gh-149202: Fix frame pointer unwinding on s390x and ARM#149409
gh-149202: Fix frame pointer unwinding on s390x and ARM#149409encukou wants to merge 7 commits intopython:mainfrom
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-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 also 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 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].
encukou
requested review from
AA-Turner,
corona10,
emmatyping and
erlend-aasland
as code owners
Documentation build overview
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🤖 New build scheduled with the buildbot fleet by @encukou for commit 955261c 🤖 Results will be shown at: https://buildbot.python.org/all/#/grid?branch=refs%2Fpull%2F149409%2Fmerge The command will test the builders whose names match following regular expression: The builders matched are:
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Just read the PEP today, so I'll gently nudge for an extra fix here, or maybe I could send another PR :) ppc64le doesn't need any frame pointer compiler flags, so I'd exclude it entirely through configure. The power abi specification requires that compilers maintain a back chain by default, so unwinding already works without a dedicated frame pointer. Adding however Also following up on Diego's comment, the current unwinding model here won't work for ppc64le (stack frame layout is on page 34 of the ABI spec pdf). The LR is saved in the caller's frame (SP+16), not the current frame, so frame_pointer[RETURN_OFFSET] can't reach it. I can provide access to a power buildbot for testing it out or try to come up with a solution myself. |
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This is based on @pablogsal's #149362; I want to run CI without pushing to his branch.