Kernel: Implement addfchdir spawn file action

Changes current directory via fd. Validates the fd is a directory
and checks for execute (search) permission.

Author: firatkizilboga

Kernel/Syscalls/posix_spawn.cpp

@@ -125,6 +125,24 @@ ErrorOr<void> Process::execute_file_actions(ReadonlyBytes file_actions_data)
             });
             break;
         }
+        case SpawnFileActionType::Fchdir: {
+            if (header->record_length < sizeof(SpawnFileActionFchdir))
+                return EINVAL;
+
+            auto const* action = reinterpret_cast<SpawnFileActionFchdir const*>(header);
+            auto description = TRY(open_file_description(action->fd));
+            if (!description->is_directory())
+                return ENOTDIR;
+
+            // Check for search (+x) permission on the directory
+            if (!description->metadata().may_execute(credentials()))
+                return EACCES;
+
+            m_current_directory.with([&](auto& current_directory) {
+                current_directory = description->custody();
+            });
+            break;
+        }
         default:
             return EINVAL;
         }
@@ -157,7 +175,7 @@ ErrorOr<FlatPtr> Process::sys$posix_spawn(Userspace<Syscall::SC_posix_spawn_para
     OwnPtr<KBuffer> file_actions_buffer;
     if (params.serialized_file_actions_data.ptr() != 0 && params.serialized_file_actions_data_size != 0) {
         // Check for unsupported file actions
-        constexpr u8 SUPPORTED_SPAWN_ACTIONS = (1 << static_cast<u8>(SpawnFileActionType::Dup2)) | (1 << static_cast<u8>(SpawnFileActionType::Close)) | (1 << static_cast<u8>(SpawnFileActionType::Open)) | (1 << static_cast<u8>(SpawnFileActionType::Chdir));
+        constexpr u8 SUPPORTED_SPAWN_ACTIONS = (1 << static_cast<u8>(SpawnFileActionType::Dup2)) | (1 << static_cast<u8>(SpawnFileActionType::Close)) | (1 << static_cast<u8>(SpawnFileActionType::Open)) | (1 << static_cast<u8>(SpawnFileActionType::Chdir)) | (1 << static_cast<u8>(SpawnFileActionType::Fchdir));
         if (params.file_action_types_present & ~SUPPORTED_SPAWN_ACTIONS)
             return ENOTSUP;