Kernel: Add option to build with coverage instrumentation and KCOV

GCC and Clang allow us to inject a call to a function named
__sanitizer_cov_trace_pc on every edge. This function has to be defined
by us. By noting down the caller in that function we can trace the code
we have encountered during execution. Such information is used by
coverage guided fuzzers like AFL and LibFuzzer to determine if a new
input resulted in a new code path. This makes fuzzing much more
effective.

Additionally this adds a basic KCOV implementation. KCOV is an API that
allows user space to request the kernel to start collecting coverage
information for a given user space thread. Furthermore KCOV then exposes
the collected program counters to user space via a BlockDevice which can
be mmaped from user space.

This work is required to add effective support for fuzzing SerenityOS to
the Syzkaller syscall fuzzer. :^) :^)

Kernel/CMakeLists.txt

@@ -46,6 +46,8 @@ set(KERNEL_SOURCES
     Devices/CharacterDevice.cpp
     Devices/Device.cpp
     Devices/FullDevice.cpp
+    Devices/KCOVDevice.cpp
+    Devices/KCOVInstance.cpp
     Devices/MemoryDevice.cpp
     Devices/NullDevice.cpp
     Devices/PCISerialDevice.cpp
@@ -76,6 +78,7 @@ set(KERNEL_SOURCES
     Graphics/VirtIOGPU/GPU.cpp
     Graphics/VirtIOGPU/GraphicsAdapter.cpp
     Graphics/VGACompatibleAdapter.cpp
+    SanCov.cpp
     Storage/Partition/DiskPartition.cpp
     Storage/Partition/DiskPartitionMetadata.cpp
     Storage/Partition/EBRPartitionTable.cpp
@@ -364,6 +367,36 @@ endif()
 
 set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -pie -Wl,--no-dynamic-linker")
 
+# Kernel Coverage (KCOV) is an API to collect and expose program counters of
+# kernel code that has been run to user space. It's rather slow and likely not
+# secure to run in production builds. Useful for coverage guided fuzzing.
+if (ENABLE_KERNEL_COVERAGE_COLLECTION)
+    add_definitions(-DENABLE_KERNEL_COVERAGE_COLLECTION)
+    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fsanitize-coverage=trace-pc")
+    set(KCOV_EXCLUDED_SOURCES
+        # Make sure we don't instrument any code called from __sanitizer_cov_trace_pc
+        # otherwise we'll end up with recursive calls to that function.
+        ../AK/Format.cpp
+        ../AK/StringBuilder.cpp
+        ../Kernel/Arch/x86/${KERNEL_ARCH}/Processor.cpp
+        ../Kernel/Devices/KCOVDevice.cpp
+        ../Kernel/Devices/KCOVInstance.cpp
+        ../Kernel/FileSystem/File.cpp
+        ../Kernel/FileSystem/FileDescription.cpp
+        ../Kernel/Heap/SlabAllocator.cpp
+        ../Kernel/init.cpp
+        ../Kernel/SanCov.cpp
+        # GCC assumes that the caller saves registers for functions according
+        # to the System V ABI and happily inserts coverage calls into the
+        # function prologue for all functions. This assumption is not true for
+        # interrupt handlers because their calling convention is not compatible
+        # with the System V ABI.
+        ../Kernel/Arch/x86/common/Interrupts.cpp
+        ../Kernel/Syscall.cpp
+    )
+    set_source_files_properties(${KCOV_EXCLUDED_SOURCES} PROPERTIES COMPILE_FLAGS "-fno-sanitize-coverage=trace-pc")
+endif()
+
 # Kernel Undefined Behavior Sanitizer (KUBSAN)
 set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fsanitize=undefined")