Meta: Describe how to analyze an LLVM fuzzer crash

Meta/Lagom/ReadMe.md

@@ -10,11 +10,38 @@ If you want to bring the comfortable Serenity classes with you to another system
 
 ## Fuzzing
 
-Lagom can be used to fuzz parts of SerenityOS's code base. This requires buildling with `clang`, so it's convenient to use a different build directory for that. Run CMake like so:
+Lagom can be used to fuzz parts of SerenityOS's code base. This requires buildling with `clang`, so it's convenient to use a different build directory for that. Run CMake like this:
 
  # From the root of the SerenityOS checkout:
  mkdir BuildLagom && cd BuildLagom
  cmake -GNinja -DBUILD_LAGOM=ON -DENABLE_FUZZER_SANITIZER=ON -DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++ ..
+ ninja Meta/Lagom/all
+ # Or as a handy rebuild-rerun line:
  ninja FuzzJs && Meta/Lagom/Fuzzers/FuzzJs
 
-clang emits different warnings than gcc, so you'll likely have to remove `-Werror` in CMakeLists.txt and Meta/Lagom/CMakeLIsts.txt.
+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. `Meta/Lagom/Fuzzers/FuzzBMP ../Base/res/html/misc/bmpsuite_files/rgba32-61754.bmp` Pay attention that LLVM also likes creating new files, don't blindly commit them (yet)!
+
+### 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 ./Meta/Lagom/Fuzzers/FuzzBMP
+<... SNIP some output ...>
+(gdb) run -handle_abrt=0 -handle_segv=0 crash-27480a219572aa5a11b285968a3632a4cf25388e
+<... SNIP some output ...>
+FuzzBMP: ../../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)
+```