-fno-omit-frame-pointer is not enough to make every target walkable by the
simple manual frame pointer unwinder.
The helper used by test_frame_pointer_unwind used to assume the frame pointer
named a two-word record where fp[0] was the previous frame pointer and fp[1]
was the return address. That is only the generic layout used by some targets.
This patch keeps that default, but moves the slots behind named offsets so
architecture-specific layouts can describe where the backchain and return
address really live.
On s390x, GCC and Clang do not emit a usable backchain unless -mbackchain is
enabled. Without it, the unwinder stops at the current C frame and the test
reports no Python frames. Once backchains are present, the helper must also
stop at the current thread's known C stack bounds; otherwise it can follow the
final backchain far enough to dereference an invalid frame and segfault.
For Linux s390x backchain frames, the documented z/Architecture stack-frame
layout saves r14, the return-address register, at byte offset 112 from the
frame pointer, so read the return address from that named slot instead of fp[1].
The 112-byte offset comes from Linux's s390 debugging documentation: its Stack
Frame Layout table shows z/Architecture backchain frames with the backchain at
offset 0 and saved r14 of the caller function at offset 112:
https://www.kernel.org/doc/html/v5.3/s390/debugging390.html#stack-frame-layout
This helper remains scoped to Linux s390x backchain frames. GNU SFrame's s390x
notes state that the s390x ELF ABI does not generally mandate where RA and FP
are saved, or whether they are saved at all:
https://sourceware.org/binutils/docs/sframe-spec.html#s390x
As Jens Remus noted, -fno-omit-frame-pointer is not needed when -mbackchain is
present.
On 32-bit ARM, GCC defaults to Thumb mode on common armhf toolchains. The Thumb
prologue keeps the saved frame pointer and link register at offsets that depend
on the generated frame, which breaks the fp[0]/fp[1] walk used by the helper.
Use -marm when it is supported for frame-pointer builds, and teach the helper
the GCC ARM-mode slots where the previous frame pointer is at fp[-1] and the
saved LR return address is at fp[0].
Co-authored-by: Petr Viktorin <encukou@gmail.com>
Co-authored-by: Victor Stinner <vstinner@python.org>
Co-authored-by: Savannah Ostrowski <savannah@python.org>
Co-authored-by: Hugo van Kemenade <1324225+hugovk@users.noreply.github.com>
Co-authored-by: Emma Smith <emma@emmatyping.dev>
* gh-96132: Add some comments and minor fixes missed in the original PR
* Update Doc/using/cmdline.rst
Co-authored-by: Kumar Aditya <59607654+kumaraditya303@users.noreply.github.com>
Co-authored-by: Kumar Aditya <59607654+kumaraditya303@users.noreply.github.com>
⚠️⚠️ Note for reviewers, hackers and fellow systems/low-level/compiler engineers ⚠️⚠️
If you have a lot of experience with this kind of shenanigans and want to improve the **first** version, **please make a PR against my branch** or **reach out by email** or **suggest code changes directly on GitHub**.
If you have any **refinements or optimizations** please, wait until the first version is merged before starting hacking or proposing those so we can keep this PR productive.
