arm64: Add unwind information to _start
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Authored by dchagin on May 26 2023, 8:40 AM.

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Reviewers

andrew
emaste

Group Reviewers

arm64

Summary

The right unwinding stop indicator should be CFI-undefined PC.
http://dwarf.freestandards.org/Dwarf3.pdf - page 118:

If a Return Address register is defined in the virtual unwind table,

and its rule is undefined (for example, by DW_CFA_undefined), then
there is no return address and no call address, and the virtual
unwind of stack activations is complete.

Create an initial frame with 0 lr and fp.
Procedure Call Standard for the ARM 64-bit Architecture (AArch64)
specifies that the end of the frame record chain is indicated by
the address zero in the address for the previous frame.

This is necessary to complete fixing the libunwind for Aarch64.

MFC after: 1 month

Diff Detail

Repository

rG FreeBSD src repository

Lint

Lint Passed

Unit

No Test Coverage

Build Status

Buildable 51699
Build 48590: arc lint + arc unit

Event Timeline

dchagin created this revision.May 26 2023, 8:40 AM

Herald added subscribers: emaste, andrew, imp. · View Herald TranscriptMay 26 2023, 8:40 AM

dchagin requested review of this revision.May 26 2023, 8:40 AM

Harbormaster completed remote builds in B51699: Diff 122453.May 26 2023, 8:40 AM

dchagin added a reviewer: arm64.May 26 2023, 8:42 AM

dchagin added a project: arm64.

I plan to commit this soon, any objections?

I thought the kernel zeroed these... but I have no objections since my aarch64 fu is weak

In D40277#924038, @imp wrote:

I thought the kernel zeroed these... but I have no objections since my aarch64 fu is weak

For the dynamically linked binaries there is no other proper way to tell the end frame to the unwinder, other than bad heuristic guessing games.
_start is the entry point for binary, which is called by rtld in that case.

dchagin edited the summary of this revision. (Show Details)Jun 20 2023, 12:33 PM

Herald added a reviewer: andrew. · View Herald TranscriptJun 20 2023, 12:33 PM

emaste accepted this revision as: emaste.Jun 20 2023, 12:36 PM

This revision is now accepted and ready to land.Jun 20 2023, 12:36 PM

emaste added inline comments.Jun 20 2023, 12:37 PM