serenity/Meta/Lagom

Lagom

The Serenity C++ library, for other Operating Systems.

About

If you want to bring the comfortable Serenity classes with you to another system, look no further. This is basically a "port" of the AK and LibCore libraries to generic *nix systems.

Lagom is a Swedish word that means "just the right amount." (Wikipedia)

Fuzzing

Lagom can be used to fuzz parts of SerenityOS's code base. Fuzzers can be run locally, and they also run continuously on OSS-Fuzz.

Fuzzing locally

Lagom can be used to fuzz parts of SerenityOS's code base. This requires building with clang, so it's convenient to use a different build directory for that. Fuzzers work best with Address Sanitizer enabled. The fuzzer build requires code generators to be pre-built without fuzzing in a two stage build. To build with LLVM's libFuzzer, invoke the BuildFuzzers.sh script with no arguments. The script does the equivalent of the CMake commands below:

    # From the Meta/Lagom directory:
    # Stage 1: Build and install code generators and other tools
    cmake -GNinja -B Build/tools \
      -DBUILD_LAGOM=OFF \
      -DCMAKE_INSTALL_PREFIX=Build/tool-install \
      -Dpackage=LagomTools
    ninja -C Build/tools install
    # Stage 2: Build fuzzers, making sure the build can find the tools we just built
    cmake -GNinja -B Build/lagom-fuzzers \
      -DBUILD_LAGOM=ON \
      -DENABLE_FUZZERS_LIBFUZZER=ON \
      -DENABLE_ADDRESS_SANITIZER=ON \
      -DENABLE_UNDEFINED_SANITIZER=ON \
      -DCMAKE_PREFIX_PATH=Build/tool-install \
      -DCMAKE_CXX_COMPILER=clang++ \
      -DCMAKE_C_COMPILER=clang
    cd Build/lagom-fuzzers
    ninja
    # Or as a handy rebuild-rerun line:
    ninja FuzzJs && ./Fuzzers/FuzzJs

(Note that we require clang >= 13, see the pick_clang() function in the script for the paths that are searched)

Any fuzzing results (particularly slow inputs, crashes, etc.) will be dropped in the current directory.

clang emits different warnings than gcc, so you may have to remove -Werror in CMakeLists.txt and Meta/Lagom/CMakeLists.txt.

Fuzzers work better if you give them a fuzz corpus, e.g. ./Fuzzers/FuzzBMPLoader ../Base/res/html/misc/bmpsuite_files/rgba32-61754.bmp Pay attention that LLVM also likes creating new files, don't blindly commit them (yet)!

To run several fuzz jobs in parallel, pass -jobs=24 -workers=24.

To get less log output, pass -close_fd_mask=3 -- but that but hides assertion messages. Just 1 only closes stdout. It's good to move overzealous log output behind FOO_DEBUG macros.

Keeping track of interesting testcases

There are many quirky files that exercise a lot of interesting edge cases. We should probably keep track of them, somewhere.

We have a bmp suite and a jpg suite and several others. They are GPL'ed, and therefore not quite as compatible with the rest of Serenity. That's probably not a problem, but keeping "our" testcases separate from those GPL'ed suits sounds like a good idea.

We could keep those testcases somewhere else in the repository, like a fuzz directory. But fuzzing tends to generate more and more and more files, and they will blow up in size. Especially if we keep all interesting testcases, which is exactly what I intend to do.

So we should keep the actual testcases out of the main serenity repo, that's why we created https://github.com/SerenityOS/serenity-fuzz-corpora

Feel free to upload lots and lots files there, or use them for great good!

Fuzzing on OSS-Fuzz

https://oss-fuzz.com/ automatically runs all fuzzers in the Fuzzers/ subdirectory whose name starts with "Fuzz" and which are added to the build in Fuzzers/CMakeLists.txt if ENABLE_FUZZERS_OSSFUZZ is set. Looking for "serenity" on oss-fuzz.com finds interesting links, in particular:

• known open bugs found by fuzzers
  • oss-fuzz bug tracker for these
• coverage report
• build logs

Here's Serenity's OSS-Fuzz Config.

To run the oss-fuzz build locally:

git clone https://github.com/google/oss-fuzz/
cd oss-fuzz
python3 infra/helper.py build_image serenity
python3 infra/helper.py build_fuzzers serenity

These commands will put the fuzzers in build/out/serenity in the oss-fuzz repo. You can run the binaries in there individually, or simply type:

python3 infra/helper.py run_fuzzer serenity FUZZER_NAME

To build the fuzzers using the oss-fuzz build process, but against a local serenity checkout:

python3 infra/helper.py build_fuzzers serenity $HOME/src/serenity/

To run a shell in oss-fuzz's serenity docker image:

docker run -it gcr.io/oss-fuzz/serenity bash

Analyzing a crash

LLVM fuzzers have a weird interface. In particular, to see the help, you need to call it with -help=1, and it will ignore --help and -help.

To reproduce a crash, run it like this: MyFuzzer crash-27480a219572aa5a11b285968a3632a4cf25388e

To reproduce a crash in gdb, you want to disable various signal handlers, so that gdb sees the actual location of the crash:

$ gdb ./Fuzzers/FuzzBMP
<... SNIP some output ...>
(gdb) run -handle_abrt=0 -handle_segv=0 crash-27480a219572aa5a11b285968a3632a4cf25388e
<... SNIP some output ...>
FuzzBMP: ../../Userland/Libraries/LibGfx/Bitmap.cpp:84: Gfx::Bitmap::Bitmap(Gfx::BitmapFormat, const Gfx::IntSize &, Gfx::Bitmap::Purgeable): Assertion `m_data && m_data != (void*)-1' failed.

Thread 1 "FuzzBMP" received signal SIGABRT, Aborted.
__GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:50
50	../sysdeps/unix/sysv/linux/raise.c: File or directory not found.
(gdb)

UBSan doesn't always give useful information. use something like export UBSAN_OPTIONS=print_stacktrace=1 to always print stacktraces.

You may run into annoying issues with the stacktrace:

==123456==WARNING: invalid path to external symbolizer!
==123456==WARNING: Failed to use and restart external symbolizer!

That means it couldn't find the executable llvm-symbolizer, which could be in your OS's package llvm. llvm-symbolizer-11 will not be recognized.

Using Lagom in an External Project

It is possible to use Lagom for your own projects outside of Serenity too!

An example of this in use can be found on Linus' LibJS test262 runner.

To implement this yourself:

• Download a copy of linusg/libjs-test262/cmake/FetchLagom.cmake and place it wherever you wish
• In your root CMakeLists.txt, add the following commands:

  include(FetchContent)
  include(cmake/FetchLagom.cmake) # If you've placed the file downloaded above differently, be sure to reflect that in this command :^)
  

• In addition, you will need to also add some compile options that Serenity uses to ensure no warnings or errors:

  add_compile_options(-Wno-literal-suffix) # AK::StringView defines operator"" sv, which GCC complains does not have an underscore.
  add_compile_options(-fno-gnu-keywords)   # JS::Value has a method named typeof, which also happens to be a GNU keyword.
  

Now, you can link against Lagom libraries.

Things to keep in mind:

• You should prefer to use a library's Lagom:: alias when linking
  • Example: Lagom::Core vs LibCore
• If you still need to use the standard library, you may have to compile with the AK_DONT_REPLACE_STD macro.
  • Serenity defines its own move and forward functions inside of AK/StdLibExtras.h that will clash with the standard library's definitions. This macro will make Serenity use the standard library's move and forward instead.