init: handle idmapped mounts

Enable the init helpers to handle idmapped mounts by passing down the
mount's user namespace. If the initial user namespace is passed nothing
changes so non-idmapped mounts will see identical behavior as before.

Link: https://lore.kernel.org/r/20210121131959.646623-21-christian.brauner@ubuntu.com
Cc: Christoph Hellwig <hch@lst.de>
Cc: David Howells <dhowells@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: linux-fsdevel@vger.kernel.org
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

fs/init.c

@@ -157,8 +157,8 @@ int __init init_mknod(const char *filename, umode_t mode, unsigned int dev)
 		mode &= ~current_umask();
 	error = security_path_mknod(&path, dentry, mode, dev);
 	if (!error)
-		error = vfs_mknod(&init_user_ns, path.dentry->d_inode, dentry,
-				  mode, new_decode_dev(dev));
+		error = vfs_mknod(mnt_user_ns(path.mnt), path.dentry->d_inode,
+				  dentry, mode, new_decode_dev(dev));
 	done_path_create(&path, dentry);
 	return error;
 }
@@ -167,6 +167,7 @@ int __init init_link(const char *oldname, const char *newname)
 {
 	struct dentry *new_dentry;
 	struct path old_path, new_path;
+	struct user_namespace *mnt_userns;
 	int error;
 
 	error = kern_path(oldname, 0, &old_path);
@@ -181,14 +182,15 @@ int __init init_link(const char *oldname, const char *newname)
 	error = -EXDEV;
 	if (old_path.mnt != new_path.mnt)
 		goto out_dput;
-	error = may_linkat(&init_user_ns, &old_path);
+	mnt_userns = mnt_user_ns(new_path.mnt);
+	error = may_linkat(mnt_userns, &old_path);
 	if (unlikely(error))
 		goto out_dput;
 	error = security_path_link(old_path.dentry, &new_path, new_dentry);
 	if (error)
 		goto out_dput;
-	error = vfs_link(old_path.dentry, &init_user_ns,
-			 new_path.dentry->d_inode, new_dentry, NULL);
+	error = vfs_link(old_path.dentry, mnt_userns, new_path.dentry->d_inode,
+			 new_dentry, NULL);
 out_dput:
 	done_path_create(&new_path, new_dentry);
 out:
@@ -207,8 +209,8 @@ int __init init_symlink(const char *oldname, const char *newname)
 		return PTR_ERR(dentry);
 	error = security_path_symlink(&path, dentry, oldname);
 	if (!error)
-		error = vfs_symlink(&init_user_ns, path.dentry->d_inode, dentry,
-				    oldname);
+		error = vfs_symlink(mnt_user_ns(path.mnt), path.dentry->d_inode,
+				    dentry, oldname);
 	done_path_create(&path, dentry);
 	return error;
 }
@@ -231,8 +233,8 @@ int __init init_mkdir(const char *pathname, umode_t mode)
 		mode &= ~current_umask();
 	error = security_path_mkdir(&path, dentry, mode);
 	if (!error)
-		error = vfs_mkdir(&init_user_ns, path.dentry->d_inode, dentry,
-				  mode);
+		error = vfs_mkdir(mnt_user_ns(path.mnt), path.dentry->d_inode,
+				  dentry, mode);
 	done_path_create(&path, dentry);
 	return error;
 }