Kernel+LibC+UserspaceEmulator: Bring back sys$dup2()

This is racy in userspace and non-racy in kernelspace so let's keep
it in kernelspace.

The behavior change where CLOEXEC is preserved when dup2() is called
with (old_fd == new_fd) was good though, let's keep that.

Kernel/API/Syscall.h

@@ -86,6 +86,7 @@ namespace Kernel {
     S(getpgrp)                \
     S(fork)                   \
     S(execve)                 \
+    S(dup2)                   \
     S(sigaction)              \
     S(umask)                  \
     S(getgroups)              \

Kernel/CMakeLists.txt

@@ -100,6 +100,7 @@ set(KERNEL_SOURCES
     Syscalls/clock.cpp
     Syscalls/debug.cpp
     Syscalls/disown.cpp
+    Syscalls/dup2.cpp
     Syscalls/execve.cpp
     Syscalls/exit.cpp
     Syscalls/fcntl.cpp

Kernel/Process.h

@@ -258,6 +258,7 @@ public:
     int sys$ptsname(int fd, Userspace<char*>, size_t);
     pid_t sys$fork(RegisterState&);
     int sys$execve(Userspace<const Syscall::SC_execve_params*>);
+    int sys$dup2(int old_fd, int new_fd);
     int sys$sigaction(int signum, const sigaction* act, sigaction* old_act);
     int sys$sigprocmask(int how, Userspace<const sigset_t*> set, Userspace<sigset_t*> old_set);
     int sys$sigpending(Userspace<sigset_t*>);

Kernel/Syscalls/dup2.cpp

@@ -0,0 +1,46 @@
+/*
+ * 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 <Kernel/FileSystem/FileDescription.h>
+#include <Kernel/Process.h>
+
+namespace Kernel {
+
+int Process::sys$dup2(int old_fd, int new_fd)
+{
+    REQUIRE_PROMISE(stdio);
+    auto description = file_description(old_fd);
+    if (!description)
+        return -EBADF;
+    if (old_fd == new_fd)
+        return 0;
+    if (new_fd < 0 || new_fd >= m_max_open_file_descriptors)
+        return -EINVAL;
+    m_fds[new_fd].set(*description);
+    return new_fd;
+}
+
+}