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/setuid.cpp

@@ -0,0 +1,156 @@
+/*
+ * 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/Process.h>
+
+namespace Kernel {
+
+int Process::sys$seteuid(uid_t euid)
+{
+    REQUIRE_PROMISE(id);
+
+    if (euid != m_uid && euid != m_suid && !is_superuser())
+        return -EPERM;
+
+    m_euid = euid;
+    return 0;
+}
+
+int Process::sys$setegid(gid_t egid)
+{
+    REQUIRE_PROMISE(id);
+
+    if (egid != m_gid && egid != m_sgid && !is_superuser())
+        return -EPERM;
+
+    m_egid = egid;
+    return 0;
+}
+
+int Process::sys$setuid(uid_t uid)
+{
+    REQUIRE_PROMISE(id);
+
+    if (uid != m_uid && uid != m_euid && !is_superuser())
+        return -EPERM;
+
+    m_uid = uid;
+    m_euid = uid;
+    m_suid = uid;
+    return 0;
+}
+
+int Process::sys$setgid(gid_t gid)
+{
+    REQUIRE_PROMISE(id);
+
+    if (gid != m_gid && gid != m_egid && !is_superuser())
+        return -EPERM;
+
+    m_gid = gid;
+    m_egid = gid;
+    m_sgid = gid;
+    return 0;
+}
+
+int Process::sys$setresuid(uid_t ruid, uid_t euid, uid_t suid)
+{
+    REQUIRE_PROMISE(id);
+
+    if (ruid == (uid_t)-1)
+        ruid = m_uid;
+    if (euid == (uid_t)-1)
+        euid = m_euid;
+    if (suid == (uid_t)-1)
+        suid = m_suid;
+
+    auto ok = [this](uid_t id) { return id == m_uid || id == m_euid || id == m_suid; };
+    if ((!ok(ruid) || !ok(euid) || !ok(suid)) && !is_superuser())
+        return -EPERM;
+
+    m_uid = ruid;
+    m_euid = euid;
+    m_suid = suid;
+    return 0;
+}
+
+int Process::sys$setresgid(gid_t rgid, gid_t egid, gid_t sgid)
+{
+    REQUIRE_PROMISE(id);
+
+    if (rgid == (gid_t)-1)
+        rgid = m_gid;
+    if (egid == (gid_t)-1)
+        egid = m_egid;
+    if (sgid == (gid_t)-1)
+        sgid = m_sgid;
+
+    auto ok = [this](gid_t id) { return id == m_gid || id == m_egid || id == m_sgid; };
+    if ((!ok(rgid) || !ok(egid) || !ok(sgid)) && !is_superuser())
+        return -EPERM;
+
+    m_gid = rgid;
+    m_egid = egid;
+    m_sgid = sgid;
+    return 0;
+}
+
+int Process::sys$setgroups(ssize_t count, const gid_t* user_gids)
+{
+    REQUIRE_PROMISE(id);
+    if (count < 0)
+        return -EINVAL;
+    if (!is_superuser())
+        return -EPERM;
+    if (count && !validate_read(user_gids, count))
+        return -EFAULT;
+
+    if (!count) {
+        m_extra_gids.clear();
+        return 0;
+    }
+
+    Vector<gid_t> gids;
+    gids.resize(count);
+    copy_from_user(gids.data(), user_gids, sizeof(gid_t) * count);
+
+    HashTable<gid_t> unique_extra_gids;
+    for (auto& gid : gids) {
+        if (gid != m_gid)
+            unique_extra_gids.set(gid);
+    }
+
+    m_extra_gids.resize(unique_extra_gids.size());
+    size_t i = 0;
+    for (auto& gid : unique_extra_gids) {
+        if (gid == m_gid)
+            continue;
+        m_extra_gids[i++] = gid;
+    }
+    return 0;
+}
+
+}