linux-kernel - [PATCH v2 1/2] fs/super: function to prevent super re-use

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Message-Id: <20220512082832.v2.1.I0e579520b03aa244906b8fe2ef1ec63f2ab7eecf@changeid>
Date:   Thu, 12 May 2022 08:29:09 +1000
From:   Daniil Lunev <dlunev@...omium.org>
To:     linux-fsdevel@...r.kernel.org
Cc:     hch@...radead.org, fuse-devel@...ts.sourceforge.net, tytso@....edu,
        miklos@...redi.hu, viro@...iv.linux.org.uk,
        linux-kernel@...r.kernel.org, Daniil Lunev <dlunev@...omium.org>
Subject: [PATCH v2 1/2] fs/super: function to prevent super re-use

The function is called from generic_shutdown_super upon super
destruction, but also can be called by a filesystem during unmount to
ensure a new superblock is created upon the next mount. This is
necessary for the cases, where force unmount of a filesystem renders
superblock disfunctional, in a state that can not be re-used (e.g. FUSE
severes the connection of the userspace daemon to the superblock which
prevents any further communications between them).

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

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

 fs/super.c         | 51 ++++++++++++++++++++++++++++++++++++----------
 include/linux/fs.h |  1 +
 2 files changed, 41 insertions(+), 11 deletions(-)

diff --git a/fs/super.c b/fs/super.c
index f1d4a193602d6..f23e45434de15 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -422,6 +422,45 @@ bool trylock_super(struct super_block *sb)
 	return false;
 }
 
+/**
+ *	retire_locked_super	-	prevent superblock from re-use
+ *	@sb: superblock that shouldn't be re-used
+ *
+ *	retire_locked_super excludes the super from the list of superblocks and
+ *	removes the link between superblock and it bdi. After a call to this
+ *	function, any subsequent mount will allocate a new superblock, even if
+ *	the retired one would have been re-used otherwise. It is safe to call
+ *	the function multiple times for the same superblock, any subsequent
+ *	invocations after the first one have no effect.
+ */
+static void retire_locked_super(struct super_block *sb)
+{
+	spin_lock(&sb_lock);
+	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)
+			bdi_unregister(sb->s_bdi);
+		bdi_put(sb->s_bdi);
+		sb->s_bdi = &noop_backing_dev_info;
+	}
+}
+
+/**
+ *	retire_super	-	prevent superblock from re-use
+ *	@sb: superblock that shouldn't be re-used
+ *
+ *	Variant of retire_locked_super, except that superblock is *not* locked
+ *	by caller.
+ */
+void retire_super(struct super_block *sb)
+{
+	down_write(&sb->s_umount);
+	retire_locked_super(sb);
+}
+EXPORT_SYMBOL(retire_super);
+
 /**
  *	generic_shutdown_super	-	common helper for ->kill_sb()
  *	@sb: superblock to kill
@@ -467,17 +506,7 @@ void generic_shutdown_super(struct super_block *sb)
 			   sb->s_id);
 		}
 	}
-	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)
-			bdi_unregister(sb->s_bdi);
-		bdi_put(sb->s_bdi);
-		sb->s_bdi = &noop_backing_dev_info;
-	}
+	retire_locked_super(sb);
 }
 
 EXPORT_SYMBOL(generic_shutdown_super);
diff --git a/include/linux/fs.h b/include/linux/fs.h
index bbde95387a23a..dc9151b7f0102 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -2424,6 +2424,7 @@ 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