dwarf-expert

# DWARF Expert Expertise for DWARF debug info: parsing and searching it, verifying its integrity, answering questions about the standard, and writing code that consumes it. Out of scope: runtime debugging (use gdb/lldb), reverse engineering beyond the DWARF sections (use Ghidra/IDA), and compiler-specific DWARF generation bugs. # Authoritative Sources When precision matters, look standard details up instead of answering from memory: 1. **dwarfstd.org** — the official specification. Web-search specific sections, e.g. "DWARF5 DW_TAG_subprogram attributes site:dwarfstd.org". 2. **LLVM** — `llvm/lib/DebugInfo/DWARF/` is a reliable reference implementation: `DWARFDie.cpp` (DIE and attribute access), `DWARFUnit.cpp` (compilation units), `DWARFDebugLine.cpp` (line tables), `DWARFVerifier.cpp` (validation). 3. **libdwarf** — the reference C implementation at github.com/davea42/libdwarf-code. # Parsing and Searching with dwarfdump Prefer `dwarfdump` over `readelf` for DWARF-specific work. Two implementations exist — libdwarf's `dwarfdump` and LLVM's `llvm-dwarfdump` — with different options, and a bare `dwarfdump` command may be either: check `dwarfdump --version` first. The options below are LLVM's. On macOS, linked Mach-O executables do not carry DWARF: it stays in the `.o` files until `dsymutil` collects it into a `.dSYM` bundle. Point `dwarfdump` at the dSYM (or the object files), not the executable. `pyelftools` is ELF-only — for Mach-O scripted work, stay with the LLVM tools. - `--all`: dump every DWARF section; `--debug-info`, `--debug-line`, etc. dump one - `--show-children [--recurse-depth=<n>]`: include child DIEs when printing selected entries — parameters, locals, and struct members are children of function and type DIEs - `--show-parents [--parent-recurse-depth=<n>]`: include parent DIEs - `--show-form`: print attribute form types, for when encoding details matter - `--find=<name>`: exact-name lookup via the accelerator tables — fast but not exhaustive; fall back to `--name` when it misses - `--name=<pattern> [--ignore-case] [--regex]`: exhaustive DIE-name search - `--lookup=<address>`: find the DIE covering an address - `--verbose`: print low-level encoding detail ## Searching DIEs Escalate through these strategies as the query grows more complex: 1. **Name or address match**: `--find`, then `--name`; `--lookup` for addresses. 2. **Attribute or type queries** (e.g. all parameters of type `float *`): dump and filter. `grep -B` pulls in the header line carrying each DIE's offset: `llvm-dwarfdump file | grep -B 5 "float \*" | grep DW_TAG_formal_parameter`, then print each DIE at its offset with `--debug-info=<offset> --show-children` (`--lookup` takes a program address, not a DIE offset). 3. **Multi-attribute or structural queries**: when grep pipelines turn brittle, write a Python script using `pyelftools` instead. # Verifying DWARF Integrity - `llvm-dwarfdump --verify <binary>`: structural checks (unit chains, DIE relationships, address ranges). `--error-display=<quiet|summary|details|full>` controls detail; `--verify-json=<path>` writes a machine-readable error summary; `--quiet` for exit-code-only checks. - `llvm-dwarfdump --statistics <binary>`: debug-info quality metrics as JSON — compare across compiler versions or optimization levels to catch regressions. Verify after producing DWARF (compilers, binary rewriters), when a debugger misbehaves on a binary, and when developing DWARF tooling against known-good files. When a current-generation compiler emitted an old DWARF version, the build explicitly passed `-gdwarf-N` — modern gcc and clang default to v4/v5, so check the build system rather than assuming a toolchain default. GCC embeds its flags in `DW_AT_producer`, so the pin is often readable right there; clang's producer string carries no flags. Old versions remain common in the wild and read the same way apart from surface forms: in v2 output, member offsets appear as location expressions (`DW_OP_plus_uconst`) and linkage names as `DW_AT_MIPS_linkage_name`. # readelf For general ELF structure, or when `dwarfdump` is unavailable: - `--debug-dump=<section>`: dump a DWARF section (`info`, `line`, ...) - `--dwarf-depth=<n>` / `--dwarf-start=<n>`: limit DIE depth / start offset # Writing Code That Parses DWARF Prefer an existing library over parsing by hand: | Library | Language | Notes | |---------|----------|-------| | `libdwarf` | C/C++ | github.com/davea42/libdwarf-code — low-level; used to implement `dwarfdump` | | `pyelftools` | Python | github.com/eliben/pyelftools — also parses ELF in general | | `gimli` | Rust | github.com/gimli-rs/gimli — pair with `object` to load container files | | `debug/dwarf` | Go | standard library | | `LibObjectFile` | .NET | github.com/xoofx/LibObjectFile — also handles ELF/PE object files | Default to Python with `pyelftools` for one-off scripts unless the task dictates otherwise. DWARF-specific pitfalls to handle — and to check for when reviewing DWARF code: - Attributes are optional: a DIE may omit `DW_AT_name`, `DW_AT_type`, ranges, etc. - Attribute indirection: a DIE's attributes may live on the DIE referenced by its `DW_AT_abstract_origin` (inlined instances) or `DW_AT_specification` (out-of-line definitions) — resolve the chain before concluding data is absent. - Type chains: qualifiers and modifiers (`DW_TAG_const_type`, `DW_TAG_pointer_type`, ...) wrap the underlying type; walk `DW_AT_type` links to reach the base type.