Issue 26799: gdb support fails with "Invalid cast." (original) (raw)
Created on 2016-04-18 21:10 by tilsche, last changed 2022-04-11 14:58 by admin. This issue is now closed.
Messages (10)
Author: Thomas (tilsche) *
Date: 2016-04-18 21:10
Trying to use any kind of python gdb integration results in the following error:
(gdb) py-bt Traceback (most recent call first): Python Exception <class 'gdb.error'> Invalid cast.: Error occurred in Python command: Invalid cast.
I have tracked it down to the type... globals, and I am able to fix it with the following commands:
(gdb) pi
Look up the gdb.Type for some standard types:
... _type_char_ptr = gdb.lookup_type('char').pointer() # char* _type_unsigned_char_ptr = gdb.lookup_type('unsigned char').pointer() # unsigned char* _type_void_ptr = gdb.lookup_type('void').pointer() # void* _type_unsigned_short_ptr = gdb.lookup_type('unsigned short').pointer() _type_unsigned_int_ptr = gdb.lookup_type('unsigned int').pointer()
After this, it works correctly. I was able to workaround it by making a fix_globals that resets the globals on each gdb.Command. I do not understand why the originally initialized types are not working properly. It feels like gdb-inception trying to debug python within a gdb that debugs cpython while executing python code.
I have tried this using hg/default cpython (--with-pydebug --without-pymalloc --with-valgrind --enable-shared)
- System install of gdb 7.11, linked against system libpython 3.5.1.
- Custom install of gdb 7.11.50.20160411-git, the debug cpython I am trying to debug
Author: STINNER Victor (vstinner) * 
Date: 2016-04-19 15:05
I tried Python 3.6 with "./configure --with-pydebug --without-pymalloc --with-valgrind". The py-bt commands works as expect in gdb.
Can you try to modify python-gdb.py to get a traceback of your Invalid cast error?
I am able to fix it with the following commands: (...) _type_char_ptr = gdb.lookup_type('char').pointer() # char*
These defines are already in Tools/gdb/libpython.py (python-gdb.py is just a copy). I don't understand why you have to redeclare these variables again.
Author: Thomas (tilsche) *
Date: 2016-04-19 16:42
I have done a bit more digging, turns out it is actually no problem at all to debug python in gdb with gdb with python support (at least using a fixed python-gdb-py).
Turns out the type->length of the the globally initialized ptr types is wrong: It is 4 instead of 8, causing the cast to fail. I suspect the initialization is done before the executable is loaded and gdb is using some default. To verify, I have put two prints in the global initialization and in a command invocation:
GOBAL INITIALIZATION: gdb.lookup_type('char').pointer().sizeof == 4 COMMAND INVOKE: gdb.lookup_type('char').pointer().sizeof == 8
I guess to be fully portable those types need to be looked up at least whenever gdb changes it's binary, but I have no idea if there is a hook for that. But it seems reasonable to just replace those globals with on-demand lookup functions or properties.
If you are interested in the actual python/c stack traces for the error:
Thread 1 "gdb" hit Breakpoint 1, value_cast (type=type@entry=0x2ef91e0, arg2=arg2@entry=0x32b13f0) at ../../gdb/valops.c:571 571 error (_("Invalid cast.")); (gdb) py-bt Traceback (most recent call first): <built-in method cast of gdb.Value object at remote 0x3260de0> File "[...]/python-gdb.py", line 1151, in proxyval field_str = field_str.cast(_type_unsigned_char_ptr) File "[...]/python-gdb.py", line 945, in print_traceback % (self.co_filename.proxyval(visited), File "[...]/python-gdb.py", line 1578, in print_traceback pyop.print_traceback() File "[...]/python-gdb.py", line 1761, in invoke frame.print_traceback() (gdb) bt #0 value_cast (type=type@entry=0x2ef91e0, arg2=arg2@entry=0x32b13f0) at ../../gdb/valops.c:571 #1 0x000000000052261f in valpy_do_cast (self=<gdb.Value at remote 0x3260de0>, args=, op=UNOP_CAST) at ../../gdb/python/py-value.c:525 #2 0x00007fc7ce2141de in PyCFunction_Call (func=func@entry=<built-in method cast of gdb.Value object at remote 0x3260de0>, args=args@entry=(<gdb.Type at remote 0x2ed0fb0>,), kwds=kwds@entry=0x0) at ../../Python-3.5.1/Objects/methodobject.c:109 #3 0x00007fc7ce2c8887 in call_function (pp_stack=pp_stack@entry=0x7ffffc81cec8, oparg=oparg@entry=1) at ../../Python-3.5.1/Python/ceval.c:4655 #4 0x00007fc7ce2c57ac in PyEval_EvalFrameEx (
Author: STINNER Victor (vstinner) *
Date: 2016-04-19 19:19
Turns out the type->length of the the globally initialized ptr types is wrong: It is 4 instead of 8, causing the cast to fail. I suspect the initialization is done before the executable is loaded and gdb is using some default.
Hum. I see two options:
- try late initialization: initialize types at the first usage
- remove variables: always lookup types at runtime
Would you like to try to implement these options?
Author: Thomas (tilsche) *
Date: 2016-04-20 15:50
The second option seems like the safest choice, attached is a patch that addresses just that.
Author: Roundup Robot (python-dev) 
Date: 2016-04-20 16:15
New changeset e1c6f8895fd8 by Victor Stinner in branch '3.5': python-gdb.py: get C types at runtime https://hg.python.org/cpython/rev/e1c6f8895fd8
New changeset 6425728d8dc6 by Victor Stinner in branch 'default': Merge 3.5: Issue #26799 https://hg.python.org/cpython/rev/6425728d8dc6
Author: Roundup Robot (python-dev)
Date: 2016-04-20 16:24
New changeset e4561aad29e6 by Victor Stinner in branch '2.7': Fix python-gdb.py: get C types on demand https://hg.python.org/cpython/rev/e4561aad29e6
New changeset 952c89a10be6 by Victor Stinner in branch '3.5': Issue #26799: Fix typo in Misc/NEWS https://hg.python.org/cpython/rev/952c89a10be6
Author: STINNER Victor (vstinner) *
Date: 2016-04-20 16:28
The second option seems like the safest choice, attached is a patch that addresses just that.
Thanks, I applied your patch (your a bugfix, you forgot one "return" ;-)).
I also inlined _type_void_ptr() into _sizeof_void_p(), since it was only called there.
Thanks for your bug report and your patch.
Can you please sign the PSF Contributor Agreement? https://www.python.org/psf/contrib/contrib-form/
It's required when you contribute to Python. In short, it warns you that Python is free software ;-)
Author: Thomas (tilsche) *
Date: 2016-04-21 07:19
Thank you for the quick integration and fixing the return. I have signed the electronic form yesterday.
Author: STINNER Victor (vstinner) *
Date: 2016-04-21 09:00
Great!
History
Date
User
Action
Args
2022-04-11 14:58:29
admin
set
github: 70986
2016-04-21 09:00:28
vstinner
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messages: +
2016-04-21 07:19:56
tilsche
set
messages: +
2016-04-20 16:28:59
vstinner
set
status: open -> closed
resolution: fixed
messages: +
2016-04-20 16:24:10
python-dev
set
messages: +
2016-04-20 16:15:15
python-dev
set
nosy: + python-dev
messages: +
2016-04-20 15:50:47
tilsche
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files: + gdb-python-invalid-cast.patch
keywords: + patch
messages: +
2016-04-19 19:19:12
vstinner
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messages: +
2016-04-19 16:42:28
tilsche
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messages: +
2016-04-19 15:05:50
vstinner
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messages: +
2016-04-19 06:25:39
SilentGhost
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nosy: + vstinner
2016-04-18 21:10:40
tilsche
create