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GDB’s builtin disassembler can be extended, or even replaced,
using the Python API. The disassembler related features are contained
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:
A read-only integer containing the address at which GDB wishes to disassemble a single instruction.
gdb.Architecture (see Architectures In Python) for
which GDB is currently disassembling, this property is
gdb.Progspace (see Program Spaces
In Python) for which GDB is currently disassembling, this
property is read-only.
True if the
DisassembleInfo object is valid,
False if not. A
DisassembleInfo object will become
invalid once the disassembly call for which the
was created, has returned. Calling other
methods, or accessing
DisassembleInfo properties, will raise a
RuntimeError exception if it is invalid.
This can be used to create a new
DisassembleInfo object that is
a copy of info. The copy will have the same
progspace values as info, and
will become invalid at the same time as info.
This method exists so that sub-classes of
be created, these sub-classes must be initialized as copies of an
DisassembleInfo object, but sub-classes might choose
to override the
read_memory method, and so control what
GDB sees when reading from memory
This method allows the disassembler to read the bytes of the
instruction to be disassembled. The method reads length bytes,
starting at offset from
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,
(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
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
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
not itself raise a
Any other exception type raised in
read_memory will propagate
back and be re-raised by
This is a base class from which all user implemented disassemblers must inherit.
The constructor takes name, a string, which should be a short name for this disassembler.
__call__ method must be overridden by sub-classes to
perform disassembly. Calling
__call__ on this base class will
The info argument is an instance of
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
that represents the disassembled instruction, this type is described
in more detail below.
__call__ method can raise a
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
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
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).
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:
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.
A read-only property containing the length of the disassembled instruction in bytes, this will always be greater than zero.
A read-only property containing a non-empty string representing the disassembled instruction.
The following functions are also contained in the
The disassembler must be a sub-class of
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
(see gdb.Architecture.name), or from
The disassembler will be installed for the architecture named by
architecture, or if architecture is
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
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.
This function calls back into GDB’s builtin disassembler to
disassemble the instruction identified by info, an instance, or
When the builtin disassembler needs to read memory the
read_memory method on info will be called. By
DisassembleInfo and overriding the
read_memory method, it is possible to intercept calls to
read_memory from the builtin disassembler, and to modify the
It is important to understand that, even when
DisassembleInfo.read_memory raises a
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
alternatively, if something goes wrong, an exception will be raised.
MemoryError will be raised if
unable to read some memory that is required in order to perform
Any exception that is not a
MemoryError, that is raised in a
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, 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
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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