Kernel: Move syscall implementations out of Process.cpp

This is something I've been meaning to do for a long time, and here we
finally go. This patch moves all sys$foo functions out of Process.cpp
and into files in Kernel/Syscalls/.

It's not exactly one syscall per file (although it could be, but I got
a bit tired of the repetitive work here..)

This makes hacking on individual syscalls a lot less painful since you
don't have to rebuild nearly as much code every time. I'm also hopeful
that this makes it easier to understand individual syscalls. :^)

Kernel/Syscalls/futex.cpp

@@ -0,0 +1,113 @@
+/*
+ * Copyright (c) 2018-2020, Andreas Kling <kling@serenityos.org>
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice, this
+ *    list of conditions and the following disclaimer.
+ *
+ * 2. Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include <AK/Time.h>
+#include <Kernel/Process.h>
+
+namespace Kernel {
+
+void compute_relative_timeout_from_absolute(const timeval& absolute_time, timeval& relative_time)
+{
+    // Convert absolute time to relative time of day.
+    timeval_sub(absolute_time, kgettimeofday(), relative_time);
+}
+
+void compute_relative_timeout_from_absolute(const timespec& absolute_time, timeval& relative_time)
+{
+    timeval tv_absolute_time;
+    timespec_to_timeval(absolute_time, tv_absolute_time);
+    compute_relative_timeout_from_absolute(tv_absolute_time, relative_time);
+}
+
+WaitQueue& Process::futex_queue(i32* userspace_address)
+{
+    auto& queue = m_futex_queues.ensure((FlatPtr)userspace_address);
+    if (!queue)
+        queue = make<WaitQueue>();
+    return *queue;
+}
+
+int Process::sys$futex(const Syscall::SC_futex_params* user_params)
+{
+    REQUIRE_PROMISE(thread);
+
+    Syscall::SC_futex_params params;
+    if (!validate_read_and_copy_typed(&params, user_params))
+        return -EFAULT;
+
+    i32* userspace_address = params.userspace_address;
+    int futex_op = params.futex_op;
+    i32 value = params.val;
+    const timespec* user_timeout = params.timeout;
+
+    if (!validate_read_typed(userspace_address))
+        return -EFAULT;
+
+    if (user_timeout && !validate_read_typed(user_timeout))
+        return -EFAULT;
+
+    switch (futex_op) {
+    case FUTEX_WAIT: {
+        i32 user_value;
+        copy_from_user(&user_value, userspace_address);
+        if (user_value != value)
+            return -EAGAIN;
+
+        timespec ts_abstimeout { 0, 0 };
+        if (user_timeout && !validate_read_and_copy_typed(&ts_abstimeout, user_timeout))
+            return -EFAULT;
+
+        WaitQueue& wait_queue = futex_queue(userspace_address);
+        timeval* optional_timeout = nullptr;
+        timeval relative_timeout { 0, 0 };
+        if (user_timeout) {
+            compute_relative_timeout_from_absolute(ts_abstimeout, relative_timeout);
+            optional_timeout = &relative_timeout;
+        }
+
+        // FIXME: This is supposed to be interruptible by a signal, but right now WaitQueue cannot be interrupted.
+        Thread::BlockResult result = Thread::current()->wait_on(wait_queue, "Futex", optional_timeout);
+        if (result == Thread::BlockResult::InterruptedByTimeout) {
+            return -ETIMEDOUT;
+        }
+
+        break;
+    }
+    case FUTEX_WAKE:
+        if (value == 0)
+            return 0;
+        if (value == 1) {
+            futex_queue(userspace_address).wake_one();
+        } else {
+            futex_queue(userspace_address).wake_n(value);
+        }
+        break;
+    }
+
+    return 0;
+}
+
+}