Changes/DebugInfoStandardization

Debug Info Standardization (from DWZ to -fdebug-types-section)

Summary

Fedora 18 implemented Features/DwarfCompressor. As the format did not get widespread and the tool is not much maintained it became burden to make existing debugging tools compatible with Fedora debug info.

Owner

• Name: Jan Kratochvil
• Email: jan.kratochvil@redhat.com

Current status

• Targeted release: Fedora 34
• Last updated: 2020-09-23
• FESCo issue: <will be assigned by the Wrangler>
• Tracker bug: <will be assigned by the Wrangler>
• Release notes tracker: <will be assigned by the Wrangler>

Detailed Description

Debug info files *.debug contained in *-debuginfo.rpm are very big in general (x86_64 Fedora 32 distribution has debug/ directory of 82GB while all its other files are only 75GB). There exist several methods how to make the *-debuginfo.rpms at least a bit smaller. Fedora 18 started using DWZ tool (from Features/DwarfCompressor) while Google implemented a similar but different approach called -fdebug-types-section.

Almost nobody uses existing Fedora DWZ (only Fedora/CentOS/RHEL and SuSE OSes) and so its support is missing in tools like LLDB, llvm-dwarfdump or binutils readelf. -fdebug-types-section is used internally by Google (produced by clang). Debian does not store any debug info archives. Ubuntu uses neither -fdebug-types-section nor DWZ.

• DWZ advantage: On the whole Fedora distro it saves 3.3% (5GB of the 157GB distribution size)
  • If the 3.3% size increase is a concern I can implement a different optimization (talk (2)) as a GCC post-processing phase which would require no changes in any DWARF consumers.
• DWZ disadvantage: DWZ has currently less support across consumers (LLDB, llvm-dwarfdump, binutils readelf)
• DWZ disadvantage: DWZ requires 8x times more complicated (LoC count) support in consumers than -fdebug-types-section.
• DWZ disadvantage: DWZ cannot update LLVM .debug_names index which can be generated only by clang (it cannot be regenerated later for DWZ-compressed file)
• DWZ disadvantage: DWZ DWARF-5 support is a work-in-progress. DWZ has been blocking DWARF-5 for Fedora for 3.5 years and only after I have now proposed to drop DWZ Mark Wielaard has started porting DWZ to DWARF-5. It can be expected next DWARF extensions will remain unsupported again. Even currently there is no plan to support DWARF-5 features used by clang which may need -fdebug-types-section for clang-built binaries or no size optimization of clang-built debug info at all.
• DWZ disadvantage: Compilation (linking) requires for C++ up to 2x as big disk space (as DWZ is processing files after linker and DWZ is incompatible with -fdebug-types-section)
• DWZ disadvantage: Compilation (linking) is slower

This proposed DWARF format was originally submitted already for Fedora 18 as Features/DebugTypesSections.

Benefit to Fedora

• Better compatibility with existing debugging and tracing tools, primarily LLDB.
• Less resource-intensive rebuilds of C++ packages (in disk space, memory requirements and compilation time).

Scope

• Proposal owners: It affects all packages generating *-debuginfo.rpm, that is compiled (not scripted) languages.

• Other developers: Report any possible debuginfo incompatibility (unexpected).

• Release engineering: #Releng issue number (a check of an impact with Release Engineering is needed)

• Policies and guidelines: All the needed changes should be done in redhat-rpm-config. The dwz package can be then retired.

• Trademark approval: N/A (not needed for this Change)

• Alignment with Objectives: The size differences are only for *-debuginfo.rpm which is outside of scope of the listed objectives.

Upgrade/compatibility impact

As *-debuginfo.rpm must exactly match NVRA of its binary package the compatibility is not relevant. Existing tools supporting DWZ will still support the DWZ file format in packages which have not been rebuilt.

How To Test

It is not a real risk as -fdebug-types-section is more widespread and more simple than DWZ. Also this whole proposal affects only debug info. Still I will run:

• libabigail DWARF comparisons
• testsuites of tools like: clang, lldb, gcc, binutils, gdb, elfutils, rpm

User Experience

No user visible change. This affects what tools can developers use.

Dependencies

none

Contingency Plan

• Contingency mechanism: Revert the change in redhat-rpm-config. Fedora can continue using DWZ, just some debugging/tracing tools will stay incompatible.
• Contingency deadline: beta freeze
• Blocks release? No
• Blocks product? N/A

Documentation

• DWARF-5 E.2 Using Type Units
• GCC -fdebug-types-section