linux-kernel - Re: [PATCH] binderfs: rework superblock destruction

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Message-ID: <20220817150527.g5l5cwhphx6wdvxr@wittgenstein>
Date:   Wed, 17 Aug 2022 17:05:27 +0200
From:   Christian Brauner <brauner@...nel.org>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Dongliang Mu <mudongliangabcd@...il.com>,
        Dongliang Mu <dzm91@...t.edu.cn>,
        Arve Hjønnevåg <arve@...roid.com>,
        Todd Kjos <tkjos@...roid.com>,
        Martijn Coenen <maco@...roid.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Carlos Llamas <cmllamas@...gle.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        syzkaller <syzkaller@...glegroups.com>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] binderfs: rework superblock destruction

On Wed, Aug 17, 2022 at 03:32:03PM +0100, Al Viro wrote:
> On Wed, Aug 17, 2022 at 03:19:13PM +0100, Al Viro wrote:
> > On Wed, Aug 17, 2022 at 04:01:49PM +0200, Christian Brauner wrote:
> > > On Wed, Aug 17, 2022 at 02:59:02PM +0100, Al Viro wrote:
> > > > On Wed, Aug 17, 2022 at 03:03:06PM +0200, Christian Brauner wrote:
> > > > 
> > > > > +static void binderfs_kill_super(struct super_block *sb)
> > > > > +{
> > > > > +	struct binderfs_info *info = sb->s_fs_info;
> > > > > +
> > > > > +	if (info && info->ipc_ns)
> > > > > +		put_ipc_ns(info->ipc_ns);
> > > > > +
> > > > > +	kfree(info);
> > > > > +	kill_litter_super(sb);
> > > > > +}
> > > > 
> > > > Other way round, please - shut the superblock down, *then*
> > > > free the objects it'd been using.  IOW,
> > > 
> > > I wondered about that but a lot of places do it the other way around.
> > > So maybe the expected order should be documented somewhere.
> > 
> > ???
> > 
> > "If you are holding internal references to dentries/inodes/etc., drop them
> > first; if you are going to free something that is used by filesystem
> > methods, don't do that before the filesystem is shut down"
> > 
> > That's just common sense...  Which filesystems are doing that "the other
> > way around"?
> 
> Note that something like e.g. ramfs, where we have a dynamically allocated
> object ->s_fs_info is pointing to and gets freed early in their ->kill_sb()
> is somewhat misleading - it's used only for two things, one is the
> creation of root directory inode (obviously not going to happen at any
> point after mount) and another - ->show_options().  By the point we get
> around to killing a superblock, it would better *NOT* have mounts pointing
> to it that might show up in /proc/mounts and make us call ->show_options().
> 
> So there we really know that nothing during the shutdown will even look
> at that thing we'd just freed.  Not that there'd ever been a point allocating
> it - all that object contains is one unsigned short, so we might as well
> just have stored (void *)root_mode in ->s_fs_info.  Oh, well...

Binderfs was really the first fs I ever wrote and back then I was trying
to be as close to best practice at possible. One thing I remember being
unclear about was what the best practice for filesystem shutdown would
be. That included ->put_super() vs just ->kill_sb() but also the order
in which kill_litter_super() and sb->s_fs_info cleanup should happen.

For binderfs the order does matter and that's also the reason I
originally decided to use ->put_super() as it's called after
evict_inodes() and gives the required ordering.