VM debugging trick: browsing object memory snapshots made from the C debugger in the simulator

Recently I made use of a virtual machine debugging trick that I don’t think I’ve written about here. The virtual machine simulator is a great way to browse around the state of object memory at the point of a VM crash, but it’s relatively slow (I measured it at about 3,000 instructions per second in my sonification work). The time it takes to get to that point might be prohibitively long.

Instead, you can run to that point in the C debugger (I use lldb on Mac OS), and make a snapshot by invoking primitiveSnapshot() manually. Then, open a simulator on that snapshot and browse the objects to your heart’s content. Depending on the error, you might even be able to modify the objects such that the active process may proceed, and you can snapshot a repaired object memory from the simulator.

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2 Responses to “VM debugging trick: browsing object memory snapshots made from the C debugger in the simulator”

  1. Frank Shearar Says:

    I’m pretty sure Eliot described this trick the other month, in the context of his own VM hacking adventures. It’s certainly a very neat trick!

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