating machine code straight from the data flow graph, so this is where things get interesting. The JIT compiler guesses types, and if the JIT believes that types are not changing, the JIT can decide to remove certain checks. Which of course can speed up the code drastically if it's a function that is called But that's not all.

After the DFG JIT there is still a JIT. The FTL - faster than light. When this animal was introduced, he used the well-known compiler backend LLVM to apply much more typical compiler optimizations FTL JIT was developed to bring aggressive C-like optimizations into JavaScript.

At some point, LLVM was replaced by B3, but the idea is the same. And this JIT compiler could make even more assumptions about the code. But let's look at that a bit, which is again where the article on the crash investigation is excellent.

It introduces several environment variables that can be used to control the behavior of the JIT and enable debugging output. For example, we could use JSC_useJIT to disable JIT entirely, or use JSC_useFTLJITJ to disable only FTL, the final stage. Or we can disable threads that do JIT in parallel.

Or we can report and print every time a JIT does an optimization. So in lldb I set the environment variable to turn everything on and then restart JSC, this already results in some JIT optimization debug prints. If you look at the function names that have been jitted, it looks like things like charAt, abs, etc. have already been tweaked.

But now we want to make our own JIT function, eoverflow function, which takes a simple number as a parameter, then prepares a result variable and loops from i = 0 to n and sums it up into result. hot ”. We can do this with a simple loop calling this function.

Let's start with just four versions. It shows some output, but not what we want. Even executing it 10 times in loops does nothing.

But executing the function 100 times will trigger the baseline JIT. Here is a compiled assembly code equivalent of the Javascript function. Let's try to trigger an even more aggressive JIT. let's increase the loop.

And Boom DFG Jit ...

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And now let's get totally crazy, adjust the loop. FTL Jit. boom, there is so much more output, but to be honest, no idea what it all means.

But the most important thing is just that we learned about various debugging methods and tricks to dig deeper. Now you know a little more about the JIT compiler. From the last article you also know how JavaScript objects, arrays and values ​​are represented in memory.

Now consider the following idea. If the JIT compiler guesses and assumes types in the code and removes checks and, for example, simply moves them from a certain memory offset, could that be abused? Just hypothetically, say, JIT code expects a JavaScript array of doubles and reacts to them directly Values. The JIT compiler then optimizes all checks, but then you will find a way to replace an entry in the array with an object.

Now an object would be placed in this array as a pointer. So if jied code has no checks and returns the first entry of this array, for example, it would return that pointer as a double, right? That would be pretty bad, and the 'It's actually one of the typical browser vulnerability patterns and we'll see that in action soon. But how does the JIT try to prevent such things? Well, it turns out that the developers are trying to model every function that affects the assumptions of. has the JIT compiler.

So if there is something that could change the layout of this array, for example if an object is placed in a double array, then such a function should be marked as dangerous. Let me tell you a brief excerpt from this ZDI article about. read out 'INVERTING YOUR ASSUMPTIONS: A GUIDE TO JIT COMPARISONS'.

Here they write: The way to say that an operation is potentially dangerous to prevent future optimizations is to call a function called clobberWorld which, among other things, breaks all assumptions about the types of all arrays within the graph. So the Javascript engine tries to mark anything that could have potential side effects by calling clobberWorld. Side effects could be things we were just thinking about like changing the type of a value and something else was not expecting that change.

So here is the clobberWorld function, which is implemented in the DFG-JIT part. This calls clobberStructures, which sets setStructureClobberState to StructuresAreClobbered very carefully are obviously things where, for example, the structure of an object changes. Let's say the JIT optimizes access to a .x property on an object, and suddenly you remove that, it needs to be marked that the structure has changed so that JIT code can be discarded, otherwise you will get memory corruption when you reuse this jitter function.

But that's enough now. In the next article we move on to Linus' exploit, which abuses such a case.

When JIT debugging is enabled any unhandled exception will be sent to the JIT?

To enable Just In Time (JIT) debugging, the . config file for this application or computer (machine. config) must have the jitDebugging value set in the system. When JIT debugging is enabled, any unhandled exception will be sent to the JIT debugger registered on the computer rather than be handled by this dialog box."

How do I disable JIT debugging?

In Windows Control Panel > Network and Internet > Internet Options, select Disable script debugging (Internet Explorer) and Disable script debugging (other).23 mei 2018

How do I fix No registered JIT debugger was specified?

No registered JIT debugger was specified. Click on Retry to have the process wait while attaching a debugger manually. Click on Cancel to abort the JIT debug request.10 mei 2018

How to enable " JIT debugging " for Microsoft.NET?

'Unhandled exception has occurred in your application.....The system cannot find the file specified...To enable Just In Time (JIT) debugging, the .config file for this application or computer (machine.config) must have the jitDebugging value set in the system.windows.forms section. The application must also be compiled with debugging enabled.

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Is there a just in time ( JIT ) debugging dialog box?

just-in-time (JIT) debugging instead of this dialog box. System.UnauthorizedAccessException: Access to the path 'C:Program Files (x86)BotBoyprefs.xml' is denied. jitDebugging value set in the system.windows.forms section. enabled. rather than be handled by this dialog box. This thread is locked.

How to enable or disable just in time debugging?

To enable or disable Just-In-Time debugging: 1 On the Tools or Debug menu, select Options > Debugging > Just-In-Time. 2 In the Enable Just-In-Time debugging for these types of code box, select the types of code you want Just-In-Time debugging to debug: Managed, Native, and/or Script. 3 Select OK. See More...

How do I fix this problem in JIT?

How do i fix this problem? just-in-time (JIT) debugging instead of this dialog box. System.UnauthorizedAccessException: Access to the path 'C:Program Files (x86)BotBoyprefs.xml' is denied. jitDebugging value set in the system.windows.forms section. enabled. rather than be handled by this dialog box. This thread is locked.

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