UserspaceEmulator: Catch use-after-frees by tracking malloc/free :^)

This patch introduces a "MallocTracer" to the UserspaceEmulator.
If this object is present on the Emulator, it can be notified whenever
the emulated program does a malloc() or free().

The notifications come in via a magic instruction sequence that we
embed in the LibC malloc() and free() functions. The sequence is:

    "salc x2, push reg32 x2, pop reg32 x3"

The data about the malloc/free operation is in the three pushes.
We make sure the sequence is harmless when running natively.

Memory accesses on MmapRegion are then audited to see if they fall
inside a known-to-be-freed malloc chunk. If so, we complain loud
and red in the debugger output. :^)

This is very, very cool! :^)

It's also a whole lot slower than before, since now we're auditing
memory accesses against a new set of metadata. This will need to be
optimized (and running in this mode should be opt-in, perhaps even
a separate program, etc.)

DevTools/UserspaceEmulator/MallocTracer.h

@@ -0,0 +1,64 @@
+/*
+ * Copyright (c) 2020, Andreas Kling <kling@serenityos.org>
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice, this
+ *    list of conditions and the following disclaimer.
+ *
+ * 2. Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#pragma once
+
+#include <AK/Badge.h>
+#include <AK/Types.h>
+#include <AK/Vector.h>
+
+namespace UserspaceEmulator {
+
+class SoftCPU;
+
+class MallocTracer {
+public:
+    MallocTracer();
+
+    void target_did_malloc(Badge<SoftCPU>, FlatPtr address, size_t);
+    void target_did_free(Badge<SoftCPU>, FlatPtr address);
+
+    void audit_read(FlatPtr address, size_t);
+    void audit_write(FlatPtr address, size_t);
+
+private:
+    struct Mallocation {
+        bool contains(FlatPtr a) const
+        {
+            return a >= address && a < (address + size);
+        }
+
+        FlatPtr address { 0 };
+        size_t size { 0 };
+        bool freed { false };
+    };
+
+    Mallocation* find_mallocation(FlatPtr);
+
+    Vector<Mallocation> m_mallocations;
+};
+
+}