utmpupdate: Add a program for updating /var/run/utmp

To keep track of ongoing terminal sessions, we now have a sort-of
traditional /var/run/utmp file, like other Unix systems.
Unlike other Unix systems however, ours is of course JSON. :^)

The /bin/utmpupdate program is used to update the file, which is
not writable by regular user accounts. This helper program is
set-GID "utmp".

Base/etc/fstab

@@ -7,6 +7,7 @@
 /etc	/etc	bind	bind,nodev,nosuid
 /home	/home	bind	bind,nodev,nosuid
 /root	/root	bind	bind,nodev,nosuid
+/var	/var	bind	bind,nodev,nosuid
 
 none	/proc	proc	nosuid
 none	/dev/pts	devpts	noexec,nosuid,ro

Base/etc/group

@@ -3,6 +3,7 @@ wheel:x:1:anon
 tty:x:2:
 phys:x:3:window,anon
 audio:x:4:anon
+utmp:x:5:
 lookup:x:10:protocol,anon
 protocol:x:11:webcontent,anon
 notify:x:12:anon

Meta/build-root-filesystem.sh

@@ -6,6 +6,7 @@ wheel_gid=1
 tty_gid=2
 phys_gid=3
 audio_gid=4
+utmp_gid=5
 window_uid=13
 window_gid=13
 
@@ -51,11 +52,13 @@ chmod 4755 mnt/bin/ping
 chmod 4750 mnt/bin/reboot
 chmod 4750 mnt/bin/shutdown
 chmod 4750 mnt/bin/keymap
+chown 0:$utmp_gid mnt/bin/utmpupdate
+chmod 2755 mnt/bin/utmpupdate
 
 echo "done"
 
 printf "creating initial filesystem structure... "
-for dir in bin etc proc mnt tmp boot mod; do
+for dir in bin etc proc mnt tmp boot mod var/run; do
     mkdir -p mnt/$dir
 done
 chmod 700 mnt/boot
@@ -63,6 +66,12 @@ chmod 700 mnt/mod
 chmod 1777 mnt/tmp
 echo "done"
 
+printf "creating utmp file... "
+touch mnt/var/run/utmp
+chown 0:$utmp_gid mnt/var/run/utmp
+chmod 664 mnt/var/run/utmp
+echo "done"
+
 printf "setting up device nodes... "
 mkdir -p mnt/dev
 mkdir -p mnt/dev/pts

Userland/utmpupdate.cpp

@@ -0,0 +1,82 @@
+#include <AK/JsonObject.h>
+#include <AK/JsonValue.h>
+#include <LibCore/ArgsParser.h>
+#include <LibCore/File.h>
+#include <LibCore/DateTime.h>
+
+// utmpupdate -c /dev/pts/0
+// utmpupdate -d /dev/pts/0
+
+int main(int argc, char** argv)
+{
+    pid_t pid = 0;
+    bool flag_create = false;
+    bool flag_delete = false;
+    const char* tty_name = nullptr;
+    const char* from = nullptr;
+
+    Core::ArgsParser args_parser;
+    args_parser.add_option(flag_create, "Create entry", "create", 'c');
+    args_parser.add_option(flag_delete, "Delete entry", "delete", 'd');
+    args_parser.add_option(pid, "PID", "PID", 'p', "PID");
+    args_parser.add_option(from, "From", "from", 'f', "From");
+    args_parser.add_positional_argument(tty_name, "TTY name", "tty");
+
+    args_parser.parse(argc, argv);
+
+    if (flag_create && flag_delete) {
+        warn() << "-c and -d are mutually exclusive";
+        return 1;
+    }
+
+    dbg() << "Updating utmp from UID=" << getuid() << " GID=" << getgid() << " EGID=" << getegid() << " PID=" << pid;
+
+    auto file_or_error = Core::File::open("/var/run/utmp", Core::IODevice::ReadWrite);
+    if (file_or_error.is_error()) {
+        dbg() << "Error: " << file_or_error.error();
+        return 1;
+    }
+
+    auto& file = *file_or_error.value();
+
+    auto file_contents = file.read_all();
+    auto previous_json = JsonValue::from_string(file_contents);
+
+    JsonObject json;
+
+    if (!previous_json.has_value() || !previous_json.value().is_object()) {
+        dbg() << "Error: Could not parse JSON";
+    } else {
+        json = previous_json.value().as_object();
+    }
+
+    if (flag_create) {
+        JsonObject entry;
+        entry.set("pid", pid);
+        entry.set("uid", getuid());
+        entry.set("from", from);
+        entry.set("login_at", Core::DateTime::now().to_string());
+        json.set(tty_name, move(entry));
+    } else {
+        ASSERT(flag_delete);
+        dbg() << "Removing " << tty_name << " from utmp";
+        json.remove(tty_name);
+    }
+
+    if (!file.seek(0)) {
+        dbg() << "Seek failed";
+        return 1;
+    }
+
+    if (!file.truncate(0)) {
+        dbg() << "Truncation failed";
+        return 1;
+    }
+
+    if (!file.write(json.to_string())) {
+        dbg() << "Write failed";
+        return 1;
+    }
+
+    return 0;
+}