How to write native (i.e. C/C++) plugin for gdb?

How to write native (i.e. C/C++) plugin for gdb?

[Dmitry Samersoff]

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Hi Everybody,

I'm about to write a plugin to deal with complicated data (Java VM
coredump).

Unfortunately, python is not an option, as I plan to re-use already
existing C++ code of JVM to access internal structures.

Is there API, documentation, examples of such plugin?

Any advice is much appreciated.

- -Dmitry

- -- 
Dmitry Samersoff
Saint Petersburg, Russia, http://devnull.samersoff.net
* There will come soft rains  ...
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Re: How to write native (i.e. C/C++) plugin for gdb?

[Hui Zhu]

https://sourceware.org/gdb/current/onlinedocs/gdb/JIT-Interface.html
Maybe this is helpful.

On Tue, Dec 24, 2013 at 8:31 PM, Dmitry Samersoff <dms@samersoff.net> wrote:
> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA1
>
> Hi Everybody,
>
> I'm about to write a plugin to deal with complicated data (Java VM
> coredump).
>
> Unfortunately, python is not an option, as I plan to re-use already
> existing C++ code of JVM to access internal structures.
>
> Is there API, documentation, examples of such plugin?
>
> Any advice is much appreciated.
>
> - -Dmitry
>
> - --
> Dmitry Samersoff
> Saint Petersburg, Russia, http://devnull.samersoff.net
> * There will come soft rains  ...
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Re: How to write native (i.e. C/C++) plugin for gdb?

[Dmitry Samersoff]

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Hui,

Thank you for the pointer. It useful information but not exactly what
I need.

I need to implement scenario like one below:

(gdb) load-plugin  libgdbjvm.so
(gdb) attach PID
(gdb) jvm show loaded-classes
(gdb) jvm show java-threads
(gdb) jvm show code-cache

etc.

- -Dmitry


On 2013-12-24 17:15, Hui Zhu wrote:
> https://sourceware.org/gdb/current/onlinedocs/gdb/JIT-Interface.html
>
> 
Maybe this is helpful.
> 
> On Tue, Dec 24, 2013 at 8:31 PM, Dmitry Samersoff
> <dms@samersoff.net> wrote: Hi Everybody,
> 
> I'm about to write a plugin to deal with complicated data (Java VM 
> coredump).
> 
> Unfortunately, python is not an option, as I plan to re-use
> already existing C++ code of JVM to access internal structures.
> 
> Is there API, documentation, examples of such plugin?
> 
> Any advice is much appreciated.
> 
> -Dmitry
> 

- -- 
Dmitry Samersoff
Saint Petersburg, Russia, http://devnull.samersoff.net
* There will come soft rains  ...
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Re: How to write native (i.e. C/C++) plugin for gdb?

[Joel Brobecker]

> I need to implement scenario like one below:
> 
> (gdb) load-plugin  libgdbjvm.so
> (gdb) attach PID
> (gdb) jvm show loaded-classes
> (gdb) jvm show java-threads
> (gdb) jvm show code-cache

Maybe you could add some routines in your program that are solely
intended for debugging purposes, and then define commands that
call those routines. Off hand, that's the only way I can think of.
Otherwise, interface the C++ stuff you need from Python - if necessary,
export some stuff to C?

-- 
Joel

Re: How to write native (i.e. C/C++) plugin for gdb?

[Dmitry Samersoff]

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Joel,

Thank you for the response.

I found this patch
https://sourceware.org/ml/gdb-patches/2012-05/msg00500.html - many
thanks to Abhijit.

1. What is the reason to keep it off main trunk? Is there a chance to
have it changed?

2. What API is available from within a plugin? i.e. how I can access
coredump memory?

- -Dmitry

On 2013-12-24 18:47, Joel Brobecker wrote:
>> I need to implement scenario like one below:
>> 
>> (gdb) load-plugin  libgdbjvm.so (gdb) attach PID (gdb) jvm show
>> loaded-classes (gdb) jvm show java-threads (gdb) jvm show
>> code-cache
> 
> Maybe you could add some routines in your program that are solely 
> intended for debugging purposes, and then define commands that call
> those routines. Off hand, that's the only way I can think of. 
> Otherwise, interface the C++ stuff you need from Python - if
> necessary, export some stuff to C?
> 


- -- 
Dmitry Samersoff
Saint Petersburg, Russia, http://devnull.samersoff.net
* There will come soft rains  ...
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Re: How to write native (i.e. C/C++) plugin for gdb?

[Dmitry Samersoff]

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Everyone,

Just found a year old discussion about Abhijit proposal.

So answering to upcoming question - why not a Python.

Python is very good staff if your speak about source level debugging.
If you have to debug optimized binary python doesn't help match for
two reason -

  1) you have to port all native data structure you plan to work with
 to python.

  2) python performance is not always acceptable and you not always
ever able to run a python.

To get a better picture of native plugins in action, please, take a
look at solaris mdb.

- -Dmitry

On 2013-12-25 19:32, Dmitry Samersoff wrote:
> Joel,
> 
> Thank you for the response.
> 
> I found this patch 
> https://sourceware.org/ml/gdb-patches/2012-05/msg00500.html - many 
> thanks to Abhijit.
> 
> 1. What is the reason to keep it off main trunk? Is there a chance
> to have it changed?
> 
> 2. What API is available from within a plugin? i.e. how I can
> access coredump memory?
> 
> -Dmitry
> 
> On 2013-12-24 18:47, Joel Brobecker wrote:
>>> I need to implement scenario like one below:
>>> 
>>> (gdb) load-plugin  libgdbjvm.so (gdb) attach PID (gdb) jvm
>>> show loaded-classes (gdb) jvm show java-threads (gdb) jvm show 
>>> code-cache
> 
>> Maybe you could add some routines in your program that are solely
>>  intended for debugging purposes, and then define commands that
>> call those routines. Off hand, that's the only way I can think
>> of. Otherwise, interface the C++ stuff you need from Python - if 
>> necessary, export some stuff to C?
> 
> 
> 
> 

- -- 
Dmitry Samersoff
Saint Petersburg, Russia, http://devnull.samersoff.net
* There will come soft rains  ...
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Re: How to write native (i.e. C/C++) plugin for gdb?

[Doug Evans]

On Wed, Dec 25, 2013 at 8:51 AM, Dmitry Samersoff <dms@samersoff.net> wrote:
> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA1
>
> Everyone,
>
> Just found a year old discussion about Abhijit proposal.
>
> So answering to upcoming question - why not a Python.
>
> Python is very good staff if your speak about source level debugging.
> If you have to debug optimized binary python doesn't help match for
> two reason -
>
>   1) you have to port all native data structure you plan to work with
>  to python.

Hi.

This typically isn't that hard an exercise, have you looked into it?

>   2) python performance is not always acceptable and you not always
> ever able to run a python.

I'd like to see the data that shows the performance isn't acceptable.
This could just be mistaken impressions.

> To get a better picture of native plugins in action, please, take a
> look at solaris mdb.
>
> - -Dmitry
>
> On 2013-12-25 19:32, Dmitry Samersoff wrote:
>> Joel,
>>
>> Thank you for the response.
>>
>> I found this patch
>> https://sourceware.org/ml/gdb-patches/2012-05/msg00500.html - many
>> thanks to Abhijit.
>>
>> 1. What is the reason to keep it off main trunk? Is there a chance
>> to have it changed?
>>
>> 2. What API is available from within a plugin? i.e. how I can
>> access coredump memory?
>>
>> -Dmitry
>>
>> On 2013-12-24 18:47, Joel Brobecker wrote:
>>>> I need to implement scenario like one below:
>>>>
>>>> (gdb) load-plugin  libgdbjvm.so (gdb) attach PID (gdb) jvm
>>>> show loaded-classes (gdb) jvm show java-threads (gdb) jvm show
>>>> code-cache
>>
>>> Maybe you could add some routines in your program that are solely
>>>  intended for debugging purposes, and then define commands that
>>> call those routines. Off hand, that's the only way I can think
>>> of. Otherwise, interface the C++ stuff you need from Python - if
>>> necessary, export some stuff to C?
>>
>>
>>
>>
>
> - --
> Dmitry Samersoff
> Saint Petersburg, Russia, http://devnull.samersoff.net
> * There will come soft rains  ...
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Re: How to write native (i.e. C/C++) plugin for gdb?

[Dmitry Samersoff]

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Doug,

Unfortunately, sustaining - i.e. fixing customer side issues, - has
different requirements comparing to development - you have much less
control on environment and have to support more versions and
configuration than usual.

see also below.

On 2013-12-27 02:55, Doug Evans wrote:
> On Wed, Dec 25, 2013 at 8:51 AM, Dmitry Samersoff
> <dms@samersoff.net> wrote: Everyone,
> 
> Just found a year old discussion about Abhijit proposal.
> 
> So answering to upcoming question - why not a Python.
> 
> Python is very good staff if your speak about source level
> debugging. If you have to debug optimized binary python doesn't
> help match for two reason -
> 
> 1) you have to port all native data structure you plan to work
> with to python.

>> This typically isn't that hard an exercise, have you looked into
>> it?

I really like python (e.g. here is pure python elf reader and
disassemblers for arm, sparc and x86
http://www.samersoff.net/hg/pydas/ written as a part of larger
project) but ...

Typical deadlock case requires me to deal with 10-20 structures.

 Counting different versions and separate big- and little- endian
representation in Python, it creates a real zoo. And it's a pain to
keep this zoo in sync with C version.

In some case I can invoke target process code and it gives an
alleviation but it not always possible.

Use appropriate c headers directly and write simple native plugin is
much easy and safer approach.

Both windbg and mdb supports native plugins so I had a chance to
compare different options in practice.

> 2) python performance is not always acceptable and you not always 
> ever able to run a python.
> 
>> I'd like to see the data that shows the performance isn't
>> acceptable. This could just be mistaken impressions.

On many of arm or mips boards overhead of python interpreter within
debugging session is not acceptable. Not mentioning the fact, that I
have to install python to this board first as it typically not there.

 (Yes, emulation helps but it's not always an option - e.g. it doesn't
help to find that the program doesn't save one of FPU register on some
version of hardware)

Other example - walking through java heap. Re-implementation of
garbage collector code in Python to optimize the walking is rather
costly but without optimization it takes a while.

- -Dmitry

> To get a better picture of native plugins in action, please, take
> a look at solaris mdb.
> 
> -Dmitry
> 
> On 2013-12-25 19:32, Dmitry Samersoff wrote:
>>>> Joel,
>>>> 
>>>> Thank you for the response.
>>>> 
>>>> I found this patch 
>>>> https://sourceware.org/ml/gdb-patches/2012-05/msg00500.html -
>>>> many thanks to Abhijit.
>>>> 
>>>> 1. What is the reason to keep it off main trunk? Is there a
>>>> chance to have it changed?
>>>> 
>>>> 2. What API is available from within a plugin? i.e. how I
>>>> can access coredump memory?
>>>> 
>>>> -Dmitry
>>>> 
>>>> On 2013-12-24 18:47, Joel Brobecker wrote:
>>>>>> I need to implement scenario like one below:
>>>>>> 
>>>>>> (gdb) load-plugin  libgdbjvm.so (gdb) attach PID (gdb)
>>>>>> jvm show loaded-classes (gdb) jvm show java-threads (gdb)
>>>>>> jvm show code-cache
>>>> 
>>>>> Maybe you could add some routines in your program that are
>>>>> solely intended for debugging purposes, and then define
>>>>> commands that call those routines. Off hand, that's the
>>>>> only way I can think of. Otherwise, interface the C++ stuff
>>>>> you need from Python - if necessary, export some stuff to
>>>>> C?
>>>> 
>>>> 
>>>> 
>>>> 
> 

- -- 
Dmitry Samersoff
Saint Petersburg, Russia, http://devnull.samersoff.net
* There will come soft rains  ...
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