Don't open-code the guts of replacing current's fs struct.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 fs/fs_struct.c            | 18 ++++++++++++++++++
 include/linux/fs_struct.h |  2 ++
 kernel/fork.c             | 22 ++++++----------------
 3 files changed, 26 insertions(+), 16 deletions(-)

diff --git a/fs/fs_struct.c b/fs/fs_struct.c
index 394875d06fd6..c441586537e7 100644
--- a/fs/fs_struct.c
+++ b/fs/fs_struct.c
@@ -147,6 +147,24 @@ int unshare_fs_struct(void)
 }
 EXPORT_SYMBOL_GPL(unshare_fs_struct);
 
+struct fs_struct *switch_fs_struct(struct fs_struct *new_fs)
+{
+	struct fs_struct *fs;
+
+	scoped_guard(task_lock, current) {
+		fs = current->fs;
+		read_seqlock_excl(&fs->seq);
+		current->fs = new_fs;
+		if (--fs->users)
+			new_fs = NULL;
+		else
+			new_fs = fs;
+		read_sequnlock_excl(&fs->seq);
+	}
+
+	return new_fs;
+}
+
 /* to be mentioned only in INIT_TASK */
 struct fs_struct init_fs = {
 	.users		= 1,
diff --git a/include/linux/fs_struct.h b/include/linux/fs_struct.h
index 0070764b790a..ade459383f92 100644
--- a/include/linux/fs_struct.h
+++ b/include/linux/fs_struct.h
@@ -40,6 +40,8 @@ static inline void get_fs_pwd(struct fs_struct *fs, struct path *pwd)
 	read_sequnlock_excl(&fs->seq);
 }
 
+struct fs_struct *switch_fs_struct(struct fs_struct *new_fs);
+
 extern bool current_chrooted(void);
 
 static inline int current_umask(void)
diff --git a/kernel/fork.c b/kernel/fork.c
index 65113a304518..67e57ee44548 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -3123,7 +3123,7 @@ static int unshare_fd(unsigned long unshare_flags, struct files_struct **new_fdp
  */
 int ksys_unshare(unsigned long unshare_flags)
 {
-	struct fs_struct *fs, *new_fs = NULL;
+	struct fs_struct *new_fs = NULL;
 	struct files_struct *new_fd = NULL;
 	struct cred *new_cred = NULL;
 	struct nsproxy *new_nsproxy = NULL;
@@ -3198,23 +3198,13 @@ int ksys_unshare(unsigned long unshare_flags)
 		if (new_nsproxy)
 			switch_task_namespaces(current, new_nsproxy);
 
-		task_lock(current);
+		if (new_fs)
+			new_fs = switch_fs_struct(new_fs);
 
-		if (new_fs) {
-			fs = current->fs;
-			read_seqlock_excl(&fs->seq);
-			current->fs = new_fs;
-			if (--fs->users)
-				new_fs = NULL;
-			else
-				new_fs = fs;
-			read_sequnlock_excl(&fs->seq);
-		}
-
-		if (new_fd)
+		if (new_fd) {
+			guard(task_lock)(current);
 			swap(current->files, new_fd);
-
-		task_unlock(current);
+		}
 
 		if (new_cred) {
 			/* Install the new user namespace */

-- 
2.47.3