Debugging Nim

Reference article: https://nim-lang.org/blog/2017/10/02/documenting-profiling-and-debugging-nim-code.html

GDB / LLDB

Nim can be instrumented with sourcemaps:

• by passing --debugger:native to the compiler, so that the stacktraces in gdb shows the Nim source code
• by passing --passC:"-g" to the compiler, so that the stacktraces in gdb shows the C source code

Sanitizers & Valgrind

LLVM and GCC sanitizers can be used with

nim c --cc:clang -r -d:release --debugger:native \
  --passC:"-fsanitize=address" --passL:"-fsanitize=address" \
  --outdir:build target_application.nim

Note on deactivating Nim memory allocator:

As mentioned in the memory management section, Nim has

• a garbage collector, by default deferred reference counting + cycle detection via mark-and-sweep if the types can have cycles (and is not tagged {.acyclic.})
• an allocator based on TLSF

Instead of Nim custom allocators, the sys malloc/free can be used by passing -d:useMalloc in the command-line

Some GC/versions might not properly accept the flag, this is a Nim bug and we can patch upstream and our own fork in that case

Fuzzers

TODO

Internal fuzzing

We are currently adding fuzzing to our repositories via libFuzzer and AFL.

• Fuzzing abstraction: https://github.com/status-im/nim-testutils
• Network lib fuzzing: https://github.com/status-im/nim-eth/tree/4f533eb5/tests/fuzzing
• Eth2 core/Validator core fuzzing: https://github.com/status-im/nim-blscurve/tree/271a5738/tests/fuzzing

External "Consensus" fuzzing

Sigma Prima is fuzzing all Ethereum 2 clients on the spec (Eth2 core/Validator core)

We provide them with a simple C API

• https://github.com/status-im/nimbus-eth2/blob/4a2e1806/nfuzz/libnfuzz.h

That we implement:

• https://github.com/status-im/nimbus-eth2/blob/4a2e1806/nfuzz/libnfuzz.nim

And compile the nim code as a shared or static library.

On usage, the only extra limitation compared to a C library is the need to call NimMain() before calling any Nim function to initialize the Nim runtime.

Inspecting the generated intermediate code and objects

The intermediate C code and object files are stored in

• $HOME/.cache/nim/compiled_target_d or $HOME/.cache/nim/compiled_target_r on UNIX (d for debug, r for release)
• $HOME/nimcache/compiled_target_d or $HOME/.cache/nim/compiled_target_r on Windows

The cache directory can be set with the --nimcache=<targetdir> compiler flag.

Repositories vendoring nimbus-build-system do not use the default setting but generate directly in the (gitignored) nimcache folder of the repo.