Skip GNU backtrace test on Arm 32-bit by diegorusso · Pull Request #149493 · python/cpython · GitHub

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.

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.

!buildbot ARM Raspbian

🤖 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

still failing...

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 :/

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

!buildbot ARM Raspbian

🤖 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

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

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

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.

!buildbot ARM Raspbian

🤖 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

!buildbot ARM Raspbian

Updating to fix some errors we introduced on the main branch.

!buildbot ARM Raspbian

🤖 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

Looks good, and much simpler now

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

GH-149562 is a backport of this pull request to the 3.15 branch.

bedevere-app Bot added tests Tests in the Lib/test dir awaiting core review labels May 7, 2026

diegorusso added skip issue skip news labels May 7, 2026

diegorusso requested a review from pablogsal May 7, 2026 12:43

diegorusso mentioned this pull request May 7, 2026

gh-149202: Fix frame pointer unwinding on ppc64le and armv7/clang #149409

Merged

encukou reviewed May 7, 2026

View reviewed changes

Refine ARM GNU backtrace unwind-table detection

cb52a24

encukou reviewed May 7, 2026

View reviewed changes

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.

Merge branch 'main' into backtrace-arm

04bd87a

Merge branch 'main' into backtrace-arm

42177ce

diegorusso requested a review from markshannon May 8, 2026 10:36

markshannon approved these changes May 8, 2026

View reviewed changes

bedevere-app Bot added awaiting merge and removed awaiting core review labels May 8, 2026

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 merged commit 2a8cece into python:main May 8, 2026
54 checks passed

bedevere-app Bot removed the awaiting merge label May 8, 2026

diegorusso added the needs backport to 3.15 bugs and security fixes label May 8, 2026

bedevere-app Bot removed the needs backport to 3.15 bugs and security fixes label May 8, 2026

diegorusso deleted the backtrace-arm branch May 8, 2026 13:07

diegorusso pushed a commit that referenced this pull request May 8, 2026

[3.15] Skip GNU backtrace test on Arm 32-bit (GH-149493 ) (#149562 )

54a187d

+                  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



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

Skip GNU backtrace test on Arm 32-bit by diegorusso · Pull Request #149493 · python/cpython · GitHub

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Skip GNU backtrace test on Arm 32-bit - #149493

Skip GNU backtrace test on Arm 32-bit#149493diegorusso merged 5 commits intopython:mainpython/cpython:mainfrom diegorusso:backtrace-armdiegorusso/cpython:backtrace-armCopy head branch name to clipboard

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Skip GNU backtrace test on Arm 32-bit#149493
diegorusso merged 5 commits into
python:mainfrom
diegorusso:backtrace-arm

diegorusso commented May 7, 2026 •
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