added range check to readBlobHeapUncached

Obfuscated assemblies can have a bogus extra reference to mscorlib which
has incorrect index in Blob heap for HashValue - index value was out
of range of the heap.I guess the assumption was that compiler will not
try to realize the whole set of information for this AssemblyRef entry
but decompiler will try to do. The problem was that F# compiler is
eagerly trying to populate information about referenced assemblies for the
given assembly so it crashes during the attempt to read this malformed entry.

fixes dotnet/fsharp#517
closes dotnet/fsharp#519

commit 326a7fc8fe346e1ee65fad9f60b3d7b84cf18a42
Author: Vladimir Matveev <vladima@microsoft.com>
Date:   Wed Jul 1 00:27:11 2015 -0700

    added tests

commit 1d8dd07e7652fd20cce3300bf7099919a1d900a2
Author: Vladimir Matveev <vladima@microsoft.com>
Date:   Mon Jun 29 00:14:27 2015 -0700

    fix incorrect condition

commit 0c0c69624da25ca3453b3fc84207a86e75cb015d
Author: Vladimir Matveev <vladima@microsoft.com>
Date:   Sun Jun 28 23:30:38 2015 -0700

    added range check to readBlobHeapUncached

Author: vladima

src/absil/ilread.fs

@@ -1049,6 +1049,7 @@ type ILReaderContext =
     userStringsStreamPhysicalLoc: int32;
     stringsStreamPhysicalLoc: int32;
     blobsStreamPhysicalLoc: int32;
+    blobsStreamSize: int32;
     readUserStringHeap: (int32 -> string);
     memoizeString: string -> string;
     readStringHeap: (int32 -> string);
@@ -1534,9 +1535,13 @@ let readStringHeapUncached ctxtH idx =
 let readStringHeap          ctxt idx = ctxt.readStringHeap idx 
 let readStringHeapOption   ctxt idx = if idx = 0 then None else Some (readStringHeap ctxt idx) 
 
+let emptyByteArray: byte[] = [||]
 let readBlobHeapUncached ctxtH idx = 
     let ctxt = getHole ctxtH
-    seekReadBlob ctxt.is (ctxt.blobsStreamPhysicalLoc + idx) 
+    // valid index lies in range [1..streamSize)
+    // NOTE: idx cannot be 0 - Blob\String heap has first empty element that is one byte 0
+    if idx <= 0 || idx >= ctxt.blobsStreamSize then emptyByteArray
+    else seekReadBlob ctxt.is (ctxt.blobsStreamPhysicalLoc + idx) 
 let readBlobHeap        ctxt idx = ctxt.readBlobHeap idx 
 let readBlobHeapOption ctxt idx = if idx = 0 then None else Some (readBlobHeap ctxt idx) 
 
@@ -3973,6 +3978,7 @@ let rec genOpenBinaryReader infile is opts =
                  userStringsStreamPhysicalLoc   = userStringsStreamPhysicalLoc;
                  stringsStreamPhysicalLoc       = stringsStreamPhysicalLoc;
                  blobsStreamPhysicalLoc         = blobsStreamPhysicalLoc;
+                 blobsStreamSize                = blobsStreamSize;
                  memoizeString                  = Tables.memoize id;
                  readUserStringHeap             = cacheUserStringHeap (readUserStringHeapUncached ctxtH);
                  readStringHeap                 = cacheStringHeap (readStringHeapUncached ctxtH);

tests/fsharp/core/ilread/Library.dll

@@ -0,0 +1,35 @@
+@if "%_echo%"=="" echo off
+
+setlocal
+REM Configure the sample, i.e. where to find the F# compiler and C# compiler.
+if EXIST build.ok DEL /f /q build.ok
+
+call %~d0%~p0..\..\..\config.bat
+@if ERRORLEVEL 1 goto Error
+
+if NOT "%FSC:NOTAVAIL=X%" == "%FSC%" ( 
+  REM Skipping test for FSI.EXE
+  goto Skip
+)
+
+"%FSC%" %fsc_flags% /r:Library.dll /out:test.exe test.fs
+@if ERRORLEVEL 1 goto Error
+
+"%PEVERIFY%" test.exe
+@if ERRORLEVEL 1 goto Error
+
+:Ok
+echo Built fsharp %~f0 ok.
+echo. > build.ok
+endlocal
+exit /b 0
+
+:Skip
+echo Skipped %~f0
+endlocal
+exit /b 0
+
+
+:Error
+endlocal
+exit /b %ERRORLEVEL%

tests/fsharp/core/ilread/build.bat

@@ -0,0 +1,22 @@
+@if "%_echo%"=="" echo off
+
+setlocal 
+dir build.ok > NUL ) || (
+  @echo 'build.ok' not found.
+  goto :ERROR
+)
+
+call %~d0%~p0..\..\..\config.bat
+
+%CLIX% .\test.exe
+if ERRORLEVEL 1 goto Error
+
+:Ok
+echo Ran fsharp %~f0 ok.
+endlocal
+exit /b 0
+
+:Error
+endlocal
+exit /b %ERRORLEVEL%
+

tests/fsharp/core/ilread/run.bat

@@ -0,0 +1,8 @@
+open Library
+
+[<EntryPoint>]
+let main _ =
+    let cls = Class1()
+    let len = cls.GetLength("123")
+    printfn "%O" len
+    if len = 3 then 0 else 1

tests/fsharp/core/ilread/test.fs

@@ -35,6 +35,7 @@ Core03		fsharp\core\innerpoly
 Core03,Smoke		fsharp\core\int32
 Core03		fsharp\core\internalsvisible
 Core03		fsharp\core\interop
+Core03		fsharp\core\ilread
 Core03		fsharp\core\lazy
 Core03		fsharp\core\letrec
 Core03,Smoke		fsharp\core\libtest