Re: [Python-Dev] GDB macros in Misc/gdbinit are broken

[Victor Stinner]

2017-02-28 20:38 GMT+01:00 Alexander Belopolsky
:
> First, I had to rename python-gdb.py to python3.6-gdb.py to make it load.

There is nothing specific to Python here, the file command of gdb (or
gdb file) tries to load "file-gdb.py". So if your program is called
python3.6, gdb looks for python3.6-gdb.py, yeah.

> Then running backtrace gave me a bunch of error messages:
> (...)
> except gdb.error:
> AttributeError: 'module' object has no attribute 'error'

Oh, I never tried python-gdb.py on RHEL. Sadly, I know that the Python
binding of gdb changes depending on the gdb version, but I also
suspect that it changes depending on the OS! I'm quite sure that
Fedora backports gdb changes for its Python API.

I'm only using python-gdb.py on Fedora, usually even the latest Fedora
version. Fedora always has bleeding edge versions of development
tools!

It shouldn't be hard to catch this "except gdb.error:" line. We should
find the right exception and patch libpython.py.


> It looks like there is a mismatch between python-gdb.py  and the gdb module
> that I assume comes with gdb.

python-gdb.py (libpython.py) should work on any gdb version. It
shouldn't be hard to support multiple gdb versions in same code base,
we already support Python 2 and Python 3 in the same code base!
(Depending on the OS and OS version, gdb may use Python 2 or Python 3
to run python-gdb.py, this script is run by gdb, not by test debugged
Python binary! ;-))

Victor
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Re: [Python-Dev] GDB macros in Misc/gdbinit are broken

[Victor Stinner]

Ok, it seems like very few people know how to use python-gdb.py :-/
Sorry, I expect that everybody was using it!

python-gdb.py was written by Dave Malcolm, it includes:

* a pretty printer for many builtin Python types: str, tuple, list,
dict, frame, etc. It means that a regular gdb "print obj" command
displays the Python object content instead of a raw pointer. This
feature is super useful, it avoids to call _PyObject_Dump(obj) which
is likely to crash!
* commands to navigate between Python frames: py-up, py-down
* command to dump the Python traceback: py-bt, py-bt-full (also
include C frames)
* a few more commands

For me, these commands are really amazing! It's impressive to be able
to that in a debugger which doesn't know Python internals at all! It's
cool to be able to "program" (extend) a debugger!

I never tried to install it, I always use it with a ./python binary
compiled in the source code tree:
---
$ make
$ gdb ./python
# load ./python-gdb.py
...
---

Note: ./python-gdb.py is simply a copy of Tools/gdb/libpython.py,
copied by Makefile.

On Fedora, gdb doesn't load python-gdb.py because it doesn't trust my
$HOME/prog directory (root of all my development directories). I had
to trust it using this ~/.gdbinit config:
---
add-auto-load-safe-path ~/prog/
---

More generally, when gdb loads a "program", it tries to load
"program-gdb.py" in the same directory. Maybe it can load
"program-gdb.py" from other directories, but I never understood this
part, and hopefully I never had to understand it :-D

Maybe the debug package of Python on Debian and Fedora installs
pythonX.Y-gdb.py in the right directory, I didn't check, but I always
debug using a freshly compiled Python, so I never tried to understand
how these things work.

Example:

haypo@selma$ gdb ./python
(gdb) b _PyEval_EvalFrameDefault
(gdb) run

Breakpoint 1, _PyEval_EvalFrameDefault (
f=Frame 0x77f22058, for file ,
line 25, in  (), throwflag=0) at Python/ceval.c:678
678PyObject *retval = NULL;/* Return value */

=> gdb displays the content of the frame "f", you can see immediately
the filename and the line number (well, this frame is a little bit
special, it's the frozen module importlib)

(gdb) py-bt
Traceback (most recent call first):
  File "", line 25, in 
---

Example of Python traceback:

haypo@selma$ gdb -args ./python -m test -r
(gdb) run
...
^C
(gdb) py-bt
Traceback (most recent call first):
  File "/home/haypo/prog/python/master/Lib/test/test_long.py", line
947, in test_bit_length
self.assertEqual(k, len(bin(x).lstrip('-0b')))
  File "/home/haypo/prog/python/master/Lib/unittest/case.py", line 601, in run
testMethod()
  File "/home/haypo/prog/python/master/Lib/unittest/case.py", line
649, in __call__
return self.run(*args, **kwds)
  File "/home/haypo/prog/python/master/Lib/unittest/suite.py", line 122, in run
test(result)
  File "/home/haypo/prog/python/master/Lib/unittest/suite.py", line
84, in __call__
return self.run(*args, **kwds)
  ...

=> you get filename, line number, function name and even the Python
line! It seems obvious to get such information, but remind that you
are in gdb, not in Python

(gdb) py-list
 942def test_bit_length(self):
 943tiny = 1e-10
 944for x in range(-65000, 65000):
 945k = x.bit_length()
 946# Check equivalence with Python version
>947self.assertEqual(k, len(bin(x).lstrip('-0b')))
 948# Behaviour as specified in the docs
 949if x != 0:
 950self.assertTrue(2**(k-1) <= abs(x) < 2**k)
 951else:
 952self.assertEqual(k, 0)

# move to the parent Python frame (= skip mutiple C frames until the
next Python frame)
(gdb) py-up
(...)

(gdb) py-list
 596with outcome.testPartExecutor(self):
 597self.setUp()
 598if outcome.success:
 599outcome.expecting_failure = expecting_failure
 600with outcome.testPartExecutor(self, isTest=True):
>601testMethod()
 602outcome.expecting_failure = False
 603with outcome.testPartExecutor(self):
 604self.tearDown()
 605
 606self.doCleanups()

(gdb) py-locals
self =  dump all variables of the current Python frame!


Victor
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Re: [Python-Dev] GDB macros in Misc/gdbinit are broken

[Skip Montanaro]

> First, I had to rename python-gdb.py ...

Okay, I found a copy of python-gdb.py at the top level in a 2.7.13
snapshot I have at-hand. I have no idea where it came from. A file
search on GitHub in the python/cpython repo on both master and 2.7
branches yielded nothing.

It does seem to be working for me on a debug build of Python 2.7.13 (I
ran strings over my GDB executable first to see if it knew anything
about Python, and it did).

Thx,

Skip
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Re: [Python-Dev] GDB macros in Misc/gdbinit are broken

[Alexander Belopolsky]

On Tue, Feb 28, 2017 at 3:33 AM, Victor Stinner 
wrote:

>
> Alexander, Skip: Oh, which kind of issues do you have with
> python-gdb.py? It doesn't work? You are unable to dump some data?


First, I had to rename python-gdb.py to python3.6-gdb.py to make it load.
Then running backtrace gave me a bunch of error messages:


(gdb) bt
#0  builtin_hex (module=, number=0) at
Python/bltinmodule.c:1436
#1  0x77a9304c in _PyCFunction_FastCallDict (func_obj=, args=0x662cf8,
nargs=1, kwargs=0x0)
at Objects/methodobject.c:209
#2  0x77a9344b in _PyCFunction_FastCallKeywords (func=, stack=0x662cf8,
nargs=1, kwnames=0x0)
at Objects/methodobject.c:295
#3  0x77bac6d9 in call_function (pp_stack=0x7fffb6d8, oparg=1,
kwnames=0x0) at Python/ceval.c:4788
#4  0x77ba3e6c in _PyEval_EvalFrameDefault (f=Frame 0x662b68, for
file r.py, line 2, in  (), throwflag=0) at Python/ceval.c:3275
#5  0x77b8cc67 in PyEval_EvalFrameEx (f=Frame 0x662b68, for file
r.py, line 2, in  (), throwflag=0) at Python/ceval.c:718
#6  0x77ba9745 in _PyEval_EvalCodeWithName (_co=, globals=Traceback (most recent call last):
  File "/home/a/.virtualenvs/3.6g/bin/python3.6-gdb.py", line 1358, in
to_string
return pyop.get_truncated_repr(MAX_OUTPUT_LEN)
  File "/home/a/.virtualenvs/3.6g/bin/python3.6-gdb.py", line 243, in
get_truncated_repr
self.write_repr(out, set())
  File "/home/a/.virtualenvs/3.6g/bin/python3.6-gdb.py", line 702, in
write_repr
for pyop_key, pyop_value in self.iteritems():
  File "/home/a/.virtualenvs/3.6g/bin/python3.6-gdb.py", line 669, in
iteritems
entries, nentries = self._get_entries(keys)
  File "/home/a/.virtualenvs/3.6g/bin/python3.6-gdb.py", line 717, in
_get_entries
except gdb.error:
AttributeError: 'module' object has no attribute 'error'
...

It looks like there is a mismatch between python-gdb.py  and the gdb module
that I assume comes with gdb.

$ gdb --version
GNU gdb (GDB) Red Hat Enterprise Linux (7.2-90.el6)
..
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Re: [Python-Dev] GDB macros in Misc/gdbinit are broken

[Barry Warsaw]

On Feb 28, 2017, at 09:09 AM, Skip Montanaro wrote:

>I haven't tried using Python support in gdb in a long time. I don't
>know what python-gdb.py is. How is that different than
>Tools/gdb/libpython.py? Is it distributed with Python? (I'm kind of
>blind at work without a source distribution I can grep through, and
>didn't see it in Tools/gdb or Misc.)
>
>I'm a simple man. I need the kind of stuff Misc/gdbinit provides
>(smart stack traces and variable printing), but I most frequently use
>the pyo user-defined command to print the contents of PyObject
>pointers. When they are working () pystack and pyframe are also
>useful.

I think I'm largely in the same boat as Skip although it's been a while since
I stepped through the C code.  I also had trouble (ages ago) getting the gdb
integration working, so I relied on the simple stuff in gdbinit.  I think
there's still utility in that if we can keep it working.

-Barry
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Re: [Python-Dev] GDB macros in Misc/gdbinit are broken

[Skip Montanaro]

I haven't tried using Python support in gdb in a long time. I don't
know what python-gdb.py is. How is that different than
Tools/gdb/libpython.py? Is it distributed with Python? (I'm kind of
blind at work without a source distribution I can grep through, and
didn't see it in Tools/gdb or Misc.)

I'm a simple man. I need the kind of stuff Misc/gdbinit provides
(smart stack traces and variable printing), but I most frequently use
the pyo user-defined command to print the contents of PyObject
pointers. When they are working () pystack and pyframe are also
useful.

Skip

On Tue, Feb 28, 2017 at 2:33 AM, Victor Stinner
 wrote:
> 2017-02-28 3:00 GMT+01:00 Skip Montanaro :
>> Alexander> I find them useful.  I've never had success with python-gdb.py.
>
> Alexander, Skip: Oh, which kind of issues do you have with
> python-gdb.py? It doesn't work? You are unable to dump some data?
>
> python-gdb.py doesn't run code to analyze memory, it prevents crashes
> when analysing a running process, and it works on core dumps. It has
> many features, it works well. I never had major issues with it, and I
> enhance it regulary. Recently, I added support for "method-wrapper".
>
> Victor
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Re: [Python-Dev] GDB macros in Misc/gdbinit are broken

[Victor Stinner]

2017-02-28 3:00 GMT+01:00 Skip Montanaro :
> Alexander> I find them useful.  I've never had success with python-gdb.py.

Alexander, Skip: Oh, which kind of issues do you have with
python-gdb.py? It doesn't work? You are unable to dump some data?

python-gdb.py doesn't run code to analyze memory, it prevents crashes
when analysing a running process, and it works on core dumps. It has
many features, it works well. I never had major issues with it, and I
enhance it regulary. Recently, I added support for "method-wrapper".

Victor
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Re: [Python-Dev] GDB macros in Misc/gdbinit are broken

[Skip Montanaro]

Alexander> I find them useful.  I've never had success with python-gdb.py.

As the original author, and occasional user (just in the last week or
two) I still find the current crude hack useful. I tried to get the
Python support in GDB working a couple years ago, but gave up in
frustration. I hope it's better now.

I added a comment to Alexander's bug report.

Skip
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Re: [Python-Dev] GDB macros in Misc/gdbinit are broken

[Alexander Belopolsky]

On Mon, Feb 27, 2017 at 7:34 PM, Victor Stinner 
wrote:

> Does someone still need gdbinit macros for gdb without python binding?
>

I find them useful.  I've never had success with python-gdb.py.
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Re: [Python-Dev] GDB macros in Misc/gdbinit are broken

[Victor Stinner]

I suggest to remove them. I didn't use them since python-gdb.py exist.

Does someone still need gdbinit macros for gdb without python binding?

Victor

Le 28 févr. 2017 1:27 AM, "Alexander Belopolsky" <
alexander.belopol...@gmail.com> a écrit :

> I have opened an issue on b.p.o., [1] but I wonder whether Misc/gdbinit is
> still supported in 3.6.
>
> [1]: http://bugs.python.org/issue29673
>
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[Python-Dev] GDB macros in Misc/gdbinit are broken

[Alexander Belopolsky]

I have opened an issue on b.p.o., [1] but I wonder whether Misc/gdbinit is
still supported in 3.6.

[1]: http://bugs.python.org/issue29673
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