Implement backtraces on Windows.

Much !!FUN!! was had

WinBacktrace.cpp

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+/* Copyright 2014 MultiMC Contributors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+// CAUTION:
+// This file contains all manner of hackery and insanity.
+// I will not be responsible for any loss of sanity due to reading this code.
+// Here be dragons!
+
+#include "WinBacktrace.h"
+
+#include <windows.h>
+
+#ifndef __i386__
+#error WinBacktrace is only supported on x86 architectures.
+#endif
+
+
+void dumpInfo(StackFrame* frame, const BYTE* caller, HINSTANCE hInst);
+
+// We need to do some crazy shit to walk through the stack.
+// Windows unwinds the stack when an exception is thrown, so we
+// need to examine the EXCEPTION_POINTERS's CONTEXT.
+size_t getBacktrace(StackFrame *stack, size_t size, CONTEXT ctx)
+{
+	// Written using information and a bit of pseudocode from
+	// http://www.eptacom.net/pubblicazioni/pub_eng/except.html
+	// This is probably one of the most horrifying things I've ever written.
+
+	// This tracks whether the current EBP is valid.
+	// When an invalid EBP is encountered, we stop walking the stack.
+	bool validEBP = true;
+	DWORD ebp =	ctx.Ebp; // The current EBP (Extended Base Pointer)
+	DWORD eip = ctx.Eip;
+	int i;
+	for (i = 0; i < size; i++)
+	{
+		if (ebp & 3)
+			validEBP = false;
+		// FIXME: This function is obsolete, according to MSDN.
+		else if (IsBadReadPtr((void*) ebp, 8))
+			validEBP = false;
+
+		if (!validEBP) break;
+
+		// Find the caller.
+		// On the first iteration, the caller is whatever EIP points to.
+		// On successive iterations, the caller is the byte after EBP.
+		BYTE* caller = !i ? (BYTE*)eip : *((BYTE**) ebp + 1);
+		// The first ebp is the EBP from the CONTEXT.
+		// On successive iterations, the EBP is the DWORD that the previous EBP points to.
+		ebp	= !i ? ebp : *(DWORD*)ebp;
+
+		// Find the caller's module.
+		// We'll use VirtualQuery to get information about the caller's address.
+		MEMORY_BASIC_INFORMATION mbi;
+		VirtualQuery(caller, &mbi, sizeof(mbi));
+
+		// We can get the instance handle from the allocation base.
+		HINSTANCE hInst = (HINSTANCE)mbi.AllocationBase;
+
+		// If the handle is 0, then the EBP is invalid.
+		if (hInst == 0) validEBP = false;
+		// Otherwise, dump info about the caller.
+		else stack[i].address = (void*)caller;
+	}
+
+	return i;
+}