[PATCH v3 1/2] fs/super: function to prevent super re-use

[Daniil Lunev <dlunev@chromium.org>]

The function is to be called from filesystem-specific code to mark a
superblock to be ignored by superblock test and thus never re-used. The
function also unregisters bdi if the bdi is per-superblock to avoid
collision if a new superblock is created to represent the filesystem.
generic_shutdown_super() skips unregistering bdi for a retired
superlock as it assumes retire function has already done it.

Signed-off-by: Daniil Lunev <dlunev@chromium.org>
---

Changes in v3:
- Back to state tracking from v1
- Use s_iflag to mark superblocked ignored
- Only unregister private bdi in retire, without freeing

Changes in v2:
- Remove super from list of superblocks instead of using a flag

 fs/super.c         | 32 ++++++++++++++++++++++++++++----
 include/linux/fs.h |  2 ++
 2 files changed, 30 insertions(+), 4 deletions(-)

diff --git a/fs/super.c b/fs/super.c
index f1d4a193602d6..cb092fc6d6d34 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -422,6 +422,30 @@ bool trylock_super(struct super_block *sb)
 	return false;
 }
 
+/**
+ *	retire_super	-	prevernts superblock from being reused
+ *	@sb: superblock to retire
+ *
+ *	The function marks superblock to be ignored in superblock test, which
+ *	prevents it from being reused for any new mounts. If the superblock has
+ *	a private bdi, it also unregisters it, but doesn't reduce the refcount
+ *	of the superblock to prevent potential races. The refcount is reduced
+ *	by generic_shutdown_super(). The function can not be called concurrently
+ *	with generic_shutdown_super(). It is safe to call the function multiple
+ *	times, subsequent calls have no effect.
+ */
+void retire_super(struct super_block *sb)
+{
+	down_write(&sb->s_umount);
+	if (sb->s_bdi != &noop_backing_dev_info) {
+		if (sb->s_iflags & SB_I_PERSB_BDI && !(sb->s_iflags & SB_I_RETIRED))
+			bdi_unregister(sb->s_bdi);
+	}
+	sb->s_iflags |= SB_I_RETIRED;
+	up_write(&sb->s_umount);
+}
+EXPORT_SYMBOL(retire_super);
+
 /**
  *	generic_shutdown_super	-	common helper for ->kill_sb()
  *	@sb: superblock to kill
@@ -468,12 +492,12 @@ void generic_shutdown_super(struct super_block *sb)
 		}
 	}
 	spin_lock(&sb_lock);
-	/* should be initialized for __put_super_and_need_restart() */
 	hlist_del_init(&sb->s_instances);
 	spin_unlock(&sb_lock);
 	up_write(&sb->s_umount);
 	if (sb->s_bdi != &noop_backing_dev_info) {
-		if (sb->s_iflags & SB_I_PERSB_BDI)
+		/* retire should have already unregistered bdi */
+		if (sb->s_iflags & SB_I_PERSB_BDI && !(sb->s_iflags & SB_I_RETIRED))
 			bdi_unregister(sb->s_bdi);
 		bdi_put(sb->s_bdi);
 		sb->s_bdi = &noop_backing_dev_info;
@@ -1216,7 +1240,7 @@ static int set_bdev_super_fc(struct super_block *s, struct fs_context *fc)
 
 static int test_bdev_super_fc(struct super_block *s, struct fs_context *fc)
 {
-	return s->s_bdev == fc->sget_key;
+	return !(s->s_iflags & SB_I_RETIRED) && s->s_bdev == fc->sget_key;
 }
 
 /**
@@ -1307,7 +1331,7 @@ EXPORT_SYMBOL(get_tree_bdev);
 
 static int test_bdev_super(struct super_block *s, void *data)
 {
-	return (void *)s->s_bdev == data;
+	return !(s->s_iflags & SB_I_RETIRED) && (void *)s->s_bdev == data;
 }
 
 struct dentry *mount_bdev(struct file_system_type *fs_type,
diff --git a/include/linux/fs.h b/include/linux/fs.h
index bbde95387a23a..ef392fd2361bd 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1411,6 +1411,7 @@ extern int send_sigurg(struct fown_struct *fown);
 #define SB_I_SKIP_SYNC	0x00000100	/* Skip superblock at global sync */
 #define SB_I_PERSB_BDI	0x00000200	/* has a per-sb bdi */
 #define SB_I_TS_EXPIRY_WARNED 0x00000400 /* warned about timestamp range expiry */
+#define SB_I_RETIRED	0x00000800	/* superblock shouldn't be reused */
 
 /* Possible states of 'frozen' field */
 enum {
@@ -2424,6 +2425,7 @@ extern struct dentry *mount_nodev(struct file_system_type *fs_type,
 	int flags, void *data,
 	int (*fill_super)(struct super_block *, void *, int));
 extern struct dentry *mount_subtree(struct vfsmount *mnt, const char *path);
+void retire_super(struct super_block *sb);
 void generic_shutdown_super(struct super_block *sb);
 void kill_block_super(struct super_block *sb);
 void kill_anon_super(struct super_block *sb);
-- 
2.31.0