Debugging with GDB: Disassembly In Python

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#### 23.3.2.38 Instruction Disassembly In Python

GDB’s builtin disassembler can be extended, or even replaced, using the Python API. The disassembler related features are contained within the `gdb.disassembler` module:

class: gdb.disassembler.DisassembleInfo

  Disassembly is driven by instances of this class. Each time GDB needs to disassemble an instruction, an instance of this class is created and passed to a registered disassembler. The disassembler is then responsible for disassembling an instruction and returning a result.

  Instances of this type are usually created within GDB, however, it is possible to create a copy of an instance of this type, see the description of `__init__` for more details.

  This class has the following properties and methods:

  Variable: DisassembleInfo.address

    A read-only integer containing the address at which GDB wishes to disassemble a single instruction.

  Variable: DisassembleInfo.architecture

    The `gdb.Architecture` (see Architectures In Python) for which GDB is currently disassembling, this property is read-only.

  Variable: DisassembleInfo.progspace

    The `gdb.Progspace` (see Program Spaces In Python) for which GDB is currently disassembling, this property is read-only.

  Function: DisassembleInfo.is_valid ()

    Returns `True` if the `DisassembleInfo` object is valid, `False` if not. A `DisassembleInfo` object will become invalid once the disassembly call for which the `DisassembleInfo` was created, has returned. Calling other `DisassembleInfo` methods, or accessing `DisassembleInfo` properties, will raise a `RuntimeError` exception if it is invalid.

  Function: DisassembleInfo.__init__ (info)

    This can be used to create a new `DisassembleInfo` object that is a copy of info. The copy will have the same `address`, `architecture`, and `progspace` values as info, and will become invalid at the same time as info.

    This method exists so that sub-classes of `DisassembleInfo` can be created, these sub-classes must be initialized as copies of an existing `DisassembleInfo` object, but sub-classes might choose to override the `read_memory` method, and so control what GDB sees when reading from memory (see builtin_disassemble).

  Function: DisassembleInfo.read_memory (length, offset)

    This method allows the disassembler to read the bytes of the instruction to be disassembled. The method reads length bytes, starting at offset from `DisassembleInfo.address`.

    It is important that the disassembler read the instruction bytes using this method, rather than reading inferior memory directly, as in some cases GDB disassembles from an internal buffer rather than directly from inferior memory, calling this method handles this detail.

    Returns a buffer object, which behaves much like an array or a string, just as `Inferior.read_memory` does (see Inferior.read_memory). The length of the returned buffer will always be exactly length.

    If GDB is unable to read the required memory then a `gdb.MemoryError` exception is raised (see Exception Handling).

    This method can be overridden by a sub-class in order to control what GDB sees when reading from memory (see builtin_disassemble). When overriding this method it is important to understand how `builtin_disassemble` makes use of this method.

    While disassembling a single instruction there could be multiple calls to this method, and the same bytes might be read multiple times. Any single call might only read a subset of the total instruction bytes.

    If an implementation of `read_memory` is unable to read the requested memory contents, for example, if there’s a request to read from an invalid memory address, then a `gdb.MemoryError` should be raised.

    Raising a `MemoryError` inside `read_memory` does not automatically mean a `MemoryError` will be raised by `builtin_disassemble`. It is possible the GDB’s builtin disassembler is probing to see how many bytes are available. When `read_memory` raises the `MemoryError` the builtin disassembler might be able to perform a complete disassembly with the bytes it has available, in this case `builtin_disassemble` will not itself raise a `MemoryError`.

    Any other exception type raised in `read_memory` will propagate back and be re-raised by `builtin_disassemble`.

class: Disassembler

  This is a base class from which all user implemented disassemblers must inherit.

  Function: Disassembler.__init__ (name)

    The constructor takes name, a string, which should be a short name for this disassembler.

  Function: Disassembler.__call__ (info)

    The `__call__` method must be overridden by sub-classes to perform disassembly. Calling `__call__` on this base class will raise a `NotImplementedError` exception.

    The info argument is an instance of `DisassembleInfo`, and describes the instruction that GDB wants disassembling.

    If this function returns `None`, this indicates to GDB that this sub-class doesn’t wish to disassemble the requested instruction. GDB will then use its builtin disassembler to perform the disassembly.

    Alternatively, this function can return a `DisassemblerResult` that represents the disassembled instruction, this type is described in more detail below.

    The `__call__` method can raise a `gdb.MemoryError` exception (see Exception Handling) to indicate to GDB that there was a problem accessing the required memory, this will then be displayed by GDB within the disassembler output.

    Ideally, the only three outcomes from invoking `__call__` would be a return of `None`, a successful disassembly returned in a `DisassemblerResult`, or a `MemoryError` indicating that there was a problem reading memory.

    However, as an implementation of `__call__` could fail due to other reasons, e.g. some external resource required to perform disassembly is temporarily unavailable, then, if `__call__` raises a `GdbError`, the exception will be converted to a string and printed at the end of the disassembly output, the disassembly request will then stop.

    Any other exception type raised by the `__call__` method is considered an error in the user code, the exception will be printed to the error stream according to the `set python print-stack` setting (see `set python print-stack`).

class: DisassemblerResult

  This class is used to hold the result of calling `Disassembler.__call__`, and represents a single disassembled instruction. This class has the following properties and methods:

  Function: DisassemblerResult.__init__ (length, string)

    Initialize an instance of this class, length is the length of the disassembled instruction in bytes, which must be greater than zero, and string is a non-empty string that represents the disassembled instruction.

  Variable: DisassemblerResult.length

    A read-only property containing the length of the disassembled instruction in bytes, this will always be greater than zero.

  Variable: DisassemblerResult.string

    A read-only property containing a non-empty string representing the disassembled instruction.

The following functions are also contained in the `gdb.disassembler` module:

Function: register_disassembler (disassembler, architecture)

  The disassembler must be a sub-class of `gdb.disassembler.Disassembler` or `None`.

  The optional architecture is either a string, or the value `None`. If it is a string, then it should be the name of an architecture known to GDB, as returned either from `gdb.Architecture.name` (see gdb.Architecture.name), or from `gdb.architecture_names` (see gdb.architecture_names).

  The disassembler will be installed for the architecture named by architecture, or if architecture is `None`, then disassembler will be installed as a global disassembler for use by all architectures.

  GDB only records a single disassembler for each architecture, and a single global disassembler. Calling `register_disassembler` for an architecture, or for the global disassembler, will replace any existing disassembler registered for that architecture value. The previous disassembler is returned.

  If disassembler is `None` then any disassembler currently registered for architecture is deregistered and returned.

  When GDB is looking for a disassembler to use, GDB first looks for an architecture specific disassembler. If none has been registered then GDB looks for a global disassembler (one registered with architecture set to `None`). Only one disassembler is called to perform disassembly, so, if there is both an architecture specific disassembler, and a global disassembler registered, it is the architecture specific disassembler that will be used.

  GDB tracks the architecture specific, and global disassemblers separately, so it doesn’t matter in which order disassemblers are created or registered; an architecture specific disassembler, if present, will always be used in preference to a global disassembler.

  You can use the `maint info python-disassemblers` command (see maint info python-disassemblers) to see which disassemblers have been registered.

Function: builtin_disassemble (info)

  This function calls back into GDB’s builtin disassembler to disassemble the instruction identified by info, an instance, or sub-class, of `DisassembleInfo`.

  When the builtin disassembler needs to read memory the `read_memory` method on info will be called. By sub-classing `DisassembleInfo` and overriding the `read_memory` method, it is possible to intercept calls to `read_memory` from the builtin disassembler, and to modify the values returned.

  It is important to understand that, even when `DisassembleInfo.read_memory` raises a `gdb.MemoryError`, it is the internal disassembler itself that reports the memory error to GDB. The reason for this is that the disassembler might probe memory to see if a byte is readable or not; if the byte can’t be read then the disassembler may choose not to report an error, but instead to disassemble the bytes that it does have available.

  If the builtin disassembler is successful then an instance of `DisassemblerResult` is returned from `builtin_disassemble`, alternatively, if something goes wrong, an exception will be raised.

  A `MemoryError` will be raised if `builtin_disassemble` is unable to read some memory that is required in order to perform disassembly correctly.

  Any exception that is not a `MemoryError`, that is raised in a call to `read_memory`, will pass through `builtin_disassemble`, and be visible to the caller.

  Finally, there are a few cases where GDB’s builtin disassembler can fail for reasons that are not covered by `MemoryError`. In these cases, a `GdbError` will be raised. The contents of the exception will be a string describing the problem the disassembler encountered.

Here is an example that registers a global disassembler. The new disassembler invokes the builtin disassembler, and then adds a comment, `## Comment`, to each line of disassembly output:

    class ExampleDisassembler(gdb.disassembler.Disassembler):
        def __init__(self):
            super().__init__("ExampleDisassembler")

        def __call__(self, info):
            result = gdb.disassembler.builtin_disassemble(info)
            length = result.length
            text = result.string + "\t## Comment"
            return gdb.disassembler.DisassemblerResult(length, text)

    gdb.disassembler.register_disassembler(ExampleDisassembler())

The following example creates a sub-class of `DisassembleInfo` in order to intercept the `read_memory` calls, within `read_memory` any bytes read from memory have the two 4-bit nibbles swapped around. This isn’t a very useful adjustment, but serves as an example.

    class MyInfo(gdb.disassembler.DisassembleInfo):
        def __init__(self, info):
            super().__init__(info)

        def read_memory(self, length, offset):
            buffer = super().read_memory(length, offset)
            result = bytearray()
            for b in buffer:
                v = int.from_bytes(b, 'little')
                v = (v << 4) & 0xf0 | (v >> 4)
                result.append(v)
            return memoryview(result)

    class NibbleSwapDisassembler(gdb.disassembler.Disassembler):
        def __init__(self):
            super().__init__("NibbleSwapDisassembler")

        def __call__(self, info):
            info = MyInfo(info)
            return gdb.disassembler.builtin_disassemble(info)

    gdb.disassembler.register_disassembler(NibbleSwapDisassembler())

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