[PATCH] 4851444: Exposing sun.reflect.Reflection#getCallerClass as a public API in Java 8

[Peter Levart]

On 09/09/2013 07:02 PM, David Chase wrote:
> Take this lightly informed suggestion with a grain of salt, but why not, for purposes of performance and security,
> change the logging-specific getCallerClass methods so that their "class" references are instead wrapped in some sort of proxy object that only forwards certain operations quickly without a security check?  For example, equals, hashcode, and toString are probably not security-sensitive.
>
> i.e.
> class SafeClass {
>   private final Class clazz;
>   public SafeClass(Class clazz) { this.clazz = class; }
>   public String toString() { return clazz.toString(); }
>   public int hashCode() { return clazz.hashCode(); }
>   public boolean equals(Object o) { return clazz.equals(o); }
>   public Class maybeWeLetYouLookAtTheRealClass() { ... a bunch of security checks ... }
> }
>
> If necessary, do the same for classloaders.
> And them, no security checks needed, as long as the "safe" methods are enough to get the job done.

Hi,

This is a good idea. It got me thinking that there are a bunch of 
methods in j.l.Class that are not security-sensitive. So instead of 
proxy-ing those in a wrapper SafeClass, let's just identify "unsafe" 
methods 1st. If the security checks that are planned for obtaining 
j.l.Class instances from call-stack are transferred to those unsafe 
methods instead, then holding a reference to a j.l.Class instance 
becomes a security-nonissue.

Just a quick example - presumably the "unsafe" methods of j.l.Class are: 
get(Declared)Method(s), get(Declared)Field(s) and 
get(Declared)Constructor(s), because they enable you to call/access 
public methods/fields/constructors of a class represented by j.l.Class 
object. If this class is from a restricted package (say sun.misc) then 
you could get access to restricted methods/fields/instances. Now if the 
security checks for obtaining reflective object would include checking 
the "class visibility/restrictability", then j.l.Class object of say 
sun.misc.Unsafe class would not represent any security issue. It's just 
about delaying the security check. What do you think? Are there any 
other security-sensitive j.l.Class methods? Are there any public methods 
in JDK that take j.l.Class instances and delegate to internal logic 
assuming that the caller can only pass-in security-pre-checked j.l.Class 
instances?

Regards, Peter

> David
>
> On 2013-09-09, at 10:54 AM, Mandy Chung <mandy.chung at oracle.com> wrote:
>
>> Nick,
>>
>> Do you have any information to some of the questions I asked below that can help the API discussion?
>>
>> We need to decide on the permission check and also the hotspot change has to be integrated and the jdk change will need to wait until it shows in a promoted build.  Schedule-wise, to make JDK 8, we should finalize the API this week so that you can update the patch soon.
>>
>> Mandy
>>
>> On 9/3/2013 5:02 PM, Mandy Chung wrote:
>>> Nick,
>>>
>>> I skimmed through the changes.  Congratulations for your first patch making changes in both hotspot and jdk code.
>>>
>>> In my mind, the Log4J use case accessing Class instance to obtain additional information for diagnosability is different than the use case of obtaining the caller's class / loader to lookup resources.  While the new APIs might be in the same class, I will discuss them individually and keep us focus one at a time.
>>>
>>> Ralph has pointed out [1] that Log4j also needs the ability to find an appropriate ClassLoader which is used for logging separation (thank you Ralph). That will be the caller-sensitivity (i.e. obtain caller's class/loader) discussion.
>>>
>>> There are a couple of RFEs:
>>> https://bugs.openjdk.java.net/browse/JDK-4942697
>>> https://bugs.openjdk.java.net/browse/JDK-6349551
>>>
>>> Performance is critical for Log4j to traverse the stack trace and obtain Class information.  I like your patch to speed up the generation of StackTraceElement[] (and StackTraceFrame[] - essentially same code with different type). java.util.logging.LogRecord has workaround the performance overhead and go to a specific frame directly and avoid the cost of generating the entire array. JDK-6349551 requests to lazily instantiate the StackTraceElement object unless that frame is requested.  Does Log4J always walk all frames and log all information?  Do you just log only some max number of frames rather than the entire stack trace?
>>>
>>> Class<?> getDeclaringClass() method is the key method you need to enhance the diagnosability.  This method opens up a new way to access a Class instance that untrusted code wouldn't be able in the past.  A permission check is needed as Alan points out early.  Performance as well as logging framework can't access all class loaders are two factors to be considered when defining the permission check.
>>>
>>> I saw your other comment about permission check (cut-n-paste below).  It seems to me that you don't agree a permission check is needed for the getDeclaringClass() method (regardless of which class it belongs to) and if that's the case, no point to continue.
>>>
>>> I want to make it very clear that I have agreed to take this on and provide a replacement of sun.reflect.Reflection.getCallerClass(int) in JDK 8 to address the use cases.  It will take time for the API and security discussion and please be prepared for that (also I am working on other things at the same time).
>>>
>>> The second comment on your patch is that there are lot of duplicated code in hotspot in order to support two similar but different types (StackTraceFrame and StackTraceElement). Serialization is the reason leading you to have a new StackTraceFrame class.  Maybe some refactoring can help but this is the cost of having the VM directly creating the instance.  One other option, as suggested in the previous thread, is to keep the declaring class in the StackTraceElement as a transient field.  If we add the getDeclaringClass method in the StackTraceElement class, it would need to specify to be optional that it only returns the Class instance when it's available.
>>>
>>> There are currently three different ways to get a stack trace:
>>> 1. Throwable.getStackTrace()
>>> 2. Thread.getStackTrace() and Thread.getAllStackTraces()
>>> 3. java.lang.management.ThreadMXBean.getThreadInfo(long id, int maxDepth).getStackTrace() and multiple thread IDs version
>>> (a) local (b) remote
>>>
>>> Since it's a new StackTraceFrame class, you have to provide a new method replacing #1 and #2.  I don't see any need to provide a new API equivalent to Thread.getAllStackTraces() and java.lang.management.
>>>
>>> The proposal I originally have last week was to keep declaring class as transient and add a method in StackTraceElement with a permission check at every call. Tom raises a good question about the cost of permission check.  Would that be a concern to Log4j? Is log4j on bootclasspath or extension directory?  I assume not. So for log4j to work with security manager installed, it would have torequire the application to grant certain permissions - can you confirm?  For example it calls sun.reflect.Reflection.getCallerClass method that will require RuntimePermission("accessClassInPackage.sun.reflect") permission. Calling Class.getProtectionDomain and Class.getClassLoader() requires RuntimePermission("getProtectionDomain") and
>>> RuntimePermission("getClassLoader") respectively.  That gives me an impression that permission check on individual stack frame might be a non-issue?
>>>
>>> Mandy
>>> [1] http://mail.openjdk.java.net/pipermail/core-libs-dev/2013-September/020525.html
>>>
>>> On 9/3/13 5:24 AM, Nick Williams wrote:
>>>> On Sep 3, 2013, at 4:07 AM, Alan Bateman wrote:
>>>>>> I'm not voicing any objection to any kind of security/permissions checks in these methods. Before I can pass judgement one way or another, I'd want to know 1) specifically what type of permissions check you are looking for, and 2) what you're looking to achieve with said permissions check.
>>>>> I would say this is TBD and start by asking the question as to whether there are concerns about leaking reference to Class objects that untrusted code wouldn't normally be able to get a reference to. Tom brings up the cost of the permission check and also whether any API should be tied to class loader. There are clearly discussion points here that could potentially influence the API.
>>>> As I have said before, there are MANY ways to get a Class object that aren't security checked. It's when you try to USE that class object to impersonate it or invoke methods that security checks begin to happen. As they should!
>>>>
>>>> Nick
>>> On 9/1/13 1:16 AM, Nick Williams wrote:
>>>> I have completed and am proposing a patch for replacing sun.reflect.Reflection#getCallerClass(...) with a public API in Java 8. I saw no point in duplicating an issue, even though it's over 10 years old, so I'm indicating that this is a patch for 4851444 (http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=4851444).
>>>>
>>>> I welcome and solicit support/+1s and a sponsor. Someone about a month ago had mentioned that they would be willing to be a sponsor if the patch looked good, but I can't remember who it was and I can't find the email. I want to say it was someone with RedHat, but my memory could be faulty, so please don't hold it against me if I'm wrong.
>>>>
>>>> *Summary of Changes*
>>>> Added the new class java.lang.StackTraceFrame. It's a virtual mirror of StackTraceElement, except that it contains a Class<?> declaringClass property instead of a String className property. Since the list members expressed reluctance to add methods to Thread and Throwable, StackTraceFrame also contains several static methods for retrieving Classes and StackTraceFrames from the stack. These methods are as follows:
>>>>
>>>> Class<?> getCallerClass(): Retrieves the class of the caller of the method calling getCallerClass(). This is identical to the new Reflection#getCallerClass() added in Java 7u25/8.
>>>>
>>>> Class<?> getCallerClass(int): Retrieves the class of the caller n frames down the stack. This is identical to the old Reflection#getCallerClass(int) that was deprecated in Java 7u25 and removed in Java 8.
>>>>
>>>> StackTraceFrame getCallerFrame(): Retrieves the StackTraceFrame of the line of code that called the method calling getCallerClass(). This is similar to the new Reflection#getCallerClass() added in Java 7u25/8, except that it returns a StackTraceFrame.
>>>>
>>>> StackTraceFrame getCallerFrame(int): Retrieves the StackTraceFrame of the caller n frames down the stack. This is similar to the old Reflection#getCallerClass(int), except that it returns a StackTraceFrame.
>>>>
>>>> StackTraceFrame[] getCurrentStackTrace(): Functionally equivalent to Thread#getStackTrace(), except that it returns an array of StackTraceFrames.
>>>>
>>>> StackTraceFrame[] getStackTrace(Throwable throwable): Functionally equivalent to Throwable#getStackTrace(), except that it returns an array of StackTraceFrames. It uses the same save point (backtrace) created when the Throwable is created that Throwable#getStackTrace() uses when it's first called. It caches the array of StackTraceFrames in the Throwable just like the array of StackTraceElements are cached, so that multiple calls for the same Throwable are fast.
>>>>
>>>> As a result of this change, sun.reflect.CallerSensitive has been moved to java.lang.CallerSensitive.
>>>>
>>>> I spent considerable time reviewing, revising, considering, and testing these changes. I included several unit tests that establish the proper behavior. I also spent considerable time benchmarking the changes. While benchmarking, I noticed some things. First, getCallerClass() and getCallerClass(int) are as fast as their counterparts in sun.reflect.Reflection, and they easily inline when appropriate. Second, getCallerFrame() and getCallerFrame(int) are /almost/ as fast as the Class versions, but there is some additional overhead for the construction of the StackTraceFrame. This is minuscule (1,000,000 invocations consume around 500 ms total on my machine). At this point, all of the benchmarking was as expected.
>>>>
>>>> However, I then encountered a surprise. The getCurrentStackTrace() and getStackTrace(Throwable) methods executed (again, 1,000,000 times) in about 70% of the time that Thread#getStackTrace() and Throwable#getStackTrace() did, respectively. Theoretically, they should have executed in the same amount of time, not faster. After extensive analysis, I discovered (what I considered to be) a serious flaw in how the stack trace is filled in within Throwable (which also affects how Thread#getStackTrace() works).
>>>>
>>>> Instead of simply iterating over the entire save point and filling in the Throwable stack trace in native code (which is what I did when I implemented the StackTraceFrame methods), the Java code in Throwable first called a native method to figure out how deep the stack was, then called another native method once for every frame in the stack to retrieve each element individually. This native method that is called repeatedly iterates over the entire backtrace once for each call, stopping only when it finds the matching element (so it's O(1) for the first call, O(2) for the second call, O(3) for the third call, and so on). While my StackTraceFrame methods were iterating over the backtrace exactly 1 time (O(n)), the Throwable methods were iterating over the backtrace 1+(n/2) times (worse than O(nlogn) but not as bad as O(n^2)). This problem would not have been extremely apparent over small stack traces (the 30% improvement was a stack trace of 6 elements), but over a large (200+ elements) stack traces the performance difference would be huge and noticeable. Seeing a room for improvement, I refactored the code that fills in the stack trace for Throwable, improving its performance accordingly to match the performance of the StackTraceFrame methods.
>>>>
>>>> I'm very pleased with how this turned out, and both the unit tests and my extensive functional testing show that the new class and its methods are working great. I just need someone willing to review and sponsor my patch!
>>>>
>>>> *The Code Changes*
>>>> I couldn't get WebRev to run without all kinds of errors. So I ran `hg diff -g` on every repository in the forest with changes. Here are the four patch files for the four repositories that had changes (no other repositories had changes):
>>>>
>>>> https://java.nicholaswilliams.net/Java8GetCallerClass/jdk8.patch
>>>> https://java.nicholaswilliams.net/Java8GetCallerClass/jdk8_jdk.patch
>>>> https://java.nicholaswilliams.net/Java8GetCallerClass/jdk8_hotspot.patch
>>>> https://java.nicholaswilliams.net/Java8GetCallerClass/jdk8_nashorn.patch
>>>>
>>>> I believe I have followed all of the procedures as closely as possible. I await feedback and hope for some support on this, so that we can get a public replacement for this method in Java 8. Let me know if you have any questions.
>>>>
>>>> Thanks!
>>>>
>>>> Nick