[release/9.0] Enhance createdump to detect alt stack execution

[hoyosjs]

Fixes .NET 9 version of #126981

Description

createdump's UnwindNativeFrames fails to capture the original thread stack when a crash occurs on a thread using an alternate signal stack. The native unwinder's monotonic-SP guard breaks we cross crosses the signal trampoline back to the original stack, because the SP legitimately decreases. This causes the unwinder to stop early, omitting the original stack memory from the dump.

The fix uses libunwind's unw_is_signal_frame in PAL_VirtualUnwindOutOfProc to detect signal trampoline frames. When a signal frame is detected, UnwindNativeFrames allows a SP decrease, enabling the unwinder to cross back to the original stack and capture its memory.

Customer Impact

Minidumps collected via createdump for crashes on alternate signal stacks are missing the original thread's stack memory. This makes the dumps incomplete and difficult to debug - native frames below the signal handler are absent from the stack walk, and you can only get the managed stack separately via clrstack. Watson and WinDBG both fail to do this automatically.

Regression

We added the SP monotonic check ~7 years ago to prevent corruption unwinding issues.

Testing

The following scenario was tested as proxy of customer's issue: Pinvoke into native library with some frames before hitting a nullref on a secondary thread. Pre-fix the repro shows the early bail unwind. The fix captured the full unwind across the signal trampoline, identifies the libc trampoline, and includes original stack memory in the dump. I also validated the fix works both with and without dwarf unwind info in the crashing native library.

Risk

Low. The change is narrowly scoped to createdump's native unwind path and some of the DAC's lazy state machine unwinding. unw_get_proc_info reuses the same cache unw_step populates - no additional remote memory reads. If unw_is_signal_frame returns false due to some libc not marking the trampoline correctly, behavior is identical to before - no regression.

[Copilot]

Pull request overview

Fixes createdump native stack unwinding when crashes occur on an alternate signal stack (SA_ONSTACK) by detecting signal trampoline frames and allowing a single stack-pointer decrease to unwind back onto the original thread stack.

Changes:

• Extend PAL_VirtualUnwindOutOfProc to optionally report whether the current frame is a signal trampoline (via unw_is_signal_frame).
• Update createdump’s UnwindNativeFrames monotonic-SP guard to allow one SP decrease when crossing a signal trampoline.
• Plumb the updated unwind API through createdump PAL shim and DAC minimal prototypes.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/coreclr/pal/src/exception/remote-unwind.cpp	Detects signal trampoline frames in out-of-proc unwinding and surfaces that via a new out-parameter.
src/coreclr/pal/inc/pal.h	Updates the PAL API signature for PAL_VirtualUnwindOutOfProc to include isSignalFrame.
src/coreclr/debug/daccess/dacfn.cpp	Updates DAC-side minimal prototype and call site to pass the new parameter (null).
src/coreclr/debug/createdump/threadinfo.cpp	Allows a single SP decrease after unwinding a signal trampoline to capture the original stack.
src/coreclr/debug/createdump/createdumppal.cpp	Updates the dynamically-loaded PAL function pointer signature and forwards the new argument.

[Copilot]

Pull request overview

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

[janvorli]

LGTM, thank you!

[Copilot]

Pull request overview

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

[steveisok]

/ba-g Known issues