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GeezerVison is showing github.com — gh-129813: Check size in PyBytesWriter_FinishWithSize() by vstinner · Pull Request #157226 · python/cpython

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@vstinner

@vstinner vstinner commented Sep 9, 2026 •
edited by bedevere-app Bot
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Reject size larger than the allocated size.

Also, check negative size in PyBytesWriter_FinishWithSize() to always raise ValueError. Previously, the function raised SystemError or ValueError depending on the code path.

Replace _PyBytesWriter_GetAllocated() optimization with _PyBytesWriter_ResizeToAllocated() to update the writer size to its allocated size.

  • Issue: [C API] PEP 782: Add PyBytesWriter API #129813

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@vstinner
pythongh-129813: Check size in PyBytesWriter_FinishWithSize()
07185df 
Reject size larger than the allocated size.

Also, check negative size in PyBytesWriter_FinishWithSize() to always
raise ValueError. Previously, the function raised SystemError or
ValueError depending on the code path.

Replace _PyBytesWriter_GetAllocated() optimization with
_PyBytesWriter_ResizeToAllocated() to update the writer size to its
allocated size.
@vstinner vstinner added skip news needs backport to 3.15 bugs and security fixes labels Sep 9, 2026
@bedevere-app bedevere-app Bot mentioned this pull request Sep 9, 2026
[C API] PEP 782: Add PyBytesWriter API #129813
Closed
@bedevere-app bedevere-app Bot added the awaiting core review label Sep 9, 2026
@vstinner

vstinner commented Sep 9, 2026

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Currently, if PyBytesWriter_FinishWithSize() size is larger than the writer size, it reads uninitialized memory (which can lead to a segfault) which is bad.
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vstinner added 4 commits September 9, 2026 19:08
@vstinner
Update test_capi
b9da2ce 
test_capi: only run the 3 "example" tests in BytesWriterTest. There
is no need to rerun the same tests in ByteArrayWriterTest.
@vstinner
Fix memory leak
d572dad
@vstinner
Add tests on the Pointer API
2091c90
@vstinner
Avoid redundant checks in FinishWithPointer()
c810e97 
FinishWithSize() already checks the size.
@vstinner
vstinner merged commit 4e530b8 into python:main Sep 9, 2026
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@vstinner
vstinner deleted the writer_size branch September 9, 2026 18:59
@bedevere-app bedevere-app Bot removed the awaiting core review label Sep 9, 2026
@miss-islington-app

miss-islington-app Bot commented Sep 9, 2026

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Thanks @vstinner for the PR 🌮🎉.. I'm working now to backport this PR to: 3.15.
🐍🍒⛏🤖
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@miss-islington-app

miss-islington-app Bot commented Sep 9, 2026

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Sorry, @vstinner, I could not cleanly backport this to 3.15 due to a conflict.
Please backport using cherry_picker on command line.

cherry_picker 4e530b82f37ec89e3b54bc5d0a1612c643bff1e0 3.15
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@miss-islington-app miss-islington-app Bot assigned vstinner Sep 9, 2026
@vstinner

vstinner commented Sep 9, 2026

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There are conflicts on the 3.15 backport. Some recent PyBytesWriter changes are not backported to 3.15 yet. I will backport this change when the 3.15 branch will be unblocked (after 3.15.0 final release).
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clin1234 pushed a commit to clin1234/cpython that referenced this pull request Sep 12, 2026
@vstinner @clin1234
pythongh-129813: Check size in PyBytesWriter_FinishWithSize() (python…
aac317c 
…#157226)

Reject size larger than the allocated size.

Check also negative size in PyBytesWriter_FinishWithSize() to always
raise ValueError. Previously, the function raised SystemError or
ValueError depending on the code path.

Replace _PyBytesWriter_GetAllocated() optimization with
_PyBytesWriter_ResizeToAllocated() to update the writer size to its
allocated size.

test_capi: only run the 3 "example" tests in BytesWriterTest. There
is no need to rerun the same tests in ByteArrayWriterTest.
johnslavik pushed a commit to johnslavik/cpython that referenced this pull request Sep 14, 2026
@vstinner @johnslavik
pythongh-129813: Check size in PyBytesWriter_FinishWithSize() (python…
3d36fb8 
…#157226)

Reject size larger than the allocated size.

Check also negative size in PyBytesWriter_FinishWithSize() to always
raise ValueError. Previously, the function raised SystemError or
ValueError depending on the code path.

Replace _PyBytesWriter_GetAllocated() optimization with
_PyBytesWriter_ResizeToAllocated() to update the writer size to its
allocated size.

test_capi: only run the 3 "example" tests in BytesWriterTest. There
is no need to rerun the same tests in ByteArrayWriterTest.
@bedevere-app

bedevere-app Bot commented Oct 12, 2026

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GH-159261 is a backport of this pull request to the 3.15 branch.
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@bedevere-app bedevere-app Bot removed the needs backport to 3.15 bugs and security fixes label Oct 12, 2026
vstinner added a commit that referenced this pull request Oct 12, 2026
@vstinner
[3.15] gh-129813: Check size in PyBytesWriter_FinishWithSize() (#157226…
c38a347 
…) (#159261)

gh-129813: Check size in PyBytesWriter_FinishWithSize() (#157226)

Reject size larger than the allocated size.

Check also negative size in PyBytesWriter_FinishWithSize() to always
raise ValueError. Previously, the function raised SystemError or
ValueError depending on the code path.

Replace _PyBytesWriter_GetAllocated() optimization with
_PyBytesWriter_ResizeToAllocated() to update the writer size to its
allocated size.

test_capi: only run the 3 "example" tests in BytesWriterTest. There
is no need to rerun the same tests in ByteArrayWriterTest.

(cherry picked from commit 4e530b8)
This was referenced Oct 24, 2026
pycriu: fix getting RPC response size with Python 3.15.1 checkpoint-restore/criu#3268
Merged
socket.recv() and recvfrom() with MSG_TRUNC raise ValueError and lose the datagram since 3.15.1 #160622
Open
socket.recv(size, MSG_TRUNC) returns more than size bytes #69121
Open
@ViderStar ViderStar mentioned this pull request Oct 25, 2026
gh-160622: Fix socket recv() and recvfrom() with MSG_TRUNC #160677
Open
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