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remove_unreachable: Assertion `target->b_predecessors == 0 || target == b->b_next' failed #124871

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15r10nk opened this issue Oct 1, 2024 · 4 comments · Fixed by #124952
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remove_unreachable: Assertion `target->b_predecessors == 0 || target == b->b_next' failed #124871

15r10nk opened this issue Oct 1, 2024 · 4 comments · Fixed by #124952
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@iritkatriel
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3.13 bugs and security fixes interpreter-core (Objects, Python, Grammar, and Parser dirs) type-crash A hard crash of the interpreter, possibly with a core dump

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@15r10nk
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15r10nk commented Oct 1, 2024 •
edited by bedevere-app bot
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Crash report

What happened?

The following code causes an assertion in a cpython debug build

name_3=5
name_2=2
name_1=0

while name_3 <= name_2 > name_1:
    try:
        raise
    except:
        pass
    finally:
        pass

output (Python 3.13.0a5+):

python3: Python/flowgraph.c:994: remove_unreachable: Assertion `target->b_predecessors == 0 || target == b->b_next' failed.

The bug is in the current 3.13 branch.
I bisected the problem down to 04697bc
@iritkatriel I think this is again one for you.

Little side note. I found this bug after fuzzing for ~3 days. I want to start to test the cpython main branch in the future and I hope to find bugs like this earlier.

CPython versions tested on:

3.13

Operating systems tested on:

No response

Output from running 'python -VV' on the command line:

No response

Linked PRs
@15r10nk 15r10nk added the type-crash A hard crash of the interpreter, possibly with a core dump label Oct 1, 2024
@ZeroIntensity ZeroIntensity added the interpreter-core (Objects, Python, Grammar, and Parser dirs) label Oct 2, 2024
@iritkatriel iritkatriel self-assigned this Oct 2, 2024
@iritkatriel
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This bug went away with fe23f8e.

@iritkatriel iritkatriel added the 3.13 bugs and security fixes label Oct 2, 2024
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iritkatriel commented Oct 3, 2024 •
edited
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The issue here is that we have a small basicblock which has SETUP_FINALLY instructions and is reachable from two other blocks via jumps - one conditional and one unconditional. The inline_small_or_no_lineno_blocks function inlines this block where it called unconditionally, but not where it's called conditionally. So the block remains, and is reachable, and when reachability calculation follows the SETUP_FINALLY target of the second time, this assertion trips.

I need to figure out whether the assertion is wrong or it's something else.

iritkatriel added a commit to iritkatriel/cpython that referenced this issue Oct 3, 2024
@iritkatriel
pythongh-124871: fix 'visited' tracking in reachability analysis
fb8a0c2
@iritkatriel iritkatriel mentioned this issue Oct 3, 2024
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I need to figure out whether the assertion is wrong it's or something else.

The assertion is wrong. The reason it was added was because this function marks blocks as visited when they are processed, and not when they are inserted to the queue (as other traversals do). The assertion ensured that we don't enter the same block to the queue more than once. The PR changes it to mark blocks as visited as soon as they are inserted to the queue, and remove the assertion.

@15r10nk
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15r10nk commented Oct 4, 2024

awesome, thank you.

iritkatriel added a commit to iritkatriel/cpython that referenced this issue Oct 4, 2024
@iritkatriel
Merge branch 'main' into pythongh-124871
680c708
iritkatriel added a commit that referenced this issue Oct 4, 2024
@iritkatriel
gh-124871: fix 'visited' tracking in compiler's reachability analysis (…
f474391 
…#124952)
miss-islington pushed a commit to miss-islington/cpython that referenced this issue Oct 4, 2024
@iritkatriel @miss-islington
pythongh-124871: fix 'visited' tracking in compiler's reachability an…
119dbe9 
…alysis (pythonGH-124952)

(cherry picked from commit f474391)

Co-authored-by: Irit Katriel <[email protected]>
@bedevere-app bedevere-app bot mentioned this issue Oct 4, 2024
Merged
Oct 4, 2024
@miss-islington @iritkatriel
[3.13] gh-124871: fix 'visited' tracking in compiler's reachability a…
b87aea6 
…nalysis (GH-124952) (#124977)

gh-124871: fix 'visited' tracking in compiler's reachability analysis (GH-124952)
(cherry picked from commit f474391)

Co-authored-by: Irit Katriel <[email protected]>
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gh-124871: fix 'visited' tracking in compiler's reachability analysis iritkatriel/cpython
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@iritkatriel @15r10nk @ZeroIntensity