[Webkit-unassigned] [Bug 140743] New: BytecodeGenerator::initializeCapturedVariable() sets a misleading value for the 5th operand of op_put_to_scope

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=140743

            Bug ID: 140743
           Summary: BytecodeGenerator::initializeCapturedVariable() sets a
                    misleading value for the 5th operand of
                    op_put_to_scope
    Classification: Unclassified
           Product: WebKit
           Version: 528+ (Nightly build)
          Hardware: Unspecified
                OS: Unspecified
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: JavaScriptCore
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: mark.lam at apple.com

Steps to reproduce:
1. Go to https://bugreport.apple.com/problem/viewproblem?problemID=18702537
2. Log in.

BytecodeGenerator::initializeCapturedVariable() is setting the 5th operand to op_put_to_scope to an inappropriate value.  As a result, the execution of put_to_scope could store a wrong inferred value into the VariableWatchpointSet for which ever captured variable is at local index 0.  In practice, this turns out to be the local for the Arguments object.  In this example, the wrong inferred value written there is the boolean true.

Subsequently, DFG compilation occurs and CreateArguments is emitted to first do a check of the local for the Arguments object.  But because that local has a wrong inferred value, the check always discovers a non-null value and we never actually create the Arguments object.  Immediately after this, an OSR exit occurs leaving the Arguments object local uninitialized.  Later on at arguments tear off, we run into a boolean true where we had expected to find an Arguments object, which in turn, leads to the crash.

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