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:

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

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:

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:

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.