Debugging with GDB: Unwinding Frames in Python

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#### 23.3.2.12 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.

An unwinder should do as little work as possible. Some otherwise innocuous operations can cause problems (even crashes, as this code is not not well-hardened yet). For example, making an inferior call from an unwinder is unadvisable, as an inferior call will reset GDB’s stack unwinding process, potentially causing re-entrant unwinding.

#### 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 reg in the frame as a `gdb.Value` object. For a description of the acceptable values of reg see Frame.read_register. If reg does not name a register for the current architecture, this method will throw an exception.

  Note that this method will always return a `gdb.Value` for a valid register name. This does not mean that the value will be valid. For example, you may request a register that an earlier unwinder could not unwind—the value will be unavailable. Instead, the `gdb.Value` returned from this method will be lazy; that is, its underlying bits will not be fetched until it is first used. So, attempting to use such a value will cause an exception at the point of use.

  The type of the returned `gdb.Value` depends on the register and the architecture. It is common for registers to have a scalar type, like `long long`; but many other types are possible, such as pointer, pointer-to-function, floating point or vector types.

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`

    The frame is identified by the given stack address and PC. The stack address must be chosen so that it is constant throughout the lifetime of the frame, so a typical choice is the value of the stack pointer at the start of the function—in the DWARF standard, this would be the “Call Frame Address”.

    This is the most common case by far. The other cases are documented for completeness but are only useful in specialized situations.

  `sp, pc, special`

    The frame is identified by the stack address, the PC, and a “special” address. The special address is used on architectures that can have frames that do not change the stack, but which are still distinct, for example the IA-64, which has a second stack for registers. Both sp and special must be constant throughout the lifetime of the frame.

  `sp`

    The frame is identified by the stack address only. Any other stack frame with a matching sp will be considered to match this frame. Inside gdb, this is called a “wild frame”. You will never need this.

  Each attribute value should be an instance of `gdb.Value`.

Function: PendingFrame.architecture ()

  Return the `gdb.Architecture` (see Architectures In Python) for this `gdb.PendingFrame`. This represents the architecture of the particular frame being unwound.

Function: PendingFrame.level ()

  Return an integer, the stack frame level for this frame. See Stack Frames.

#### 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, for a description of the acceptable values see Frame.read_register. 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__(....):
            super(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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