[issue45521] obmalloc radix tree typo in code

[Neil Schemenauer]

Change by Neil Schemenauer :


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resolution:  -> fixed
stage: patch review -> resolved
status: open -> closed

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[issue45521] obmalloc radix tree typo in code

[Neil Schemenauer]

Neil Schemenauer  added the comment:


New changeset 1cdac61065e72db60d26e03ef9286d2743d7000e by Miss Islington (bot) 
in branch '3.10':
bpo-45521: Fix a bug in the obmalloc radix tree code. (GH-29051) (GH-29122)
https://github.com/python/cpython/commit/1cdac61065e72db60d26e03ef9286d2743d7000e


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[issue45521] obmalloc radix tree typo in code

[miss-islington]

Change by miss-islington :


--
nosy: +miss-islington
nosy_count: 2.0 -> 3.0
pull_requests: +27400
pull_request: https://github.com/python/cpython/pull/29122

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[issue45521] obmalloc radix tree typo in code

[Neil Schemenauer]

Neil Schemenauer  added the comment:


New changeset 311910b31a4bd94dc79298388b7cb65ca5546438 by Neil Schemenauer in 
branch 'main':
bpo-45521: Fix a bug in the obmalloc radix tree code. (GH-29051)
https://github.com/python/cpython/commit/311910b31a4bd94dc79298388b7cb65ca5546438


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[issue45521] obmalloc radix tree typo in code

[Neil Schemenauer]

Neil Schemenauer  added the comment:

I have not yet been able to reproduce methane's crash.  My guess it it's not 
related.

An explanation of what I think the impact of this bug is:

The radix tree is used to determine if memory is from obmalloc or from the 
system malloc (i.e return value from address_in_range()).  WIth ADDRESS_BITS 
set to 48, we ignore the top 16 bits of addresses.  The next 10 bits are 
supposed to be the index into the top level node array for the radix tree.  Due 
to the bug, we mask those and only use the bottom 8 of those 10.  So, if you 
have virtual addresses that span more than that 8 bit range, we will index into 
the wrong node.  That means address_in_range() could give the wrong answer.  
Which means you might try to free memory with the wrong malloc.

I think this is likely to be triggered only if you allocate a massive amount of 
memory, like 70 TB.  However, triggering it would depend on how the kernel maps 
virtual memory to the Python process.  I.e. there might be a wierd OS that 
gives pages at 0x7f00 and then right after pages at 0x3f00.

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[issue45521] obmalloc radix tree typo in code

[Neil Schemenauer]

Change by Neil Schemenauer :


--
keywords: +patch
pull_requests: +27322
pull_request: https://github.com/python/cpython/pull/29051

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[issue45521] obmalloc radix tree typo in code

[Inada Naoki]

Inada Naoki  added the comment:

When I am trying to understand this issue, I see this segfault.

https://gist.github.com/methane/1b83e2abc6739017e0490c5f70a27b52

I am not sure this segfault is caused by this issue or not. If this is 
unrelated, I will create another issue.

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[issue45521] obmalloc radix tree typo in code

[Neil Schemenauer]

New submission from Neil Schemenauer :

There is a typo in the radix tree obmalloc code, spotted by Inada Naoki. 

-#define MAP_TOP_MASK (MAP_BOT_LENGTH - 1)
+#define MAP_TOP_MASK (MAP_TOP_LENGTH - 1)

This should be fixed both in the main branch and in 3.10.x.

--
assignee: methane
components: Interpreter Core
messages: 404268
nosy: methane, nascheme
priority: high
severity: normal
stage: patch review
status: open
title: obmalloc radix tree typo in code
versions: Python 3.10, Python 3.11

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