[PATCH v5 15/42] fs: add file_user_ns() helper

[Christian Brauner]

Add a simple helper to retrieve the user namespace associated with the
vfsmount of a file. Christoph correctly points out that this makes
codepaths (e.g. ioctls) way easier to follow that would otherwise
dereference via mnt_user_ns(file->f_path.mnt).

In order to make file_user_ns() static inline we'd need to include
mount.h in either file.h or fs.h which seems undesirable so let's simply
not force file_user_ns() to be inline.

Cc: David Howells <dhowells at redhat.com>
Cc: Al Viro <viro at zeniv.linux.org.uk>
Cc: linux-fsdevel at vger.kernel.org
Suggested-by: Christoph Hellwig <hch at lst.de>
Signed-off-by: Christian Brauner <christian.brauner at ubuntu.com>
---
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/* v5 */
patch introduced
base-commit: 7c53f6b671f4aba70ff15e1b05148b10d58c2837
---
 fs/namei.c         | 6 ++++++
 include/linux/fs.h | 1 +
 2 files changed, 7 insertions(+)

diff --git a/fs/namei.c b/fs/namei.c
index cd124e18cec5..d8dee449e92a 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -259,6 +259,12 @@ void putname(struct filename *name)
 		__putname(name);
 }
 
+struct user_namespace *file_user_ns(struct file *file)
+{
+	return mnt_user_ns(file->f_path.mnt);
+}
+EXPORT_SYMBOL(file_user_ns);
+
 /**
  * check_acl - perform ACL permission checking
  * @mnt_userns:	user namespace of the mount the inode was found from
diff --git a/include/linux/fs.h b/include/linux/fs.h
index ef993857682b..89e41a821bad 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -2585,6 +2585,7 @@ static inline struct file *file_clone_open(struct file *file)
 	return dentry_open(&file->f_path, file->f_flags, file->f_cred);
 }
 extern int filp_close(struct file *, fl_owner_t id);
+extern struct user_namespace *file_user_ns(struct file *file);
 
 extern struct filename *getname_flags(const char __user *, int, int *);
 extern struct filename *getname(const char __user *);
-- 
2.30.0