SOS: read macOS arm64 thread metadata from a valid address

[max-charlamb]

Summary

• read macOS arm64 createdump thread metadata from the new 47-bit-valid address
• fall back to the legacy address for compatibility with existing createdump output
• use the selected address for all subsequent thread-info entries

Motivation

Apple Silicon LLDB rejects the legacy 0x7fffffff00000000 synthetic segment and assigns sequential thread IDs beginning at zero. SOS cannot then correlate LLDB threads with runtime OS thread IDs or retrieve the selected thread context.

Coordinated writer change: dotnet/runtime#131962

Related precedent

This follows @steveisok's coordinated Apple Silicon fix for SpecialDiagInfoAddress:

• runtime writer: createdump: use 47-bit-valid SpecialDiagInfo address on arm64 macOS runtime#130443
• diagnostics readers: macOS SOS-in-lldb: speed up ReadVirtual and fix arm64 SpecialDiagInfo #5823

Testing

Draft pending coordinated macOS arm64 validation with the runtime writer change.

[Copilot]

Pull request overview

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Updates SOS’s macOS arm64 thread-metadata reader to probe a new 47-bit-valid synthetic address (to satisfy LLDB) while retaining a fallback to the legacy address for older createdump outputs.

Changes:

• Read SpecialThreadInfoHeader from a new macOS arm64-safe address, with fallback to the legacy address.
• Carry forward the selected base address for subsequent SpecialThreadInfoEntry reads.
• Introduce a dedicated SpecialThreadInfoLegacyAddress constant and document the rationale.

Reviewed changes

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

File	Description
src/SOS/lldbplugin/services.cpp	Adds fallback probing logic and uses the chosen base address for the thread-info table walk.
src/SOS/inc/specialthreadinfo.h	Defines a new 47-bit-valid address for Apple Silicon and a legacy fallback constant.

Suppressed comments (1)

src/SOS/lldbplugin/services.cpp:2773

• The signature validation is currently only used to decide whether to fall back; if the fallback read succeeds but returns unexpected data (or if legacy == primary), the function can proceed without ever validating header.signature. Add a post-read validation step (after the potential fallback) that returns early if header.signature does not match SPECIAL_THREADINFO_SIGNATURE.

    size_t read = process.ReadMemory(threadInfoAddress, &header, sizeof(SpecialThreadInfoHeader), error);
    if ((error.Fail() || read != sizeof(header) ||
         strncmp(header.signature, SPECIAL_THREADINFO_SIGNATURE, sizeof(SPECIAL_THREADINFO_SIGNATURE)) != 0)
        && SpecialThreadInfoAddress != SpecialThreadInfoLegacyAddress)
    {
        // Fall back to the legacy address for dumps produced by older createdump binaries.
        error.Clear();
        threadInfoAddress = SpecialThreadInfoLegacyAddress;
        read = process.ReadMemory(threadInfoAddress, &header, sizeof(SpecialThreadInfoHeader), error);
    }
    if (error.Fail() || read != sizeof(header))
    {
        return;
    }

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