passing gdb.Value pointers to ctypes function calls

passing gdb.Value pointers to ctypes function calls

[Christoph Mathys]

Hello!

I'm trying to write a pretty printer for gdb 7.2 using python, to be
concrete I would like to print an Xml node (libxml2) as text. I've
managed to get the pointers to node and document. Now I'm trying to
call libxml using ctypes module, passing the pointer values as
c_void_p. But gdb keeps crashing on me. Is this supposed to work at
all? I allocate some buffer, pass it to libxml functions along with
some addresses I obtain from the debugged process using gdb, and as a
result I expect the string representation of the node in the buffer.

For converting between gdb.Value and ctypes usable stuff I use something like:
ptr = int(str(gdbValNode), 16)
c_void_p(ptr)

I'm attached to the running process. But as far as I understand, this
should work on the running process as well as the core file.

Thanks for insight!
Christoph

Re: passing gdb.Value pointers to ctypes function calls

[Phil Muldoon]

Christoph Mathys <eraserix@gmail.com> writes:

> Hello!
>
> I'm trying to write a pretty printer for gdb 7.2 using python, to be
> concrete I would like to print an Xml node (libxml2) as text. I've
> managed to get the pointers to node and document. Now I'm trying to
> call libxml using ctypes module, passing the pointer values as
> c_void_p. But gdb keeps crashing on me.

If GDB ever crashes it is a bug.  Can you file a bug for this?  


>  Is this supposed to work at
> all? I allocate some buffer, pass it to libxml functions along with
> some addresses I obtain from the debugged process using gdb, and as a
> result I expect the string representation of the node in the buffer.
>
> For converting between gdb.Value and ctypes usable stuff I use something like:
> ptr = int(str(gdbValNode), 16)
> c_void_p(ptr)
>
> I'm attached to the running process. But as far as I understand, this
> should work on the running process as well as the core file.

It should, but without more concrete information it is difficult to
diagnose the issue.  I am concerned about the GDB crash though, that
should be fixed.

Cheers,

Phil

Re: passing gdb.Value pointers to ctypes function calls

[Christoph Mathys]

On Tue, Apr 12, 2011 at 11:17 PM, Phil Muldoon <pmuldoon@redhat.com> wrote:
> Christoph Mathys <eraserix@gmail.com> writes:
>
>> call libxml using ctypes module, passing the pointer values as
>> c_void_p. But gdb keeps crashing on me.
>
> If GDB ever crashes it is a bug.  Can you file a bug for this?

I don't mind filing a bug report, but I'm not sure what else gdb
should do considering what I'm doing: Accessing possibly invalid
memory inside gdb itself. The code below is what I think happens in my
script. The difference is that I have a more complicated way of
getting the wrong pointer and call a different C function.

This is in crash.py:
from ctypes import *

ptr = c_void_p(1)
libc = CDLL('libc.so.6')
libc.printf('%s\n', ptr)

Fire up gdb and do: python execfile('crash.py'). For me, this crashes
gdb, but this is somewhat expected.

> It should, but without more concrete information it is difficult to
> diagnose the issue.  I am concerned about the GDB crash though, that
> should be fixed.

It was suggested that what I'm trying to do can't work. gdb lives in
another address space than my inferior, so pointers from my inferior
don't make sense to a library loaded into gdbs address space.

Christoph

Re: passing gdb.Value pointers to ctypes function calls

[Tom Tromey]

>>>>> "Christoph" == Christoph Mathys <eraserix@gmail.com> writes:

Christoph> I'm trying to write a pretty printer for gdb 7.2 using python, to be
Christoph> concrete I would like to print an Xml node (libxml2) as text. I've
Christoph> managed to get the pointers to node and document. Now I'm trying to
Christoph> call libxml using ctypes module, passing the pointer values as
Christoph> c_void_p. But gdb keeps crashing on me. Is this supposed to work at
Christoph> all?

Nope, this won't work.

A gdb.Value represents a value in the inferior.  If you look at its
address, you are getting an address in some other process, not in gdb.

Tom

Re: passing gdb.Value pointers to ctypes function calls

[Tom Tromey]

>>>>> "Phil" == Phil Muldoon <pmuldoon@redhat.com> writes:

>> I'm trying to write a pretty printer for gdb 7.2 using python, to be
>> concrete I would like to print an Xml node (libxml2) as text. I've
>> managed to get the pointers to node and document. Now I'm trying to
>> call libxml using ctypes module, passing the pointer values as
>> c_void_p. But gdb keeps crashing on me.

Phil> If GDB ever crashes it is a bug.  Can you file a bug for this?  

Usually this is true, but ctypes is special.  It lets you make pretty
much unrestricted C calls from Python.  We can't guard against all of
those.

Tom