gh-78724: raise RuntimeError's when calling methods on non-ready Struct()'s by skirpichev · Pull Request #143643 · python/cpython

skirpichev · 2026-01-10T07:08:55Z

Issue: _struct.Struct: calling functions without calling __init__ results in SystemError #78724

… by dunder methods

Now modification of the Struct() while packing trigger a RuntimeError

…y Struct()'s Calling the ``Struct.__new__()`` dunder without required argument now is deprecated. Calling the ``Struct.__init__()`` dunder method on initialized object now also is deprecated.

serhiy-storchaka

This issue is relatively independed from the concurrent packing issue, so I suggest fir it separately. We should not add deprecation in maintained versions, so I suggest to only leave checks that Struct was initialized, and add the deprecation in a separate PR.

prepare_s() should be rewritten so it only sets s_codes and other fields when it is successful.

serhiy-storchaka · 2026-01-10T09:01:50Z

 {
 PyObject *self;

+ if (PyTuple_GET_SIZE(args) != 1


I do not see how can this be useful.

This is to trigger deprecation warning in a following scenario:
>>> import struct >>> classMyStruct(struct.Struct): ... def__init__(self): ... super().__init__('>h') ... >>> my_struct = MyStruct() <python-input-2>:1: DeprecationWarning: Struct().__new__() has one required argument
PS: deprecation part moved to #143659.

skirpichev · 2026-01-10T10:52:29Z

prepare_s() should be rewritten so it only sets s_codes and other fields when it is successful.

I think it's already the case. We can move a bit s_size/s_len assignments, but as far as we interested only in s_codes - I'm not sure if it's worth. There should be no failures after codes = PyMem_Malloc(...).

serhiy-storchaka · 2026-01-10T13:11:28Z

Well, it does not matter. It would reduce the chance of race condition, but nobody should run __init__() and pack() concurrently, there are no reasons to do this, and we should not support this.

serhiy-storchaka · 2026-01-10T15:55:10Z

- self.assertRaises(RuntimeError, meth, spam)
+ self.assertRaises(RuntimeError, S.iter_unpack, 1)
+ self.assertRaises(RuntimeError, S.pack, 1)
+ self.assertRaises(RuntimeError, S.pack_into, 1)


S.pack_into(buffer, offset, v1, v2, ...)

BTW, I think that argument processing in the struct's functions/methods should be refactored and transformed to the AC. E.g. S.pack_info could check required arguments before validation of the soself struct.
Though, this belongs to a separate issue.

AC cleanup goes to #143673

skirpichev · 2026-01-10T22:53:14Z

Well, it does not matter. It would reduce the chance of race condition, but nobody should run __init__() and pack() concurrently, there are no reasons to do this, and we should not support this.

I guess this belongs to the #143382 pr thread.

serhiy-storchaka

LGTM. 👍

miss-islington-app · 2026-01-11T15:52:05Z

Thanks @skirpichev for the PR, and @serhiy-storchaka for merging it 🌮🎉.. I'm working now to backport this PR to: 3.13, 3.14.
🐍🍒⛏🤖

…y Struct()'s (pythonGH-143643) (cherry picked from commit 515ae40) Co-authored-by: Sergey B Kirpichev <skirpichev@gmail.com>

miss-islington-app · 2026-01-11T15:52:11Z

Sorry, @skirpichev and @serhiy-storchaka, I could not cleanly backport this to 3.13 due to a conflict.
Please backport using cherry_picker on command line.

cherry_picker 515ae4078dffa0b74e5e5431462c2f4fe4563ffa 3.13

bedevere-app · 2026-01-11T15:52:16Z

GH-143695 is a backport of this pull request to the 3.14 branch.

bedevere-app · 2026-01-11T23:36:31Z

GH-143714 is a backport of this pull request to the 3.13 branch.

…dy Struct()'s (GH-143643) (GH-143695) (cherry picked from commit 515ae40) Co-authored-by: Sergey B Kirpichev <skirpichev@gmail.com>

…dy Struct()'s (GH-143643) (GH-143714) (cherry picked from commit 515ae40)

…y Struct()'s (pythonGH-143643)

skirpichev added 5 commits January 3, 2026 10:36

pythongh-143379: fix UAF in struct.Struct.pack() when object modified…
a1c2603
… by dunder methods

Merge branch 'master' into fix-uaf-s_pack_internal/143379
716e28c

address review: implement a different solution with a mutex flag
e611953
Now modification of the Struct() while packing trigger a RuntimeError

+ clean the mess
ab62167

pythongh-78724: raise RuntimeError's when calling methods on non-read…
3b18427
…y Struct()'s Calling the ``Struct.__new__()`` dunder without required argument now is deprecated. Calling the ``Struct.__init__()`` dunder method on initialized object now also is deprecated.

bedevere-appBot mentioned this pull request Jan 10, 2026
_struct.Struct: calling functions without calling __init__ results in SystemError #78724
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skirpichev mentioned this pull request Jan 10, 2026
gh-143379: fix UAF in struct.Struct.pack() when object modified by dunder methods #143382
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skirpichev marked this pull request as ready for review January 10, 2026 07:55

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skirpichev requested a review from serhiy-storchaka January 10, 2026 07:56

serhiy-storchaka reviewed Jan 10, 2026
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serhiy-storchaka added needs backport to 3.13 bugs and security fixes needs backport to 3.14 bugs and security fixes labels Jan 10, 2026

skirpichev marked this pull request as draft January 10, 2026 10:14

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skirpichev added 3 commits January 10, 2026 13:15

Merge branch 'master' into deprecate-struct-init/78724
54dbea8

revert stuff frompython#143382
b6c8bed

revert deprecations (TBD: a separate pr)
f542173

address review: redo patch to use s_codes
69a5eec

skirpichev marked this pull request as ready for review January 10, 2026 11:45

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skirpichev requested a review from serhiy-storchaka January 10, 2026 11:46

Merge branch 'main' into deprecate-struct-init/78724
0e6af95

serhiy-storchaka reviewed Jan 10, 2026
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Comment threadLib/test/test_struct.py Outdated
Comment threadModules/_struct.c Outdated

address review: -asserts + redo test
8ba6b33

serhiy-storchaka reviewed Jan 10, 2026
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address review: adjust tests
ea0f8e2

skirpichev requested a review from serhiy-storchaka January 11, 2026 11:09

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Comment threadModules/_struct.c
Comment threadModules/_struct.c Outdated

skirpichev added 2 commits January 11, 2026 18:21

Merge branch 'master' into deprecate-struct-init/78724
c71be45

address review: revert unrelated change
b61bddb

serhiy-storchaka approved these changes Jan 11, 2026
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serhiy-storchaka enabled auto-merge (squash) January 11, 2026 15:47

serhiy-storchaka merged commit 515ae40 into python:mainJan 11, 2026
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skirpichev deleted the deprecate-struct-init/78724 branch January 11, 2026 18:04

skirpichev self-assigned this Jan 11, 2026

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serhiy-storchaka pushed a commit that referenced this pull request Jan 12, 2026
[3.14]gh-78724: Raise RuntimeError's when calling methods on non-rea…
8f85adc
…dy Struct()'s (GH-143643) (GH-143695) (cherry picked from commit 515ae40) Co-authored-by: Sergey B Kirpichev <skirpichev@gmail.com>

serhiy-storchaka pushed a commit that referenced this pull request Jan 12, 2026
[3.13]gh-78724: Raise RuntimeError's when calling methods on non-rea…
8fd4435
…dy Struct()'s (GH-143643) (GH-143714) (cherry picked from commit 515ae40)

reidenong pushed a commit to reidenong/cpython that referenced this pull request Jan 12, 2026
pythongh-78724: Raise RuntimeError's when calling methods on non-read…
e3ef2fc
…y Struct()'s (pythonGH-143643)

thunder-coding pushed a commit to thunder-coding/cpython that referenced this pull request Feb 15, 2026
pythongh-78724: Raise RuntimeError's when calling methods on non-read…
3ab851b
…y Struct()'s (pythonGH-143643)

serhiy-storchaka removed their assignment Jul 15, 2026

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