[cDAC][wasm] Stop R2R unwinding at reverse P/Invoke frames and restore exception-frame FP

[lewing]

Stacked on #135140 (base lewing-cdac-wasm-stackwalk-fixes @ 6d044c16421). This PR closes stack-walk gaps that #135140 leaves out of scope.

Changes

Reverse P/Invoke boundary. Native RtlVirtualUnwind (src/coreclr/vm/wasm/helpers.cpp) sets callerIsNative when the frame being unwound has a reverse P/Invoke frame in its GC info and is not a funclet. WasmUnwindStackFrameCore then reports IP 0 instead of reading the native caller's stack as an R2R frame. The cDAC had no such check, so unwinding out of an UnmanagedCallersOnly R2R method could make up a managed caller from native stack bytes.

• WasmUnwinder.TryUnwindOneFrame takes the current control PC and mirrors this check.
• Like native, it now always advances SP to the caller's SP and reports a null IP when the caller isn't R2R code (an interpreter transition, native code, or the stack top). Previously it nulled SP and returned false, which lost the caller's stack position.
• WasmContext.Unwind recomputes FP only for an R2R caller, as native does.
• GCInfoHeader.HasReversePInvokeFrame is new and set by both the common decoder and the x86 decoder (InfoHdr.RevPInvokeOffset).

SoftwareExceptionFrame FP. On WASM the callee-saved register set is InterpreterFP (ENUM_CALLEE_SAVED_REGISTERS in vm/wasm/cgencpu.h), so native SoftwareExceptionFrame::UpdateRegDisplay_Impl restores FP along with IP and SP. The cDAC base handler copies only the registers listed in the CalleeSavedRegisters descriptor, which is empty on WASM, so FP was left over from the previous frame. WasmFrameHandler now restores all three from the saved context.

Inactive InlinedCallFrame. Native returns before any update when the frame has no active call. WasmFrameHandler checked only after it had already written the interpreter-frame stash.

StackWalk.md and GCInfo.md are updated.

Not included

• IStackWalk.GetRuntimeFramePointer on WASM. Native GetFramePointerWorker uses GetRegdisplayStackMark, which has no TARGET_WASM implementation (src/coreclr/inc/regdisp.h), so there's no native behavior to mirror. It needs a runtime decision first.
• Leaf FP when a walk starts inside a funclet (cDAC WASM: leaf frame pointer is not funclet-aware when a walk starts inside a funclet #135147). That depends on the leaf-funclet layout in Avoid frame identity for leaf Wasm funclets #134608.
• GetWasmFunctionIdentity. It goes in a separate PR stacked on [cDAC][wasm] Continue stack walks from a TransitionFrame into its R2R caller #135140.

Validation

• ./.dotnet/dotnet test src/native/managed/cdac/tests/UnitTests/Microsoft.Diagnostics.DataContractReader.Tests.csproj: passed, 3312/3312.
• ./build.sh -s tools.cdactests -test: passed (UnitTests, DataGeneratorTests, UsageTests).
• pwsh src/native/managed/cdac/tools/CdacUsageGraph/generate-docs.ps1 -Check: passed.
• New MockTarget tests:
  • TryUnwindOneFrame_CallerWithoutVirtualIp_PreservesCallerStackPointer
  • TryUnwindOneFrame_ReversePInvokeFrame_DoesNotReadNativeCallerAsR2RFrame: reverse P/Invoke, funclet with reverse P/Invoke GC info, and ordinary method
  • Unwind_ReversePInvokeFrame_KeepsCallerStackPointerAndClearsInstructionAndFramePointers
  • GCInfoHeader_ReportsReversePInvokeFrame: hand-encoded WASM fat header and x86 header, with and without the frame
  • UpdateContextFromFrame_WasmSoftwareExceptionFrame_RestoresFramePointer
  • UpdateContextFromFrame_WasmInactiveInlinedCallFrameOverInterpreterFrame_LeavesContextUnchanged
• Each mutation below was confirmed applied by diff, then failed the tests on exact values, then was restored:

Mutation	Fails
Drop the reverse P/Invoke check	reverse-P/Invoke theory case and the Unwind boundary test
Drop the funclet exclusion	funclet theory case
Restore "null SP on non-R2R caller"	3 tests
Compute FP without an R2R caller	Unwind boundary test
x86 HasReversePInvokeFrame always false	x86 header case
Common decoder always false	WASM header case
Remove the SoftwareExceptionFrame override	expected FP 0x41040, got the stale 0x70000
Remove the inactive-ICF early return	stash expected 0, got 0x70000

Note

This PR description was generated with GitHub Copilot.

[azure-pipelines]

Azure Pipelines:
Successfully started running 5 pipeline(s).
11 pipeline(s) were filtered out due to trigger conditions.
There may be pipelines that require an authorized user to comment /azp run to run.

[dotnet-policy-service]

Tagging subscribers to this area: @steveisok, @tommcdon, @dotnet/dotnet-diag
See info in area-owners.md if you want to be subscribed.