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GeezerVison is showing github.com — GH-139653: Only raise an exception (or fatal error) when the stack pointer is about to overflow the stack. by markshannon · Pull Request #141711 · python/cpython

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markshannon commented Nov 18, 2025 •

edited by bedevere-app Bot

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Only raises if the stack pointer is both below the limit and above the stack base. This prevents false positives for user-space threads, as the stack pointer will be outside those bounds if the stack has been swapped.

Issue: Python 3.14 stack overflow detection is incompatible with C++ Boost make_fcontext() coroutines #139653

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          Only raise an exception (or fatal error) when the stack pointer is ab…

41e9404

…out to overflow the stack.

Only raises if the stack pointer is both below the limit *and* above the stack base.
This prevents false positives for user-space threads, as the stack pointer will be outside those bounds
if the stack has been swapped.

markshannon requested a review from AA-Turner as a code owner

November 18, 2025 11:44

bedevere-app Bot added the awaiting core review label

Nov 18, 2025

bedevere-app Bot mentioned this pull request

Nov 18, 2025

Python 3.14 stack overflow detection is incompatible with C++ Boost make_fcontext() coroutines #139653

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markshannon added 3.14

bugs and security fixes

needs backport to 3.14 bugs and security fixes and removed 3.14 bugs and security fixes labels

Nov 18, 2025

encukou reviewed

Nov 18, 2025

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Python/ceval.c

		# endif
		uintptr_t here_addr = _Py_get_machine_stack_pointer();
		uintptr_t top_addr = _Py_SIZE_ROUND_UP(here_addr, 4096);
		// Add some space for caller function then round to minimum page size

encukou Nov 18, 2025

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Thanks for the comment.
Both “space for caller function” and “minimum page size” are guesses; what happens if they're wrong?

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markshannon Nov 18, 2025

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Nothing bad. We only use the upper limit (for stack growing down) for reporting stack use in case of an overflow.
If we are wrong the report value will be off by a few kb, but it is already an estimate. We are assuming that there isn't much stack above the call to create the thread state.

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encukou Nov 18, 2025

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Would it make sense to avoid calculation, and use here_addr as is? With the Py_C_STACK_SIZE guesses, 4k indeed doesn't matter.

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markshannon Nov 18, 2025

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It's not a perfect guess, but I don't see any reason to make it worse.

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encukou Nov 18, 2025

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What I see is unnecessary complexity: adding 60 bytes of faux precision to a guess that's in the order of megabytes is rather confusing. These look like more numbers to tweak if you get spurious overflow check failures

If this needs to go in, could you add a comment that this guess can be wildly wrong?

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markshannon Nov 18, 2025

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Will do.

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Python/ceval.c Resolved

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encukou added the 🔨 test-with-buildbots Test PR w/ buildbots; report in status section label

Nov 18, 2025

bedevere-bot commented Nov 18, 2025

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🤖 New build scheduled with the buildbot fleet by @encukou for commit 41e9404 🤖

Results will be shown at:

https://buildbot.python.org/all/#/grid?branch=refs%2Fpull%2F141711%2Fmerge

If you want to schedule another build, you need to add the 🔨 test-with-buildbots label again.

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bedevere-bot removed the 🔨 test-with-buildbots Test PR w/ buildbots; report in status section label

Nov 18, 2025

efimov-mikhail reviewed

Nov 18, 2025

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efimov-mikhail left a comment •

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I've noticed some typos.

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Include/internal/pycore_ceval.h Resolved

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Include/internal/pycore_ceval.h Resolved

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InternalDocs/stack_protection.md Resolved

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InternalDocs/stack_protection.md Resolved

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encukou reviewed

Nov 18, 2025

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Python/ceval.c Outdated


		/* The function _Py_EnterRecursiveCallTstate() only calls _Py_CheckRecursiveCall()
		if the recursion_depth reaches recursion_limit. */
		if the stack poineter is between the stack base and c_stack_hard_limit. */

encukou Nov 18, 2025

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Suggested change

	if the stack poineter is between the stack base and c_stack_hard_limit. */
	if the stack pointer is between the stack base and c_stack_hard_limit. */

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Python/ceval.c

		# endif
		uintptr_t here_addr = _Py_get_machine_stack_pointer();
		uintptr_t top_addr = _Py_SIZE_ROUND_UP(here_addr, 4096);
		// Add some space for caller function then round to minimum page size

encukou Nov 18, 2025

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If this needs to go in, could you add a comment that this guess can be wildly wrong?

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          Fix typo and clarify

1e7ae53

encukou approved these changes

Nov 18, 2025

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Thank you!

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bedevere-app Bot added awaiting merge and removed awaiting core review labels

Nov 18, 2025

efimov-mikhail approved these changes

Nov 18, 2025

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chris-eibl reviewed

Nov 18, 2025

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Include/internal/pycore_ceval.h Resolved

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InternalDocs/stack_protection.md Resolved

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          A couple more typos

1de4a56

markshannon merged commit c25a070 into python:main

Nov 19, 2025

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miss-islington-app Bot commented Nov 19, 2025

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Thanks @markshannon for the PR 🌮🎉.. I'm working now to backport this PR to: 3.14.
🐍🍒⛏🤖

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bedevere-app Bot removed the awaiting merge label

Nov 19, 2025

markshannon deleted the conservative-stack-check branch

November 19, 2025 10:16

miss-islington-app Bot commented Nov 19, 2025

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Sorry, @markshannon, I could not cleanly backport this to 3.14 due to a conflict.
Please backport using cherry_picker on command line.

cherry_picker c25a070759952b13f97ecc37ca2991c2669aee47 3.14

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miss-islington-app Bot assigned markshannon

Nov 19, 2025

markshannon added 2 commits

November 20, 2025 04:19


          Update comment

7dcdf04


          Fix typos

b28b230

hroncok pushed a commit to hroncok/cpython that referenced this pull request

Nov 24, 2025


          pythonGH-139653: Only raise an exception (or fatal error) when the st…

30cdcac

…ack pointer is about to overflow the stack. (pythonGH-141711)

Only raises if the stack pointer is both below the limit *and* above the stack base.
This prevents false positives for user-space threads, as the stack pointer will be outside those bounds
if the stack has been swapped.

(cherry picked from commit c25a070)

hroncok mentioned this pull request

Nov 24, 2025

[3.14] GH-139653: Only raise an exception (or fatal error) when the stack pointer is about to overflow the stack. (GH-141711) #141892

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bedevere-app Bot commented Nov 24, 2025

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GH-141892 is a backport of this pull request to the 3.14 branch.

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bedevere-app Bot removed the needs backport to 3.14 bugs and security fixes label

Nov 24, 2025

vstinner mentioned this pull request

Nov 24, 2025

[3.14] gh-139653: Add PyUnstable_ThreadState_SetStackProtection() (GH-139668) #141661

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encukou pushed a commit to encukou/cpython that referenced this pull request

Nov 25, 2025


          [3.14] pythonGH-139653: Only raise an exception (or fatal error) when…

6adfd35

… the stack pointer is about to overflow the stack. (pythonGH-141711)

Only raises if the stack pointer is both below the limit *and* above the stack base.
This prevents false positives for user-space threads, as the stack pointer will be outside those bounds
if the stack has been swapped.

Cherry-picked from commit c25a070

encukou pushed a commit to encukou/cpython that referenced this pull request

Nov 25, 2025


          [3.14] pythonGH-139653: Only raise an exception (or fatal error) when…

0728b62

… the stack pointer is about to overflow the stack. (pythonGH-141711)

Only raises if the stack pointer is both below the limit *and* above the stack base.
This prevents false positives for user-space threads, as the stack pointer will be outside those bounds
if the stack has been swapped.

Cherry-picked from commit c25a070

bedevere-app Bot commented Nov 25, 2025

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GH-141944 is a backport of this pull request to the 3.14 branch.

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encukou added a commit that referenced this pull request

Nov 26, 2025


          [3.14] GH-139653: Only raise an exception (or fatal error) when the s…

69021e9

…tack pointer is about to overflow the stack. (GH-141711) (GH-141944)

Only raises if the stack pointer is both below the limit *and* above the stack base.
This prevents false positives for user-space threads, as the stack pointer will be outside those bounds
if the stack has been swapped.

Cherry-picked from commit c25a070

Co-authored-by: Mark Shannon <[email protected]>

StanFromIreland pushed a commit to StanFromIreland/cpython that referenced this pull request

Dec 6, 2025


          pythonGH-139653: Only raise an exception (or fatal error) when the st…

341306c

…ack pointer is about to overflow the stack. (pythonGH-141711)

Only raises if the stack pointer is both below the limit *and* above the stack base.
This prevents false positives for user-space threads, as the stack pointer will be outside those bounds
if the stack has been swapped.

ashm-dev pushed a commit to ashm-dev/cpython that referenced this pull request

Dec 8, 2025


          pythonGH-139653: Only raise an exception (or fatal error) when the st…

a019bca

…ack pointer is about to overflow the stack. (pythonGH-141711)

Only raises if the stack pointer is both below the limit *and* above the stack base.
This prevents false positives for user-space threads, as the stack pointer will be outside those bounds
if the stack has been swapped.

Reviewers

chris-eibl

chris-eibl left review comments

encukou

encukou approved these changes

efimov-mikhail

efimov-mikhail approved these changes

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markshannon commented Nov 18, 2025 • edited by bedevere-app Bot Loading Uh oh! There was an error while loading. Please reload this page.

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miss-islington-app Bot commented Nov 19, 2025

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markshannon commented Nov 18, 2025 •

edited by bedevere-app Bot

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