GeezerVison

The World Wide Web, restored to 800 pixels of glory.

GeezerVison is showing github.com — gh-141311: Avoid assertion in BytesIO readinto by cmaloney · Pull Request #141333 · python/cpython

Conversation

cmaloney commented Nov 10, 2025 •

edited by bedevere-app Bot

Loading

Uh oh!

There was an error while loading. Please reload this page.

Copy link

Copy Markdown

Contributor

Account for when self->pos is equal to PY_SSIZE_T_MAX. The length of read was correctly set to zero but the asserts assumed self->pos couldn't reach PY_SSIZE_T_MAX. Return early to avoid edge cases.

Issue: Assertion failure in Modules/_io/bytesio.c _io_BytesIO_readinto_impl: Assertion 'self->pos + len < PY_SSIZE_T_MAX' failed. #141311

Sorry, something went wrong.

Uh oh!

There was an error while loading. Please reload this page.

All reactions


          pythongh-141311: Avoid assertion in BytesIO readinto

db81df6

Account for when self->pos is equal to PY_SSIZE_T_MAX. The length of
read was correctly set to zero but the asserts assumed
self->pos couldn't reach PY_SSIZE_T_MAX. Return early to avoid edge
cases.

bedevere-app Bot added the awaiting review label

Nov 10, 2025

bedevere-app Bot mentioned this pull request

Nov 10, 2025

Assertion failure in Modules/_io/bytesio.c _io_BytesIO_readinto_impl: Assertion 'self->pos + len < PY_SSIZE_T_MAX' failed. #141311

Closed


          Tweak comment for correctness.

34ed126

serhiy-storchaka reviewed

Nov 10, 2025

View reviewed changes

Comment thread

Modules/_io/bytesio.c Resolved

Uh oh!

There was an error while loading. Please reload this page.

Comment thread

Lib/test/test_io/test_memoryio.py Resolved

Uh oh!

There was an error while loading. Please reload this page.

Comment thread

Misc/NEWS.d/next/Library/2025-11-09-18-55-13.gh-issue-141311.qZ3swc.rst Resolved

Uh oh!

There was an error while loading. Please reload this page.

cmaloney added 3 commits

November 10, 2025 09:55


          Simplify by using sys.maxsize

2c84efe


          Reference io.BytesIO.readinto directly

d119904


          Resolve read_bytes_lock_held and audit other cases

d78cc84

For `read_bytes_lock_held` the `size` parameter is always set to 0 by calling
code currently if self->pos > self->string_size. I added a range check for
self->pos as an extra safety as codepaths chagne but can remove if it's too much.

`write_bytes_lock_held`: `endpos` is a `size_t` so can hold
`2 * PY_SSIZE_T_MAX`. Value is bounds checked in `resize_buffer_lock_held`.

A number of cases use `scan_eol_lock_held` to move forward. That has code which
checks `self->pos >= self->string_size` and returns `0`. Callsites all seem to
handle that correctly.

cmaloney commented Nov 11, 2025

Copy link

Copy Markdown

Contributor Author

I audited the codepaths which did self->pos + buffer.

For read_bytes_lock_held the size parameter is always set to 0 by calling code currently if self->pos > self->string_size. I added a range check for self->pos as an extra safety as codepaths change but can remove if it's too much.

write_bytes_lock_held: endpos is a size_t so can hold 2 * PY_SSIZE_T_MAX. Value is bounds checked in resize_buffer_lock_held.

A number of cases use scan_eol_lock_held to move forward. That has code which checks self->pos >= self->string_size and returns 0. Callers seem to handle that correctly (early exit or call read_bytes_lock_held)..

All reactions

Sorry, something went wrong.

Uh oh!

There was an error while loading. Please reload this page.


          Update blurb for additional change

4c12be7

serhiy-storchaka reviewed

Nov 11, 2025

View reviewed changes

Comment thread

Modules/_io/bytesio.c Resolved

Uh oh!

There was an error while loading. Please reload this page.

Comment thread

Modules/_io/bytesio.c Resolved

Uh oh!

There was an error while loading. Please reload this page.

Comment thread

Modules/_io/bytesio.c Resolved

Uh oh!

There was an error while loading. Please reload this page.

cmaloney added 2 commits

November 11, 2025 12:51


          Update for review comments

e06d274


          tweak blurb

6cc0ab3

serhiy-storchaka reviewed

Nov 11, 2025

View reviewed changes

Comment thread

Modules/_io/bytesio.c Resolved

Uh oh!

There was an error while loading. Please reload this page.


          Improve precision of comments around the undefined behavior and safeties

d205d4a

serhiy-storchaka approved these changes

Nov 11, 2025

View reviewed changes

serhiy-storchaka left a comment

Copy link

Copy Markdown

Member

There was a problem hiding this comment.

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

LGTM. 👍

Sorry, something went wrong.

Uh oh!

There was an error while loading. Please reload this page.

All reactions

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

Nov 11, 2025

serhiy-storchaka merged commit 7d54374 into python:main

Nov 12, 2025

50 checks passed

Uh oh!

There was an error while loading. Please reload this page.

bedevere-app Bot removed the awaiting merge label

Nov 12, 2025

serhiy-storchaka added needs backport to 3.13

only security fixes

needs backport to 3.14 bugs and security fixes labels

Nov 12, 2025

miss-islington-app Bot commented Nov 12, 2025

Copy link

Copy Markdown

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

All reactions

Sorry, something went wrong.

Uh oh!

There was an error while loading. Please reload this page.

miss-islington-app Bot commented Nov 12, 2025

Copy link

Copy Markdown

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

All reactions

Sorry, something went wrong.

Uh oh!

There was an error while loading. Please reload this page.

miss-islington-app Bot commented Nov 12, 2025

Copy link

Copy Markdown

Sorry, @cmaloney 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 7d54374f9c7d91e0ef90c4ad84baf10073cf1d8a 3.13

All reactions

Sorry, something went wrong.

Uh oh!

There was an error while loading. Please reload this page.

miss-islington-app Bot assigned serhiy-storchaka

Nov 12, 2025

miss-islington pushed a commit to miss-islington/cpython that referenced this pull request

Nov 12, 2025


          pythongh-141311: Avoid assertion in BytesIO.readinto() (pythonGH-141333)

7151cfb

Fix error in assertion which causes failure if pos is equal to PY_SSIZE_T_MAX.
Fix undefined behavior in read() and readinto() if pos is larger that the size
of the underlying buffer.
(cherry picked from commit 7d54374)

Co-authored-by: Cody Maloney <[email protected]>

bedevere-app Bot commented Nov 12, 2025

Copy link

Copy Markdown

GH-141457 is a backport of this pull request to the 3.14 branch.

All reactions

Sorry, something went wrong.

Uh oh!

There was an error while loading. Please reload this page.

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

Nov 12, 2025

serhiy-storchaka commented Nov 12, 2025

Copy link

Copy Markdown

Member

@cmaloney, could you please create a backport to 3.13 manually?

cmaloney reacted with thumbs up emoji

All reactions

👍 1 reaction

Sorry, something went wrong.

Uh oh!

There was an error while loading. Please reload this page.

serhiy-storchaka pushed a commit that referenced this pull request

Nov 12, 2025


          [3.14] gh-141311: Avoid assertion in BytesIO.readinto() (GH-141333) (G…

75b5157

…H-141457)

Fix error in assertion which causes failure if pos is equal to PY_SSIZE_T_MAX.
Fix undefined behavior in read() and readinto() if pos is larger that the size
of the underlying buffer.
(cherry picked from commit 7d54374)

Co-authored-by: Cody Maloney <[email protected]>

cmaloney deleted the bytesio_seek_far branch

November 12, 2025 19:57

bedevere-app Bot commented Nov 12, 2025

Copy link

Copy Markdown

GH-141478 is a backport of this pull request to the 3.13 branch.

All reactions

Sorry, something went wrong.

Uh oh!

There was an error while loading. Please reload this page.

bedevere-app Bot removed the needs backport to 3.13 only security fixes label

Nov 12, 2025

serhiy-storchaka pushed a commit that referenced this pull request

Nov 12, 2025


          [3.13] gh-141311: Avoid assertion in BytesIO.readinto() (GH-141333) (G…

7c62bd5

…H-141478)

Fix error in assertion which causes failure if pos is equal to PY_SSIZE_T_MAX.
Fix undefined behavior in read() and readinto() if pos is larger that the size
of the underlying buffer.
(cherry picked from commit 7d54374)

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

Dec 6, 2025


          pythongh-141311: Avoid assertion in BytesIO.readinto() (pythonGH-141333)

7592d29

Fix error in assertion which causes failure if pos is equal to PY_SSIZE_T_MAX.
Fix undefined behavior in read() and readinto() if pos is larger that the size
of the underlying buffer.

serhiy-storchaka removed their assignment

Jul 15, 2026

matthiasgoergens mentioned this pull request

Aug 5, 2026

gh-148286: Fix undefined behaviour in io.StringIO.read() after an overseek #154913

Open

Reviewers

serhiy-storchaka

serhiy-storchaka approved these changes

Uh oh!

There was an error while loading. Please reload this page.

Assignees

No one assigned

Labels

None yet

Projects

None yet

Milestone

No milestone

Development

Successfully merging this pull request may close these issues.

Uh oh!

There was an error while loading. Please reload this page.

2 participants

Add this suggestion to a batch that can be applied as a single commit.This suggestion is invalid because no changes were made to the code.Suggestions cannot be applied while the pull request is closed.Suggestions cannot be applied while viewing a subset of changes.Only one suggestion per line can be applied in a batch.Add this suggestion to a batch that can be applied as a single commit.Applying suggestions on deleted lines is not supported.You must change the existing code in this line in order to create a valid suggestion.Outdated suggestions cannot be applied.This suggestion has been applied or marked resolved.Suggestions cannot be applied from pending reviews.Suggestions cannot be applied on multi-line comments.Suggestions cannot be applied while the pull request is queued to merge.Suggestion cannot be applied right now. Please check back later.

You are visitor 01402503 — GeezerVison does not store the pages you look up.
Best viewed with Netscape Navigator 4.7 or Internet Explorer 6 at 800×600. No cookies required.

GeezerVison

Conversation

cmaloney commented Nov 10, 2025 • edited by bedevere-app Bot Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

cmaloney commented Nov 11, 2025

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

serhiy-storchaka left a comment

Choose a reason for hiding this comment

Uh oh!

Uh oh!

miss-islington-app Bot commented Nov 12, 2025

Uh oh!

miss-islington-app Bot commented Nov 12, 2025

Uh oh!

miss-islington-app Bot commented Nov 12, 2025

Uh oh!

bedevere-app Bot commented Nov 12, 2025

Uh oh!

serhiy-storchaka commented Nov 12, 2025

Uh oh!

bedevere-app Bot commented Nov 12, 2025

Uh oh!

Reviewers

Uh oh!

Assignees

Labels

Projects

Milestone

Development

Uh oh!

2 participants

cmaloney commented Nov 10, 2025 •

edited by bedevere-app Bot

Loading

Uh oh!

There was an error while loading. Please reload this page.