Malformed PyImport_Inittab after re-initialization

[kryheb]

BPO	44441
Nosy	@vstinner, @kryheb
PRs	bpo-44441: Set PyImport_Inittab to default on _PyImport_Fini2 #26767 bpo-44441: _PyImport_Fini2() resets PyImport_Inittab #26874 [3.10] bpo-44441: _PyImport_Fini2() resets PyImport_Inittab (GH-26874) #26877 [3.9] bpo-44441: _PyImport_Fini2() resets PyImport_Inittab (GH-26874) #26878
Files	inittab-bug.c: Source code to reproduce an issue inittab-bug_no-threads.c: Source code to reproduce an issue

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2021-06-23.15:48:43.362>
created_at = <Date 2021-06-17.10:14:12.876>
labels = ['expert-C-API', 'library', '3.9', '3.10', '3.11']
title = 'Malformed PyImport_Inittab after re-initialization'
updated_at = <Date 2021-06-24.08:48:55.234>
user = 'https://github.com/kryheb'

bugs.python.org fields:

activity = <Date 2021-06-24.08:48:55.234>
actor = 'kryheb'
assignee = 'none'
closed = True
closed_date = <Date 2021-06-23.15:48:43.362>
closer = 'vstinner'
components = ['Library (Lib)', 'C API']
creation = <Date 2021-06-17.10:14:12.876>
creator = 'kryheb'
dependencies = []
files = ['50115', '50125']
hgrepos = []
issue_num = 44441
keywords = ['patch']
message_count = 10.0
messages = ['395985', '396300', '396385', '396404', '396405', '396414', '396427', '396428', '396429', '396462']
nosy_count = 2.0
nosy_names = ['vstinner', 'kryheb']
pr_nums = ['26767', '26874', '26877', '26878']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue44441'
versions = ['Python 3.9', 'Python 3.10', 'Python 3.11']

[kryheb]

Hi all,

I observed misbehavior trying to embed the Python interpreter into a C app.
It seems that after re-initialization, PyImport_Inittab is malformed and points to the memory freed _PyImport_Fini2.

Steps to reproduce:

1. Append embedded module
2. Initialize Python from config with run_filename
3. Run main with an infinite loop
4. Interrupt script execution with async call
5. Finalize Python
6. Repeat all above

Observed behavior:
The script is executed at first iteration, but re-initialization fails with an error:
"
Traceback (most recent call last):
  File "<frozen importlib._bootstrap>", line 1187, in _install_external_importers
ModuleNotFoundError: No module named 'posix'

Error: external importer setup failed
"

Head of modules list at fist run:
------ Modules: ------
#0 'posix'
#1 'errno'
#2 'pwd'
----------------------
and after re-initialization:
------ Modules: ------
#0 'P ''
#1 'errno'
#2 'pwd'
----------------------

An issue discovered on:
Fedora 33
gcc (GCC) 10.3.1 20210422 (Red Hat 10.3.1-1)
python3-devel.x86_64 3.9.5-2.fc33

Issue still exists on the latest main and on the rc 3.10
Source code to reproduce an issue in attachment.

Best regards,
Krystian Heberlein

[vstinner]

inittab-bug.c uses the Python C API in 3 threads:

• Thread A (init_proc) calls Py_InitializeFromConfig()
• Thread B (run_proc) calls Py_RunMain()
• The main thread (main) calls Py_FinalizeEx()

Problem: the thread B (run_proc) doesn't hold the GIL and so I get a fatal error with Python built in debug mode:
------------

$ gcc inittab-bug.c -ggdb -pthread -lpython3.11d -L. -I. -I Include/
$ PYTHONPATH=$PWD/Lib LD_LIBRARY_PATH=$PWD ./a.out
------ Modules: 

---

#0 'posix'
#1 'errno'
#2 'pwd'
----------------------
Could not find platform independent libraries <prefix>
Could not find platform dependent libraries <exec_prefix>
Consider setting $PYTHONHOME to <prefix>[:<exec_prefix>]
Run Python Script
Fatal Python error: _PyMem_DebugMalloc: Python memory allocator called without holding the GIL
Python runtime state: initialized

Thread 0x00007f61ce5c1640 (most recent call first):
<no Python frame>
Abandon (core dumped)
------------

If I remove the thread B (comment the comment), I get a similar error in the main thread which calls Py_FinalizeEx().

Please fix your usage of the GIL. For example, you can try to use:

• https://docs.python.org/dev/c-api/init.html#c.PyGILState_Ensure
• https://docs.python.org/dev/c-api/init.html#c.PyGILState_Release

Usually, Python initialization and Python finalization is done in the same thread, but another thread can use the Python C API if it acquires the GIL using PyGILState_Ensure().

[kryheb]

Hi vstinner,

I apologize for the inaccurate code sample.
Please find a simplified example without threads inittab-bug_no-threads.c in attachments. Please notice that main.py has changed, there is no infinite loop anymore.

Steps to reproduce:

1. Append embedded module
2. Initialize Python from config with run_filename
3. Run main
4. Finalize
5. Repeat all above

Observed behavior:
The script is executed at the first iteration, but re-initialization fails with a segmentation fault:

gdb) r
Starting program: /home/kheb/proj/tmp/pyc/a.out 
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib64/libthread_db.so.1".
Run Python Script
>>> This is the python script
Python Script completed (0)

Program received signal SIGSEGV, Segmentation fault.
__strlen_avx2 () at ../sysdeps/x86_64/multiarch/strlen-avx2.S:65
65 VPCMPEQ (%rdi), %ymm0, %ymm1
Missing separate debuginfos, use: dnf debuginfo-install libxcrypt-4.4.20-2.fc33.x86_64
(gdb) bt
#0 __strlen_avx2 () at ../sysdeps/x86_64/multiarch/strlen-avx2.S:65
#1 0x00007ffff7d039f0 in PyUnicode_FromString (
u=0xdddddddddddddddd <error: Cannot access memory at address 0xdddddddddddddddd>)
at Objects/unicodeobject.c:2309
#2 0x00007ffff7dbf049 in list_builtin_module_names () at ./Python/sysmodule.c:2056
#3 0x00007ffff7dc1777 in _PySys_InitCore (tstate=tstate@entry=0x422ac0, sysdict=sysdict@entry=0x7fffea221070)
at ./Python/sysmodule.c:2813
#4 0x00007ffff7dc36ef in _PySys_Create (tstate=tstate@entry=0x422ac0, sysmod_p=sysmod_p@entry=0x7fffffffd9d8)
at ./Python/sysmodule.c:3087
#5 0x00007ffff7da9b32 in pycore_interp_init (tstate=0x422ac0) at Python/pylifecycle.c:824
#6 0x00007ffff7da9c8e in pyinit_config (runtime=runtime@entry=0x7ffff7fbe820 <_PyRuntime>,
tstate_p=tstate_p@entry=0x7fffffffdca8, config=config@entry=0x7fffffffdac0) at Python/pylifecycle.c:866
#7 0x00007ffff7daba3e in pyinit_core (runtime=runtime@entry=0x7ffff7fbe820 <_PyRuntime>,
src_config=src_config@entry=0x7fffffffdd40, tstate_p=tstate_p@entry=0x7fffffffdca8) at Python/pylifecycle.c:1029
#8 0x00007ffff7dabb18 in Py_InitializeFromConfig (config=0x7fffffffdd40) at Python/pylifecycle.c:1214
#9 0x00000000004012d3 in main (argc=1, argv=0x7fffffffdfc8) at inittab-bug_no-threads.c:45

I hope this example helps.

Best regards,
Krystian

[vstinner]

Thanks for inittab-bug_no-threads.c reproducer. I managed to reproduce the issue and I wrote PR 26767 to fix with a regression test.

[vstinner]

Oh, I didn't notice your PR 26767 fix! Thanks. I wrote PR 26874 which includes the same fix, but adds also a regression test.

[vstinner]

New changeset 489699c by Victor Stinner in branch 'main':
bpo-44441: _PyImport_Fini2() resets PyImport_Inittab (GH-26874)
489699c

[vstinner]

New changeset 5ed7827 by Victor Stinner in branch '3.9':
bpo-44441: _PyImport_Fini2() resets PyImport_Inittab (GH-26874) (GH-26878)
5ed7827

[vstinner]

New changeset ece3841 by Victor Stinner in branch '3.10':
bpo-44441: _PyImport_Fini2() resets PyImport_Inittab (GH-26874) (GH-26877)
ece3841

[vstinner]

Thanks kryheb for your bug report, reproducer and your fix!

The bug should now be fixed in 3.9, 3.10 and main branches.

[kryheb]

Thanks vstinner for your help and for taking care of this case!