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GeezerVison is showing github.com — Skip GNU backtrace test on Arm 32-bit by diegorusso · Pull Request #149493 · python/cpython

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@diegorusso

@diegorusso diegorusso commented May 7, 2026 •
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backtrace() and GDB use different unwinding paths.

On AArch64, runtime unwinding normally uses DWARF CFI in .eh_frame / .eh_frame_hdr.
On 32-bit ARM EABI, the runtime unwind format is .ARM.exidx / .ARM.extab, as specified by the Arm EHABI (https://github.com/ARM-software/abi-aa/blob/main/ehabi32/ehabi32.rst#the-object-producer-interface)

GDB can still unwind the 32-bit ARM binary because it can read debugger-only DWARF CFI from .debug_frame, while glibc backtrace() runs inside the process and needs runtime unwind metadata.

Without -funwind-tables, the 32-bit ARM build has effectively empty runtime unwind metadata:

  .ARM.exidx contains 1 entry:
    _start: cantunwind

With -funwind-tables, GCC emits the ARM EHABI runtime unwind tables needed by backtrace().
GCC documents -funwind-tables as generating the needed static unwind data without otherwise changing generated code.

This PR skips the GNU backtrace unwind test only on 32-bit Arm builds that lack -funwind-tables, instead of skipping all 32-bit Arm builds.

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@diegorusso
Skip GNU backtrace test on Arm 32-bit without unwind tables
3358e2d 
backtrace() on 32-bit ARM EABI depends on runtime unwind tables
emitted as .ARM.exidx/.ARM.extab. Without -funwind-tables,
glibc can return an empty stack even though GDB can still unwind using
debug-only .debug_frame data.

Only skip the GNU backtrace unwind test on Arm 32-bit builds that lack
-funwind-tables, instead of skipping all Arm 32-bit builds.
@bedevere-app bedevere-app Bot added tests Tests in the Lib/test dir awaiting core review labels May 7, 2026
@diegorusso diegorusso added skip issue skip news labels May 7, 2026
@encukou

encukou commented May 7, 2026

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!buildbot ARM Raspbian
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@bedevere-bot

bedevere-bot commented May 7, 2026

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🤖 New build scheduled with the buildbot fleet by @encukou for commit 3358e2d 🤖

Results will be shown at:

https://buildbot.python.org/all/#/grid?branch=refs%2Fpull%2F149493%2Fmerge

The command will test the builders whose names match following regular expression: ARM Raspbian

The builders matched are:

  • ARM Raspbian PR
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@diegorusso
diegorusso requested a review from pablogsal May 7, 2026 12:43
@diegorusso diegorusso mentioned this pull request May 7, 2026
gh-149202: Fix frame pointer unwinding on ppc64le and armv7/clang #149409
Merged
@diegorusso

diegorusso commented May 7, 2026

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still failing...
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encukou
encukou reviewed May 7, 2026
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Comment thread Lib/test/test_frame_pointer_unwind.py Outdated


def _gnu_backtrace_requires_unwind_tables(machine):
if not (sys.maxsize < 2**32 and machine.startswith("arm")):

@encukou encukou May 7, 2026 •
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AFAIK it's an aarch64 machine, with a 32-bit Python build?

Suggested change
if not (sys.maxsize < 2**32 and machine.startswith("arm")):
if not (sys.maxsize < 2**32 and machine.startswith(("arm", "aarch"))):

But we're deep into overfitting to Buildbot workers here :/

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@diegorusso diegorusso May 7, 2026

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mmm I don't think this is the right approach. We need to base the condition about the build of Python

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@diegorusso
Refine ARM GNU backtrace unwind-table detection
cb52a24
@diegorusso

diegorusso commented May 7, 2026

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!buildbot ARM Raspbian
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@bedevere-bot

bedevere-bot commented May 7, 2026

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🤖 New build scheduled with the buildbot fleet by @diegorusso for commit cb52a24 🤖

Results will be shown at:

https://buildbot.python.org/all/#/grid?branch=refs%2Fpull%2F149493%2Fmerge

The command will test the builders whose names match following regular expression: ARM Raspbian

The builders matched are:

  • ARM Raspbian PR
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encukou
encukou reviewed May 7, 2026
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Comment thread Lib/test/test_frame_pointer_unwind.py Outdated
Comment on lines +118 to +130
for option in options:
if option == "-funwind-tables":
unwind_tables = True
elif option == "-fno-unwind-tables":
unwind_tables = False
elif option == "-fasynchronous-unwind-tables":
asynchronous_unwind_tables = True
elif option == "-fno-asynchronous-unwind-tables":
asynchronous_unwind_tables = False
elif option == "-fexceptions":
exceptions = True
elif option == "-fno-exceptions":
exceptions = False

@encukou encukou May 7, 2026

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That's a lot of untested logic.
Should we skip the test for any 32-bit Arm?

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@diegorusso diegorusso May 7, 2026

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I tend to agree with you. On Arm 32bit you can have unwind data with any of these flags: -funwind-tables, -fasynchronous-unwind-tables, -fexceptions

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@diegorusso
Skip GNU backtrace tests on 32-bit Arm
61015e1 
GNU backtrace() can return an empty stack on 32-bit Arm
when the build lacks suitable runtime unwind tables.
Rather than trying to infer the effective unwind-table
compiler flags in the test, skip the GNU backtrace tests
on all 32-bit Arm builds.
@diegorusso

diegorusso commented May 7, 2026

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!buildbot ARM Raspbian
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@bedevere-bot

bedevere-bot commented May 7, 2026

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🤖 New build scheduled with the buildbot fleet by @diegorusso for commit 61015e1 🤖

Results will be shown at:

https://buildbot.python.org/all/#/grid?branch=refs%2Fpull%2F149493%2Fmerge

The command will test the builders whose names match following regular expression: ARM Raspbian

The builders matched are:

  • ARM Raspbian PR
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@diegorusso
Merge branch 'main' into backtrace-arm
04bd87a
@diegorusso

diegorusso commented May 7, 2026

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!buildbot ARM Raspbian
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@StanFromIreland

StanFromIreland commented May 7, 2026

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Updating to fix some errors we introduced on the main branch.
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@StanFromIreland
Merge branch 'main' into backtrace-arm
42177ce
@diegorusso

diegorusso commented May 8, 2026

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!buildbot ARM Raspbian
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@bedevere-bot

bedevere-bot commented May 8, 2026

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🤖 New build scheduled with the buildbot fleet by @diegorusso for commit 42177ce 🤖

Results will be shown at:

https://buildbot.python.org/all/#/grid?branch=refs%2Fpull%2F149493%2Fmerge

The command will test the builders whose names match following regular expression: ARM Raspbian

The builders matched are:

  • ARM Raspbian PR
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@diegorusso
diegorusso requested a review from markshannon May 8, 2026 10:36
markshannon
markshannon approved these changes May 8, 2026
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Looks good, and much simpler now

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@bedevere-app bedevere-app Bot added awaiting merge and removed awaiting core review labels May 8, 2026
@diegorusso diegorusso changed the title Skip GNU backtrace test on Arm 32-bit without unwind tables Skip GNU backtrace test on Arm 32-bit May 8, 2026
@diegorusso
diegorusso merged commit 2a8cece into python:main May 8, 2026
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@bedevere-app bedevere-app Bot removed the awaiting merge label May 8, 2026
@diegorusso diegorusso added the needs backport to 3.15 bugs and security fixes label May 8, 2026
@miss-islington-app

miss-islington-app Bot commented May 8, 2026

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

bedevere-app Bot commented May 8, 2026

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GH-149562 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 May 8, 2026
@diegorusso
diegorusso deleted the backtrace-arm branch May 8, 2026 13:07
diegorusso pushed a commit that referenced this pull request May 8, 2026
@miss-islington
[3.15] Skip GNU backtrace test on Arm 32-bit (GH-149493) (#149562)
54a187d
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@encukou encukou encukou left review comments
@markshannon markshannon markshannon approved these changes
@pablogsal pablogsal Awaiting requested review from pablogsal

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@diegorusso @encukou @bedevere-bot @StanFromIreland @markshannon
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