Re: [PATCH 2/3] ecryptfs: use private mount in path

From: Miklos Szeredi <miklos@szeredi.hu>
To: Christian Brauner <christian.brauner@ubuntu.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>,
	Christian Brauner <brauner@kernel.org>,
	Tyler Hicks <code@tyhicks.com>,
	ecryptfs@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	Amir Goldstein <amir73il@gmail.com>
Subject: Re: [PATCH 2/3] ecryptfs: use private mount in path
Date: Mon, 12 Apr 2021 10:53:00 +0200	[thread overview]
Message-ID: <CAJfpegsONBtvy+ykW2jy6e903wFqXaYvJ_pYSNSQ=eiPc=MzFQ@mail.gmail.com> (raw)
In-Reply-To: <20210410123037.qlfjyv55ncdjpwn3@wittgenstein>

On Sat, Apr 10, 2021 at 2:30 PM Christian Brauner
<christian.brauner@ubuntu.com> wrote:
>
> On Sat, Apr 10, 2021 at 12:31:02AM +0000, Al Viro wrote:
> > On Fri, Apr 09, 2021 at 06:24:21PM +0200, Christian Brauner wrote:
> >
> > > Reading through the codebase of ecryptfs it currently takes path->mnt
> > > and then retrieves that path whenever it needs to perform operations in
> > > the underlying filesystem. Simply drop the old path->mnt once we've
> > > created a private mount and place the new private mnt into path->mnt.
> > > This should be all that is needed to make this work since ecryptfs uses
> > > the same lower path's vfsmount to construct the paths it uses to operate
> > > on the underlying filesystem.
> >
> > > +   mnt = clone_private_mount(&path);
> >
> > Incidentally, why is that thing anything other than a trivial wrapper
> > for mnt_clone_internal() (if that - I'm not convinced that the check of
> > unbindable is the right thing to do here).  Miklos?
>
> The unbindable check is irrelevant at least for both ecryptfs and for
> the corresponding cachefiles change I sent out since they don't care
> about it.
> In practice it doesn't matter to be honest. MS_UNBINDABLE is wildly
> esoteric in userspace (We had a glaring bug with that some time ago that
> went completely unnoticed for years.). Especially unlikely to be used
> for a users home directory (ecryptfs) or /var/cache/fscache
> (cachefiles). So even by leaving this check in it's very unlikely for
> any regressions to appear.
>
> I hadn't seen mnt_clone_internal() but it's different in so far as it
> sets MNT_INTERNAL whereas clone_private_mount() uses MNT_NS_INTERNAL.
> Which points me to another potential problem here:
> clone_private_mount() seems to want kern_unmount() to be called instead
> of just a simple mntput()?

Yes, that's stated in a comment in the clone_private_mount() helper.

The difference is that short term mounts take a small penalty on each
mntput(), while longterm mounts take a fairly large penalty on
kern_unmount().  It's just a performance thing, AFAIK.

As for MS_UNBINDABLE, my recollection is that it was copy-pasted from
regular bind mount.  I agree that it can be moved to overlayfs (or
removed altogether, with some thought into what MS_UNBINDABLE actually
is used for).

Thanks,
Miklos