Don't crash when walking callstack roots in the debugger

[leculver]

Changeset 1fa1745 introduced a stack walking regression where we will crash trying to unwind/report registers. The real fix here is in cgenamd64.cpp/excep.cpp, which will point the context to a local copy before we try to use it.

I've also added some changes in gcinfodecoder.cpp that aren't strictly necessary but would have avoided the problem to begin with.

Fixes #124401.

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[Copilot]

Pull request overview

This pull request fixes a critical crash that occurs when debuggers walk callstack roots on Linux amd64 coredumps. The root cause is that context pointers in DAC (Data Access Component) builds resolve through a cache that can evict entries before the pointers are consumed, leading to invalid memory access.

Changes:

• Fixed AMD64 FaultingExceptionFrame to use local context pointers in DAC builds instead of frame member references
• Extended SoftwareExceptionFrame context pointer fix from X86-only to all architectures
• Added defensive checks in GC info decoder to use captured register values in DAC scenarios across all architectures

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/coreclr/vm/amd64/cgenamd64.cpp	Added DACCESS_COMPILE guards in FaultingExceptionFrame::UpdateRegDisplay_Impl to point context pointers at local pCurrentContext copy instead of m_ctx members
src/coreclr/vm/excep.cpp	Expanded DACCESS_COMPILE context pointer fix from TARGET_X86 to all architectures in SoftwareExceptionFrame::UpdateRegDisplay_Impl
src/coreclr/vm/gcinfodecoder.cpp	Added DACCESS_COMPILE && TARGET_UNIX guards to use GetCapturedRegister instead of GetRegisterSlot in ReportRegisterToGC and GetStackSlot across all architectures

[Copilot]

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated no new comments.

[janvorli]

Looking at the FaultingExceptionFrame::UpdateRegDisplay_Impl variants, it seems the other architectures would suffer from the same issue.

[leculver]

I'll take a look and keep working on it when I'm back in office Tuesday morning.

[leculver]

I've updated the other platforms with the fix as well.

[Copilot]

Pull request overview

Copilot reviewed 7 out of 7 changed files in this pull request and generated no new comments.

[leculver]

Before merging, I want JanV to have a chance to weigh in here when he's back on Monday.

[janvorli]

LGTM, thank you!

[Copilot]

Pull request overview

Copilot reviewed 7 out of 7 changed files in this pull request and generated no new comments.