Unwinding Frames in Python (Debugging with GDB)

23.3.2.13 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 maintains 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 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 (register)

This method returns the contents of register in the frame as a gdb.Value object. For a description of the acceptable values of register see Frame.read_register. If register 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 frame_id is used internally by GDB to identify the frames within the current thread’s stack. The attributes of frame_id determine what type of frame is created within GDB:

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 either be an instance of gdb.Value or an integer.

A helper class is provided in the gdb.unwinder module that can be used to represent a frame-id (see gdb.unwinder.FrameId).

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.

Function: PendingFrame.name (): Returns the function name of this pending frame, or None if it can’t be obtained.

Function: PendingFrame.is_valid ()

Returns true if the gdb.PendingFrame object is valid, false if not. A pending frame object becomes invalid when the call to the unwinder, for which the pending frame was created, returns.

All gdb.PendingFrame methods, except this one, will raise an exception if the pending frame object is invalid at the time the method is called.

Function: PendingFrame.pc (): Returns the pending frame’s resume address.

Function: PendingFrame.block (): Return the pending frame’s code block (see Blocks In Python). If the frame does not have a block – for example, if there is no debugging information for the code in question – then this will raise a RuntimeError exception.

Function: PendingFrame.function (): Return the symbol for the function corresponding to this pending frame. See Symbols In Python.

Function: PendingFrame.find_sal (): Return the pending frame’s symtab and line object (see Symbol Tables In Python).

Function: PendingFrame.language (): Return the language of this frame, as a string, or None.

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 (register, value): register identifies the register, for a description of the acceptable values see Frame.read_register. value is a register value (a gdb.Value object).

The `gdb.unwinder` Module

GDB comes with a gdb.unwinder module which contains the following classes:

class: gdb.unwinder.Unwinder

The Unwinder class is a base class from which user created unwinders can derive, though it is not required that unwinders derive from this class, so long as any user created unwinder has the required name and enabled attributes.

Function: gdb.unwinder.Unwinder.__init__(name): The name is a string used to reference this unwinder within some GDB commands (see Managing Registered Unwinders).

Variable: gdb.unwinder.name: A read-only attribute which is a string, the name of this unwinder.

Variable: gdb.unwinder.enabled: A modifiable attribute containing a boolean; when True, the unwinder is enabled, and will be used by GDB. When False, the unwinder has been disabled, and will not be used.

class: gdb.unwinder.FrameId

This is a class suitable for being used as the frame-id when calling gdb.PendingFrame.create_unwind_info. It is not required to use this class, any class with the required attribute (see gdb.PendingFrame.create_unwind_info) will be accepted, but in most cases this class will be sufficient.

gdb.unwinder.FrameId has the following method:

Function: gdb.unwinder.FrameId.__init__(sp, pc, special = None)

The sp and pc arguments are required and should be either a gdb.Value object, or an integer.

The special argument is optional; if specified, it should be a gdb.Value object, or an integer.

gdb.unwinder.FrameId has the following read-only attributes:

Variable: gdb.unwinder.sp: The sp value passed to the constructor.

Variable: gdb.unwinder.pc: The pc value passed to the constructor.

Variable: gdb.unwinder.special: The special value passed to the constructor, or None if no such value was passed.

Registering an Unwinder

Object files and program spaces can have unwinders registered with them. In addition, you can register unwinders globally.

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

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

locus specifies to which unwinder list to prepend the unwinder. It can be either an object file (see Objfiles In Python), a program space (see Progspaces In Python), or None, in which case the unwinder is registered globally. 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 an unwinder with an already existing name raises an exception unless replace is True, in which case the old unwinder is deleted and the new unwinder is registered in its place.

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

Unwinder Skeleton Code

Here is an example of how to structure a user created unwinder:

from gdb.unwinder import Unwinder, FrameId

class MyUnwinder(Unwinder):
    def __init__(self):
        super().__init___("MyUnwinder_Name")

    def __call__(self, pending_frame):
        if not <we recognize frame>:
            return None

        # Create a FrameID.  Usually the frame is identified by a
        # stack pointer and the function address.
        sp = ... compute a stack address ...
        pc = ... compute function address ...
        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-number>, <register-value>)
        ....

        # Return the result:
        return unwind_info

gdb.unwinder.register_unwinder(<locus>, MyUnwinder(), <replace>)

Managing Registered Unwinders

GDB defines 3 commands to manage registered unwinders. These are:

info unwinder [ locus [ name-regexp ] ]

Lists all registered unwinders. Arguments locus and name-regexp are both optional and can be used to filter which unwinders are listed.

The locus argument should be either global, progspace, or the name of an object file. Only unwinders registered for the specified locus will be listed.

The name-regexp is a regular expression used to match against unwinder names. When trying to match against unwinder names that include a string enclose name-regexp in quotes.

disable unwinder [ locus [ name-regexp ] ]

The locus and name-regexp are interpreted as in info unwinder above, but instead of listing the matching unwinders, all of the matching unwinders are disabled. The enabled field of each matching unwinder is set to False.

enable unwinder [ locus [ name-regexp ] ]

The locus and name-regexp are interpreted as in info unwinder above, but instead of listing the matching unwinders, all of the matching unwinders are enabled. The enabled field of each matching unwinder is set to True.