[lldb-dev] Segfault using the lldb python module with a non-Xcode python binary

[António Afonso via lldb-dev]

I'm building lldb with python3 support by using the framework that is
shipped with the latest versions of Xcode.

I'm able to build and run lldb just fine but if I try to use the lldb
python module on a python binary that is not the one from Xcode it
segfaults when creating the module. I then tried with the stock lldb from
Xcode and found the exact same issue ☹. I don’t think this was a problem
before?



I'm not sure why this happens and I wasn't able to debug the issue. I've
already tried with a binary that has the exact same version of python but
still the same problem:



Works fine with the Xcode binary:

$ PYTHONPATH=`lldb -P`
/Applications/Xcode.app/Contents/Developer/Library/Frameworks/Python3.framework/Versions/3.7/bin/python3

Python 3.7.3 (default, Sep 18 2019, 14:29:06)

[Clang 11.0.0 (clang-1100.0.33.8)] on darwin

Type "help", "copyright", "credits" or "license" for more information.

>>> import lldb

>>>



Fails with any other:

$ PYTHONPATH=`lldb -P` /Users/aadsm/.pyenv/versions/3.7.3/bin/python

Python 3.7.3 (default, Nov 12 2019, 23:19:54)

[Clang 11.0.0 (clang-1100.0.33.8)] on darwin

Type "help", "copyright", "credits" or "license" for more information.

>>> import lldb

Segmentation fault: 11



I attached lldb to see where it was failing and it's right after liblldb is
loaded and python is trying to create the module itself, in the
PyModule_Create2 function (
https://github.com/python/cpython/blob/master/Objects/moduleobject.c#L173-L179
).

The disassembly shows:



Process 89097 stopped

* thread #1, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS
(code=1, address=0x10)

    frame #0: 0x000000010f4cae5b Python3`PyModule_Create2 + 27

Python3`PyModule_Create2:

->  0x10f4cae5b <+27>: movq   0x10(%rax), %rdi

    0x10f4cae5f <+31>: callq  0x10f5823b0               ;
_PyImport_IsInitialized

    0x10f4cae64 <+36>: testl  %eax, %eax

    0x10f4cae66 <+38>: je     0x10f4cae77               ; <+55>

Target 0: (Python) stopped.

(lldb) dis

Python3`PyModule_Create2:

    0x10f4cae40 <+0>:  pushq  %rbp

    0x10f4cae41 <+1>:  movq   %rsp, %rbp

    0x10f4cae44 <+4>:  pushq  %r14

    0x10f4cae46 <+6>:  pushq  %rbx

    0x10f4cae47 <+7>:  movl   %esi, %r14d

    0x10f4cae4a <+10>: movq   %rdi, %rbx

    0x10f4cae4d <+13>: leaq   0x2226ac(%rip), %rax      ; _PyRuntime

    0x10f4cae54 <+20>: movq   0x5a0(%rax), %rax

->  0x10f4cae5b <+27>: movq   0x10(%rax), %rdi

    0x10f4cae5f <+31>: callq  0x10f5823b0               ;
_PyImport_IsInitialized

    0x10f4cae64 <+36>: testl  %eax, %eax

    0x10f4cae66 <+38>: je     0x10f4cae77               ; <+55>

    0x10f4cae68 <+40>: movq   %rbx, %rdi

    0x10f4cae6b <+43>: movl   %r14d, %esi

    0x10f4cae6e <+46>: popq   %rbx

    0x10f4cae6f <+47>: popq   %r14

    0x10f4cae71 <+49>: popq   %rbp

    0x10f4cae72 <+50>: jmp    0x10f4cae90               ;
_PyModule_CreateInitialized

    0x10f4cae77 <+55>: leaq   0x14f111(%rip), %rdi      ; "Python import
machinery not initialized"

    0x10f4cae7e <+62>: callq  0x10f593d40               ; Py_FatalError

    0x10f4cae83 <+67>: nopw   %cs:(%rax,%rax)

    0x10f4cae8d <+77>: nopl   (%rax)



Not really sure how to debug this besides trying to build my own version of
python and see if I can repro (I don't have this issue on linux). I’ve also
checked the sys.abiflags and both binaries have the same ones.

Has anyone experienced this before or has any pointers to debug it?

- Afonso
-- 
Best regards,
António Afonso
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