fs: refactor do_mount

Factor out a path_mount helper that takes a struct path * instead of the
actual file name.  This will allow to convert the init and devtmpfs code
to properly mount based on a kernel pointer instead of relying on the
implicit set_fs(KERNEL_DS) during early init.
Signed-off-by: Christoph Hellwig <hch@lst.de>

fs/namespace.c

@@ -3115,12 +3115,11 @@ char *copy_mount_string(const void __user *data)
  * Therefore, if this magic number is present, it carries no information
  * and must be discarded.
  */
-long do_mount(const char *dev_name, const char __user *dir_name,
+static int path_mount(const char *dev_name, struct path *path,
 		const char *type_page, unsigned long flags, void *data_page)
 {
-	struct path path;
 	unsigned int mnt_flags = 0, sb_flags;
-	int retval = 0;
+	int ret;
 
 	/* Discard magic */
 	if ((flags & MS_MGC_MSK) == MS_MGC_VAL)
@@ -3133,19 +3132,13 @@ long do_mount(const char *dev_name, const char __user *dir_name,
 	if (flags & MS_NOUSER)
 		return -EINVAL;
 
-	/* ... and get the mountpoint */
-	retval = user_path_at(AT_FDCWD, dir_name, LOOKUP_FOLLOW, &path);
-	if (retval)
-		return retval;
-
-	retval = security_sb_mount(dev_name, &path,
-				   type_page, flags, data_page);
-	if (!retval && !may_mount())
-		retval = -EPERM;
-	if (!retval && (flags & SB_MANDLOCK) && !may_mandlock())
-		retval = -EPERM;
-	if (retval)
-		goto dput_out;
+	ret = security_sb_mount(dev_name, path, type_page, flags, data_page);
+	if (ret)
+		return ret;
+	if (!may_mount())
+		return -EPERM;
+	if ((flags & SB_MANDLOCK) && !may_mandlock())
+		return -EPERM;
 
 	/* Default to relatime unless overriden */
 	if (!(flags & MS_NOATIME))
@@ -3172,7 +3165,7 @@ long do_mount(const char *dev_name, const char __user *dir_name,
 	    ((flags & (MS_NOATIME | MS_NODIRATIME | MS_RELATIME |
 		       MS_STRICTATIME)) == 0)) {
 		mnt_flags &= ~MNT_ATIME_MASK;
-		mnt_flags |= path.mnt->mnt_flags & MNT_ATIME_MASK;
+		mnt_flags |= path->mnt->mnt_flags & MNT_ATIME_MASK;
 	}
 
 	sb_flags = flags & (SB_RDONLY |
@@ -3185,22 +3178,32 @@ long do_mount(const char *dev_name, const char __user *dir_name,
 			    SB_I_VERSION);
 
 	if ((flags & (MS_REMOUNT | MS_BIND)) == (MS_REMOUNT | MS_BIND))
-		retval = do_reconfigure_mnt(&path, mnt_flags);
-	else if (flags & MS_REMOUNT)
-		retval = do_remount(&path, flags, sb_flags, mnt_flags,
-				    data_page);
-	else if (flags & MS_BIND)
-		retval = do_loopback(&path, dev_name, flags & MS_REC);
-	else if (flags & (MS_SHARED | MS_PRIVATE | MS_SLAVE | MS_UNBINDABLE))
-		retval = do_change_type(&path, flags);
-	else if (flags & MS_MOVE)
-		retval = do_move_mount_old(&path, dev_name);
-	else
-		retval = do_new_mount(&path, type_page, sb_flags, mnt_flags,
-				      dev_name, data_page);
-dput_out:
+		return do_reconfigure_mnt(path, mnt_flags);
+	if (flags & MS_REMOUNT)
+		return do_remount(path, flags, sb_flags, mnt_flags, data_page);
+	if (flags & MS_BIND)
+		return do_loopback(path, dev_name, flags & MS_REC);
+	if (flags & (MS_SHARED | MS_PRIVATE | MS_SLAVE | MS_UNBINDABLE))
+		return do_change_type(path, flags);
+	if (flags & MS_MOVE)
+		return do_move_mount_old(path, dev_name);
+
+	return do_new_mount(path, type_page, sb_flags, mnt_flags, dev_name,
+			    data_page);
+}
+
+long do_mount(const char *dev_name, const char __user *dir_name,
+		const char *type_page, unsigned long flags, void *data_page)
+{
+	struct path path;
+	int ret;
+
+	ret = user_path_at(AT_FDCWD, dir_name, LOOKUP_FOLLOW, &path);
+	if (ret)
+		return ret;
+	ret = path_mount(dev_name, &path, type_page, flags, data_page);
 	path_put(&path);
-	return retval;
+	return ret;
 }
 
 static struct ucounts *inc_mnt_namespaces(struct user_namespace *ns)