lower-half: resolve loader debug file by build ID

[alan-lira]

Refs #485

Background

On the Ubuntu environment used for my MANA validation, the loader's detached debug symbols are installed by libc6-dbg under the standard GNU build-ID tree:

/usr/lib/debug/.build-id/xx/yyyy.debug

MANA needs the loader's mmap and munmap offsets. My current installation works around discovery by creating a compatibility path, but that requires a root-owned filesystem change outside MANA.

This draft explores resolving the standard build-ID debug file directly.

Proposed lookup order

1. Search the resolved loader itself, preserving current behavior.
2. Search MANA's existing conventional debug path, preserving compatibility.
3. Read the loader's NT_GNU_BUILD_ID note.
4. Search /usr/lib/debug/.build-id/xx/yyyy.debug.
5. Report the attempted paths clearly if the symbols cannot be found.

Included in this draft

• a small ELF build-ID helper;
• a test-only alternate debug root so tests never write to /usr/lib/debug;
• a detached-debug-file fixture test;
• evidence collection for the actual system loader;
• integration points for the lower half.

Design question for maintainers

I would appreciate guidance on the preferred implementation mechanism. The draft uses a self-contained helper, but I am happy to adapt it to an existing MANA ELF utility, libelf/elfutils, or another dependency already preferred by the project. I do not propose invoking readelf from production code.

Validation

bash test_build_id_helper.sh /tmp/mana-build-id-test
make -j$(nproc) mana
real Ubuntu loader lookup without compatibility symlink: passed
existing MANA unit tests: baseline results recorded separately

Scope

This draft addresses only loader debug-file discovery. It does not change coordinator behavior, restart semantics, DMTCP source, or distributed test orchestration.

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