[PATCH RFC v2 05/16] fs: add switch_files_struct()

[Christian Brauner <brauner@kernel.org>]

Add switch_files_struct() to install another table on a task. It
consumes the reference to the new table and hands the old one back for
the caller to put. Convert every place that switches a descriptor table.

No functional changes.

Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
---
 fs/file.c               | 28 ++++++++++++++++------------
 include/linux/fdtable.h |  2 ++
 kernel/fork.c           |  6 ++----
 3 files changed, 20 insertions(+), 16 deletions(-)

diff --git a/fs/file.c b/fs/file.c
index 88ede1262b3e..aec2adf59031 100644
--- a/fs/file.c
+++ b/fs/file.c
@@ -515,16 +515,23 @@ void put_files_struct(struct files_struct *files)
 	}
 }
 
-void exit_files(struct task_struct *tsk)
+/* Install @files on @tsk, consuming the reference; returns the old table. */
+struct files_struct *switch_files_struct(struct task_struct *tsk,
+					 struct files_struct *files)
 {
-	struct files_struct * files = tsk->files;
+	struct files_struct *old;
 
-	if (files) {
-		task_lock(tsk);
-		tsk->files = NULL;
-		task_unlock(tsk);
-		put_files_struct(files);
-	}
+	task_lock(tsk);
+	old = tsk->files;
+	tsk->files = files;
+	task_unlock(tsk);
+	return old;
+}
+
+void exit_files(struct task_struct *tsk)
+{
+	if (tsk->files)
+		put_files_struct(switch_files_struct(tsk, NULL));
 }
 
 struct files_struct init_files = {
@@ -855,10 +862,7 @@ SYSCALL_DEFINE3(close_range, unsigned int, fd, unsigned int, max_fd,
 		 * We're done closing the files we were supposed to. Time to install
 		 * the new file descriptor table and drop the old one.
 		 */
-		task_lock(me);
-		me->files = cur_fds;
-		task_unlock(me);
-		put_files_struct(fds);
+		put_files_struct(switch_files_struct(me, cur_fds));
 	}
 
 	return 0;
diff --git a/include/linux/fdtable.h b/include/linux/fdtable.h
index 050f4a04157e..80f192412a5a 100644
--- a/include/linux/fdtable.h
+++ b/include/linux/fdtable.h
@@ -100,6 +100,8 @@ static inline bool close_on_exec(unsigned int fd, const struct files_struct *fil
 struct task_struct;
 
 void put_files_struct(struct files_struct *fs);
+struct files_struct *switch_files_struct(struct task_struct *tsk,
+					 struct files_struct *files);
 int unshare_files(void);
 struct fd_range {
 	unsigned int from, to;
diff --git a/kernel/fork.c b/kernel/fork.c
index 416758c8a3d4..b959191edf52 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -3320,10 +3320,8 @@ int ksys_unshare(unsigned long unshare_flags)
 		if (new_fs)
 			new_fs = switch_fs_struct(new_fs);
 
-		if (new_fd) {
-			guard(task_lock)(current);
-			swap(current->files, new_fd);
-		}
+		if (new_fd)
+			new_fd = switch_files_struct(current, new_fd);
 
 		if (new_cred) {
 			/* Install the new user namespace */

-- 
2.53.0