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Java6 introduces a new way for verifying byte code comparing to Java5. Now class files contain stackmaptable attribute which was designed to help byte code verifier. Verification of class files containing stackmaptable (Java6-verification) is much simpler comparing to original verification of classes not containing this attribute (we will refer to it as "Java5-verification"). This attribute is normally inserted by Java Compiler (javac).
But if some byte code instrumentor makes modification of the original code then stackmaptable inserted by javac becomes invalid. Thus instrumentor should be able to fix stackmaptable and thus there should be a tool that is able to calculate a valid stackmaptable attribute.
Briefly, stackmaptable attribute contains information about types that come to each start of liner block (to each branch target). If there are several types coming to some point then stackmaptable for this point contains a type to which all the coming types are assignable to. In particular if class A, B, and C may come to some point then stackmaptable contains their common super class.
An obstacle here is that at compile time javac has access to all the classes referenced in the class being compiled, while dynamic instrumentors might not have access to some of the referenced classes, thus it's hard to guess which class is a common super class of A, B, and C from the example above. If we knew what specifically is modified by instrumentor and how, we could modify original stackmaptable of the class, but we would like to keep flexibility of instrumentor and not limit its modifications over the code with unnecessary constraints.
We assume that either original class file contained valid byte code or if it does not then it's OK for instrumented class file to contain invalid stackmaptable (so that it will be rejected by the VM later). Thus we will use data from the original uninstrumented class file to calculate a new stackmaptable attribute.
To compute stackmaptable attribute verifier uses two types of information: either it needs access to all the classes referenced in the current class (this mode is like how javac calculates stackmaps) or it needs the original class (if the current class was obtained from some original valid class by instrumentation). In the latter case it calculates new stackmap basing on the data extracted from the original class file. There are no different modes: the verifier is always operates in a "mixed" mode: it uses both data extracted from the original class file (when it exists) and try to access referenced classes.
The main function that computes stackmap for the given method is
vf_Result vf_recompute_stackmaptable( method_handler method, uint8 **attrBytes, char **error, void *trustedPairs )
Where trustedParis (which might be 0) contains list of knowingly assignable pairs. If stackmap is recomputed in an instrumentor, referenced files are not always can be accessed at the moment of instrumentation (e.g. they could go from some URL location), so the stackmap data from original (uninstrumented) class file is used instead:
First the original class is going to
function, that makes first step of verification: it verifies integrity of the code and checks assignability of the classes available at the moment of first verification step. It creates a list of pairs that are not loaded and need to be checked later. In a normal execution there is a second verification step, where these pairs are forcedly loaded and checked (just before execution of the class).
When we recompute stackmaptable for an instrumented class, the second verification step is not performed. Instead we assume that original class file was valid and consider obtained pairs as "trusted" pairs: we assume that they are properly assignable. (There is no problem in this assumption: if it was not the case then our generated stackmaptable would be invalid either and the class would be rejected by VM).
Once we are done with the first step Java6 verification of the original class, we go to Java5 verification of the instrumented class. We also perform just the first step of verification (i.e. we don't force loading the classes), pairs of classes to be checked are stored somewhere as well, they are not mixed with our trusted pairs and they are never used later.
Once we are done with Java5 verification we don't destroy its temporary data, but start to work with it. So Java5 verifier creates its interim stackmaps at the same points where they are necessary according to the spec (Well, it's not quite true. For optimization purposes there are places for some of which interim stackmaps are not created, so I had to implement a flag to switch this optimization off when we are at recompute stackmap mode).
By design of Harmony Java5 verifier, each stackmap contains types that are incoming for the given point as well as types that are expected by the following instructions as well as list of branches where these incoming types go further to. (We propagate incoming types in one direction only: we do add incoming types to all the branch targets, but we don't add expected types to the points which have branches to the current point)
Then verifier looks for a "solution": i.e. set of values for each stackmap so that we may prove that 1) each incoming type is assignable to that value 2) the value is assignable to each type expected by the following instructions and 3) the value is assignable to the values (that also belong to the solution) for the branch target stackmaps.
Verifier may prove assignability in three ways. Class "A" is provably assignable to class "B" if either 1) we know that "B" is an interface or a super class of A (i.e. A or B is loaded) or 2) list of trusted pairs contain (A, C), where C is provably assignable to B 3) A==B
There are exclusions, when verifier does not prove assignability: e.g. if there is just a single incoming type for t he given stackmap, then solution contains this type and it is not checked against expected types or branches.
To find a solution verifier does NOT apply a brute force search. Instead constraint propagation and backtracking-like
algorithms are used:
First for each node (i.e. for each type in the resulting stackmaptable) it creates a list of types that can potentially be a solution, i.e. all types X for which there exist an incoming type I that provably assignable to X. We'll call this list as "solution candidates".
Then we reduce the list of solution candidates by removing those candidates X for which there is an expected type E so that X is not provably assignable to E.
The state when all solution candidate lists contain exactly one element, will be called "SOLUTION FOUND". The state when there is an empty candidate list, will be "INCONSISTENCY FOUND"
The following procedure will be called "propagation": remove all solution candidates X such that there is a branch B with its own solution candidates B_1, ..., B_N and for each B_i X is not provably assignable to B_i
A recursive procedure below describes the search for a solution. First we do PROPAGATION. If solution found - global stop, we are done. If inconsistency found - "roll back" to previous recursion level. (If it was the first level, then we are in trouble: the data we had was not enough to build stackmaptable attribute).
Then we remember all candidate lists (to return here in case we roll back from the next recursion level), chose a list where there is more than one candidate, select a candidate there, remove all other candidates and apply SEARCH (i.e. launch the next recursion level) with reduced list of candidates. If SEARCH found a solution then it's a global stop. If it found inconsistency then restore remembered state, select another candidate from the list, and launch the next recursion level again. If there are no more candidates and solution is not yet found - roll back to the previous recursion level (if it was the first level - we are in trouble again).
It's not a simple task to calculate stackmaptable attribute for the beginning of dead (unreachable) block of code. Despite the code dead, it's verified by Java6 verifier (required by spec) and thus it should satisfy all instructions tat follows. Moreover dead code might be in the scope of a reachable exception handler and thus may affect stackmaptable in the reachable parts of the code. So to kill the problem it's easier to modify class file rather than to build stackmaptable (which is even not always possible).
First, we avoid the situation when the dead code is the only intersection of two different try blocks (since their
handlers may have incompatible types): for each try block that starts or ends in the dead code we reduce scope to
eliminate dead code from the try block (when dead code is in the middle then it's OK).
Next, we change the code in the dead block with NOPs followed by ATHROW.
Finally, we build stackmaptable for the beginning of the dead block: its stack contains just a single element NULL, and (to make it compatible with possible exception handlers) its locals are exactly the same as locals in the stackmaptable for the first alive instruction that follows dead code.
Sometimes verifier has to know whether a field or a method referenced in the class is declared in the super class of the current class or not. This is necessary to understand which type the instruction expects. But to answer the question we have to load ether super classes of the current class or a class where referenced member is declared. class_is_extending_class method from the class interface is called to figure that out.
In our case we can't load additional classes, but recompute stackmap functionality works correct if class_is_extending_class always returns NULL (indicating that referenced class is not a super class of the current one). To make sure that stackmap table is correct verifier chooses the least general (i.e. least close to java.lang.Object) type among several possible for the SOLUTION.
// Allocates an empty verification context for a class, // to be passed back to the verifier upon verification requests. // Memory must be disposed by calling free_verification_context // (see below). // @param klass - class handler // @return a verification context for the class verification_context allocate_verification_context(class_handler klass); // Initializes the verification context with method's information. // This function must be called before instrumenting the method. // The resulting context should be passed back to the verifier // upon verification requests // @param method - method handler // @param[in,out] context - verification context of the // method's defining class // @return error code vf_Result init_verification_context_for_method(method_handler method, verification_context context); // Recomputes the StackMapTable of a method using the verification // context created and initialized prior // to method instrumentation // @param[out] attrBytes - a pointer to a newly allocated StackMapTable // attribute. Memory must be disposed by calling free_stackmaptable // (see below). // @param method - method handler // @param context - class and method verification context // @return error code vf_Result recompute_stackmaptable(uint8 **attrBytes, method_handler method, verification_context context); // Frees memory allocated for a StackMapTable attribute void free_stackmaptable(uint8 *attrBytes); // Frees memory allocated for a verification context void free_verification_context (verification_context context);