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Cp/r145614545 backtrace through frameless function which faults 611 #10745

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jasonmolenda
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Re-landing this patch with small tweaks to address CI bot failures
as it was run on many different configurations.  I think the test
may run on aarch64 Linux systems now.

When a frameless function faults or is interrupted asynchronously, the
UnwindPlan MAY have no register location rule for the return address
register (lr on arm64); the value is simply live in the lr register when
it was interrupted, and the frame below this on the stack -- e.g.
sigtramp on a Unix system -- has the full register context, including
that register.

RegisterContextUnwind::SavedLocationForRegister, when asked to find the
caller's pc value, will first see if there is a pc register location. If
there isn't, on a Return Address Register architecture like
arm/mips/riscv, we rewrite the register request from "pc" to "RA
register", and search for a location.

On frame 0 (the live frame) and an interrupted frame, the UnwindPlan may
have no register location rule for the RA Reg, that is valid. A
frameless function that never calls another may simply keep the return
address in the live register the whole way. Our instruction emulation
unwind plans explicitly add a rule (see Pavel's May 2024 change
llvm#91321 ), but an UnwindPlan
sourced from debug_frame may not.

I've got a case where this exactly happens - clang debug_frame for arm64
where there is no register location for the lr in a frameless function.
There is a fault in the middle of this frameless function and we only
get the lr value from the fault handler below this frame if lr has a
register location of `IsSame`, in line with Pavel's 2024 change.

Similar to how we see a request of the RA Reg from frame 0 after failing
to find an unwind location for the pc register, the same style of
special casing is needed when this is a function that was interrupted.

Without this change, we can find the pc of the frame that was executing
when it was interrupted, but we need $lr to find its caller, and we
don't descend down to the trap handler to get that value, truncating the
stack.

rdar://145614545
(cherry picked from commit b957cc0)
(cherry picked from commit 32b1f83)
There's something still wrong with how it's building
the test file.

(cherry picked from commit 48a814c)
(cherry picked from commit 139c00b)
@jasonmolenda jasonmolenda requested a review from a team as a code owner May 24, 2025 03:39
@jasonmolenda
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@swift-ci test

@JDevlieghere JDevlieghere merged commit 10df7e7 into swiftlang:swift/release/6.1.1 May 27, 2025
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2 participants