Re: container_of equivalent in gdb-python script

[Jan Kiszka <jan dot kiszka at web dot de>]

Daniel Jacobowitz wrote:
> On Wed, Jun 10, 2009 at 10:10:39PM +0200, Jan Kiszka wrote:
>> I want to automate 'add-symbol-file linux_module.ko 0xff...' that you
>> have to run for loading the symbols of dynamically loaded kernel
>> modules. Before that you also have to look up the module base address,
>> typically by cat'ing /proc/modules on the target. With a proper python
>> script, this will be trivial to do automatically. You just have to walk
>> the module list of the kernel you are attached to, extract names and
>> base addresses, search for the corresponding module binaries (also easy
>> with python) and issue the proper add-symbol-file commands.
> 
> FYI, I am hoping to work on this soon (in the next month).  The right
> representation is not a set of add-symbol-file commands, which doesn't
> play nicely with repeated unload/reload, but instead a synthetic list

Currently, I does. I simply reload the whole set of symbols (vmlinux +
modules) when requested by the user. Of course, one could additionally
register a triggering breakpoint on return of load_module(), but having
a simple helper that can be called on demand will already be a great
improvement.

> of shared libraries.  That calls for some extensions to the current
> set of Python hooks and shared library infrastructure, though!  And it
> will require basically everything you're doing now :-)

Looking forward nevertheless. What ever makes kernel debugging more
handy is welcome. Once finished, I will post my script here.

> 
>> But now back to the core problems, starting with the exercise to
>> implement offset_of(type, field):
>>
>> def offset_of(type, field):
>> 	container_type = gdb.lookup_type(type)
>> 	dummy_obj = gdb.selected_frame().read_var('modules')
>> 	container_obj = dummy_obj.cast(container_type)
>> 	field_obj = container_obj[field]
>> 	return int(str(field_obj.address), 16) - \
>> 	       int(str(container_obj.address), 16)
>>
>> I meanwhile discovered (reading testcases and python-*.c)
>> gdb.lookup_type() and Value.cast() to make this real. But you see, I
>> still need an ugly synthetic Value object which must have a non-'None'
>> address to do this calculation. Is there a cleaner, more generic way?
> 
> It's typical to do this the same way folks do in C: Create a null
> pointer of the right type.

You can't do that with the current upstream python interface, but the
parse_and_eval solves this nicely.

Jan

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