ClrMD Part 7 – Manipulate nested structs using dynamic

By: CriteoLabs / 28 Aug 2017

In the previous post of the ClrMD series, we’ve seen how to use dynamic to manipulate objects from a memory dump the same way as you would with actual objects. However, the code we wrote was limited to class instances. This time, we’re going to see how to extend it to structs. The associated code is part of the DynaMD library and is available on GitHub and nuget.

Part 1: Bootstrap ClrMD to load a dump.

Part 2: Find duplicated strings with ClrMD heap traversing.

Part 3: List timers by following static fields links.

Part 4: Identify timers callback and other properties.

Part 5: Use ClrMD to extend SOS in WinDBG.

Part 6: Manipulate memory structures like real objects.

 

Let’s start with a reminder of the object we’re manipulating via our proxy:

Accessing the value of proxy.Description.Id is a special case. Why? Because Size is a struct, and therefore is embedded directly inside of Sample outside of the responsibility of the managed heap. This scenario isn’t directly supported by ClrMD, and calling GetValue on the Size field will return null instead of the address of the struct. We need to compute this address ourselves.

What is the layout of those objects in memory? To find out, we can create a Sample object and then check how the memory is structured within WinDBG

 

 

The first address (4 bytes because the dump was taken on a 32-bit process) points to the “method table”; some metadata used internally by the CLR to identify the strong type corresponding to this instance in the managed heap. Next, we get the “1” stored by the Value field, followed by a pointer to the “Test” string stored by the Name field. The field values of the Description structure (in dark blue) are then embedded directly inside of the Sample object. And inside of that structure, we find the values of the fields of the Size structure (in light blue). Finally, the reference to the Child field is stored last in the memory of the Sample object.

To sum it up, to get the address of the Description field, we need to take the address of the Sample instance, add 4 bytes for the method table (8 bytes for 64-bit memory dumps), then add the offset of the field (which counts the size of the previous fields and the padding if any):

We call this method from TryGetMember when we have a struct that isn’t a primitive type. ClrMD gives that information thanks to the HasSimpleValue property of ClrField:

There’s another subtlety though. As we’ve seen in the layout, the Description struct, embedded inside of the Sample class, doesn’t have a method table of its own. The consequence is that we can’t find out its type by using ClrHeap.GetObjectType. As a workaround, we add a constructor allowing us to manually set the underlying ClrMD type of the object impersonated by the proxy:

This constructor is called when we still have the type information (taken from the ClrField):

Now we’re able to read the value of a field of a nested struct:

Are we done now? Almost. There is one last case to handle, the Size struct that is nested inside of Description, itself a struct nested inside of a Sample instance. How is that an issue? If we use the same code to compute the address of the Size struct, then we will be adding the 4/8 bytes for the method table. Except that, as we’ve seen, the nested Size struct doesn’t have a method table! We need to add a condition to the code to know whenever we are inside of a nested struct and handle this corner case:

The Boolean flag _interior is set in the second constructor that accepts a ClrType:

We’re finally able to read the value of the nested-nested struct:

Next time, we’ll see how to use the same mechanisms to manipulate arrays in a convenient way.

Post written by:


 

 

 

 

 

Christophe Nasarre

Staff Software Engineer, R&D.

 

 

 

 

 

Kevin Gosse

Staff Software Engineer, R&D.

Twitter: KooKiz

 

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