Debugging with GDB: Unwinding Frames in Python

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#### 23.2.2.12 Unwinding Frames in Python

In GDB terminology “unwinding” is the process of finding the previous frame (that is, caller’s) from the current one. An unwinder has three methods. The first one checks if it can handle given frame (“sniff” it). For the frames it can sniff an unwinder provides two additional methods: it can return frame’s ID, and it can fetch registers from the previous frame. A running GDB mantains a list of the unwinders and calls each unwinder’s sniffer in turn until it finds the one that recognizes the current frame. There is an API to register an unwinder.

The unwinders that come with GDB handle standard frames. However, mixed language applications (for example, an application running Java Virtual Machine) sometimes use frame layouts that cannot be handled by the GDB unwinders. You can write Python code that can handle such custom frames.

You implement a frame unwinder in Python as a class with which has two attributes, `name` and `enabled`, with obvious meanings, and a single method `__call__`, which examines a given frame and returns an object (an instance of `gdb.UnwindInfo class)` describing it. If an unwinder does not recognize a frame, it should return `None`. The code in GDB that enables writing unwinders in Python uses this object to return frame’s ID and previous frame registers when GDB core asks for them.

#### Unwinder Input

An object passed to an unwinder (a `gdb.PendingFrame` instance) provides a method to read frame’s registers:

Function: PendingFrame.read_register (reg)

  This method returns the contents of the register regn in the frame as a `gdb.Value` object. reg can be either a register number or a register name; the values are platform-specific. They are usually found in the corresponding `platform-tdep.h` file in the GDB source tree.

It also provides a factory method to create a `gdb.UnwindInfo` instance to be returned to GDB:

Function: PendingFrame.create_unwind_info (frame_id)

  Returns a new `gdb.UnwindInfo` instance identified by given frame_id. The argument is used to build GDB’s frame ID using one of functions provided by GDB. frame_id’s attributes determine which function will be used, as follows:

  `sp, pc, special`

    `frame_id_build_special (frame_id.sp, frame_id.pc, frame_id.special)`

  `sp, pc`

    `frame_id_build (frame_id.sp, frame_id.pc)`

    This is the most common case.

  `sp`

    `frame_id_build_wild (frame_id.sp)`

  The attribute values should be `gdb.Value`

#### Unwinder Output: UnwindInfo

Use `PendingFrame.create_unwind_info` method described above to create a `gdb.UnwindInfo` instance. Use the following method to specify caller registers that have been saved in this frame:

Function: gdb.UnwindInfo.add_saved_register (reg, value)

  reg identifies the register. It can be a number or a name, just as for the `PendingFrame.read_register` method above. value is a register value (a `gdb.Value` object).

#### Unwinder Skeleton Code

GDB comes with the module containing the base `Unwinder` class. Derive your unwinder class from it and structure the code as follows:

    from gdb.unwinders import Unwinder

    class FrameId(object):
        def __init__(self, sp, pc):
            self.sp = sp
            self.pc = pc

    class MyUnwinder(Unwinder):
        def __init__(....):
            supe(MyUnwinder, self).__init___(<expects unwinder name argument>)

        def __call__(pending_frame):
            if not <we recognize frame>:
                return None
            # Create UnwindInfo.  Usually the frame is identified by the stack
            # pointer and the program counter.
            sp = pending_frame.read_register(<SP number>)
            pc = pending_frame.read_register(<PC number>)
            unwind_info = pending_frame.create_unwind_info(FrameId(sp, pc))

            # Find the values of the registers in the caller's frame and
            # save them in the result:
            unwind_info.add_saved_register(<register>, <value>)
            ....

            # Return the result:
            return unwind_info

#### Registering a Unwinder

An object file, a program space, and the GDB proper can have unwinders registered with it.

The `gdb.unwinders` module provides the function to register a unwinder:

Function: gdb.unwinder.register_unwinder (locus, unwinder, replace=False)

  locus is specifies an object file or a program space to which unwinder is added. Passing `None` or `gdb` adds unwinder to the GDB’s global unwinder list. The newly added unwinder will be called before any other unwinder from the same locus. Two unwinders in the same locus cannot have the same name. An attempt to add a unwinder with already existing name raises an exception unless replace is `True`, in which case the old unwinder is deleted.

#### Unwinder Precedence

GDB first calls the unwinders from all the object files in no particular order, then the unwinders from the current program space, and finally the unwinders from GDB.

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