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/Devices/KCOVDevice.cpp

@@ -0,0 +1,152 @@
+/*
+ * Copyright (c) 2021, Patrick Meyer <git@the-space.agency>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <AK/Assertions.h>
+#include <AK/NonnullOwnPtr.h>
+#include <Kernel/Devices/KCOVDevice.h>
+#include <Kernel/Devices/KCOVInstance.h>
+#include <Kernel/FileSystem/FileDescription.h>
+#include <LibC/sys/ioctl_numbers.h>
+
+#include <Kernel/Panic.h>
+
+namespace Kernel {
+
+HashMap<ProcessID, KCOVInstance*>* KCOVDevice::proc_instance;
+HashMap<ThreadID, KCOVInstance*>* KCOVDevice::thread_instance;
+
+UNMAP_AFTER_INIT NonnullRefPtr<KCOVDevice> KCOVDevice::must_create()
+{
+    return adopt_ref(*new KCOVDevice);
+}
+
+UNMAP_AFTER_INIT KCOVDevice::KCOVDevice()
+    : BlockDevice(30, 0)
+{
+    proc_instance = new HashMap<ProcessID, KCOVInstance*>();
+    thread_instance = new HashMap<ThreadID, KCOVInstance*>();
+    dbgln("KCOVDevice created");
+}
+
+void KCOVDevice::free_thread()
+{
+    auto thread = Thread::current();
+    auto tid = thread->tid();
+
+    auto maybe_kcov_instance = thread_instance->get(tid);
+    if (!maybe_kcov_instance.has_value())
+        return;
+
+    auto kcov_instance = maybe_kcov_instance.value();
+    VERIFY(kcov_instance->state == KCOVInstance::TRACING);
+    kcov_instance->state = KCOVInstance::OPENED;
+    thread_instance->remove(tid);
+}
+
+void KCOVDevice::free_process()
+{
+    auto process = Process::current();
+    auto pid = process->pid();
+
+    auto maybe_kcov_instance = proc_instance->get(pid);
+    if (!maybe_kcov_instance.has_value())
+        return;
+
+    auto kcov_instance = maybe_kcov_instance.value();
+    VERIFY(kcov_instance->state == KCOVInstance::OPENED);
+    kcov_instance->state = KCOVInstance::UNUSED;
+    proc_instance->remove(pid);
+    delete kcov_instance;
+}
+
+KResultOr<NonnullRefPtr<FileDescription>> KCOVDevice::open(int options)
+{
+    auto process = Process::current();
+    auto pid = process->pid();
+    if (proc_instance->get(pid).has_value())
+        return EBUSY; // This process already open()ed the kcov device
+    auto kcov_instance = new KCOVInstance(pid);
+    kcov_instance->state = KCOVInstance::OPENED;
+    proc_instance->set(pid, kcov_instance);
+
+    return File::open(options);
+}
+
+int KCOVDevice::ioctl(FileDescription&, unsigned request, FlatPtr arg)
+{
+    int error = 0;
+    auto thread = Thread::current();
+    auto tid = thread->tid();
+    auto pid = thread->pid();
+    auto maybe_kcov_instance = proc_instance->get(pid);
+    if (!maybe_kcov_instance.has_value())
+        return ENXIO; // This proc hasn't opened the kcov dev yet
+    auto kcov_instance = maybe_kcov_instance.value();
+
+    ScopedSpinLock lock(kcov_instance->lock);
+    switch (request) {
+    case KCOV_SETBUFSIZE: {
+        if (kcov_instance->state >= KCOVInstance::TRACING) {
+            error = EBUSY;
+            break;
+        }
+        error = kcov_instance->buffer_allocate(arg);
+        break;
+    }
+    case KCOV_ENABLE: {
+        if (kcov_instance->state >= KCOVInstance::TRACING) {
+            error = EBUSY;
+            break;
+        }
+        if (!kcov_instance->has_buffer()) {
+            error = ENOBUFS;
+            break;
+        }
+        VERIFY(kcov_instance->state == KCOVInstance::OPENED);
+        kcov_instance->state = KCOVInstance::TRACING;
+        thread_instance->set(tid, kcov_instance);
+        break;
+    }
+    case KCOV_DISABLE: {
+        auto maybe_kcov_instance = thread_instance->get(tid);
+        if (!maybe_kcov_instance.has_value()) {
+            error = ENOENT;
+            break;
+        }
+        VERIFY(kcov_instance->state == KCOVInstance::TRACING);
+        kcov_instance->state = KCOVInstance::OPENED;
+        thread_instance->remove(tid);
+        break;
+    }
+    default: {
+        error = EINVAL;
+    }
+    };
+
+    return error;
+}
+
+KResultOr<Region*> KCOVDevice::mmap(Process& process, FileDescription&, const Range& range, u64 offset, int prot, bool shared)
+{
+    auto pid = process.pid();
+    auto maybe_kcov_instance = proc_instance->get(pid);
+    VERIFY(maybe_kcov_instance.has_value()); // Should happen on fd open()
+    auto kcov_instance = maybe_kcov_instance.value();
+
+    if (!kcov_instance->vmobject) {
+        return ENOBUFS; // Mmaped, before KCOV_SETBUFSIZE
+    }
+
+    return process.space().allocate_region_with_vmobject(
+        range, *kcov_instance->vmobject, offset, {}, prot, shared);
+}
+
+String KCOVDevice::device_name() const
+{
+    return "kcov"sv;
+}
+
+}