Kernel: Some improvements to the mount syscall

- You must now have superuser privileges to use mount().
- We now verify that the mount point is a valid path first, before
  trying to find a filesystem on the specified device.
- Convert some dbgprintf() to dbg().

Kernel/FileSystem/VirtualFileSystem.cpp

@@ -36,21 +36,25 @@ InodeIdentifier VFS::root_inode_id() const
     return m_root_inode->identifier();
 }
 
-bool VFS::mount(NonnullRefPtr<FS>&& file_system, StringView path)
+KResult VFS::mount(NonnullRefPtr<FS>&& file_system, Custody& mount_point)
+{
+    auto& inode = mount_point.inode();
+    dbg() << "VFS: Mounting " << file_system->class_name() << " at " << mount_point.absolute_path() << " (inode: " << inode.identifier() << ")";
+    // FIXME: check that this is not already a mount point
+    auto mount = make<Mount>(mount_point, move(file_system));
+    m_mounts.append(move(mount));
+    mount_point.did_mount_on({});
+    return KSuccess;
+}
+
+KResult VFS::mount(NonnullRefPtr<FS>&& file_system, StringView path)
 {
     auto result = resolve_path(path, root_custody());
     if (result.is_error()) {
         dbg() << "VFS: mount can't resolve mount point '" << path << "'";
-        return false;
+        return result.error();
     }
-    auto& inode = result.value()->inode();
-    dbg() << "VFS: Mounting " << file_system->class_name() << " at " << path << " (inode: " << inode.identifier() << ")";
-    // FIXME: check that this is not already a mount point
-    auto mount = make<Mount>(*result.value(), move(file_system));
-    m_mounts.append(move(mount));
-
-    result.value()->did_mount_on({});
-    return true;
+    return mount(move(file_system), result.value());
 }
 
 bool VFS::mount_root(NonnullRefPtr<FS>&& file_system)

Kernel/FileSystem/VirtualFileSystem.h

@@ -57,7 +57,8 @@ public:
     ~VFS();
 
     bool mount_root(NonnullRefPtr<FS>&&);
-    bool mount(NonnullRefPtr<FS>&&, StringView path);
+    KResult mount(NonnullRefPtr<FS>&&, StringView path);
+    KResult mount(NonnullRefPtr<FS>&&, Custody& mount_point);
 
     KResultOr<NonnullRefPtr<FileDescription>> open(RefPtr<Device>&&, int options);
     KResultOr<NonnullRefPtr<FileDescription>> open(StringView path, int options, mode_t mode, Custody& base);

Kernel/Process.cpp

@@ -2729,16 +2729,26 @@ int Process::sys$reboot()
 
 int Process::sys$mount(const char* device, const char* mountpoint)
 {
-    dbgprintf("mount: device = %s mountpoint = %s\n", device, mountpoint);
+    if (!is_superuser())
+        return -EPERM;
+
     if (!validate_read_str(device) || !validate_read_str(mountpoint))
         return -EFAULT;
 
+    dbg() << "mount: device " << device << " @ " << mountpoint;
+
+    auto custody_or_error = VFS::the().resolve_path(mountpoint, current_directory());
+    if (custody_or_error.is_error())
+        return custody_or_error.error();
+
+    auto& mountpoint_custody = custody_or_error.value();
+
     // Let's do a stat to get some information about the device..
     // Let's use lstat so we can convert devname into a major/minor device pair!
     struct stat st;
     int rc = sys$stat(device, &st);
     if (rc < 0) {
-        dbgprintf("mount: call to sys$stat failed!\n");
+        dbg() << "mount: call to sys$stat failed!";
         return -ENODEV;
     }
 
@@ -2748,7 +2758,7 @@ int Process::sys$mount(const char* device, const char* mountpoint)
     auto* dev = VFS::the().get_device(major, minor);
     auto* disk_device = static_cast<DiskDevice*>(dev);
 
-    dbgprintf("mount: attempting to mount %d,%d to %s...\n", major, minor, mountpoint);
+    dbg() << "mount: attempting to mount device (" << major << "," << minor << ") on " << mountpoint;
 
     // Do a quick check to make sure we're not passing nullptr to Ext2FS::create!
     if (dev == nullptr)
@@ -2757,15 +2767,14 @@ int Process::sys$mount(const char* device, const char* mountpoint)
     // We currently only support ext2. Sorry :^)
     auto ext2fs = Ext2FS::create(*disk_device);
     if (!ext2fs->initialize()) {
-        dbgprintf("mount: failed to create ext2fs!\n");
+        dbg() << "mount: could not find ext2 filesystem on " << device;
         return -ENODEV; // Hmmm, this doesn't seem quite right....
     }
 
     // Let's mount the volume now
-    VFS::the().mount(ext2fs, mountpoint);
-    dbgprintf("mount: successfully mounted ext2 volume on %s to %s!\n", device, mountpoint);
-
-    return ESUCCESS;
+    auto result = VFS::the().mount(ext2fs, mountpoint_custody);
+    dbg() << "mount: successfully mounted " << device << " on " << mountpoint;
+    return result;
 }
 
 ProcessTracer& Process::ensure_tracer()