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From:   "NeilBrown" <neilb@suse.de>
To:     "Christian Brauner" <christian.brauner@ubuntu.com>
Cc:     "Christoph Hellwig" <hch@infradead.org>,
        "Josef Bacik" <josef@toxicpanda.com>,
        "J. Bruce Fields" <bfields@fieldses.org>,
        "Chuck Lever" <chuck.lever@oracle.com>, "Chris Mason" <clm@fb.com>,
        "David Sterba" <dsterba@suse.com>,
        "Alexander Viro" <viro@zeniv.linux.org.uk>,
        linux-fsdevel@vger.kernel.org, linux-nfs@vger.kernel.org,
        linux-btrfs@vger.kernel.org
Subject: Re: [PATCH 11/11] btrfs: use automount to bind-mount all subvol roots.
In-reply-to: <20210728131213.pgu3r4m4ulozrcav@wittgenstein>
References: <162742539595.32498.13687924366155737575.stgit@noble.brown>,
 <162742546558.32498.1901201501617899416.stgit@noble.brown>,
 <20210728131213.pgu3r4m4ulozrcav@wittgenstein>
Date:   Thu, 29 Jul 2021 10:43:23 +1000
Message-id: <162751940386.21659.17682627731630829061@noble.neil.brown.name>
Precedence: bulk

On Wed, 28 Jul 2021, Christian Brauner wrote:
> 
> Hey Neil,
> 
> Sorry if this is a stupid question but wouldn't you want to copy the
> mount properties from path->mnt here? Couldn't you otherwise use this to
> e.g. suddenly expose a dentry on a read-only mount as read-write?

There are no stupid questions, and this is a particularly non-stupid
one!

I hadn't considered that, but having examined the code I see that it
is already handled.
The vfsmount that d_automount returns is passed to finish_automount(),
which hands it to do_add_mount() together with the mnt_flags for the
parent vfsmount (plus MNT_SHRINKABLE).
do_add_mount() sets up the mnt_flags of the new vfsmount.
In fact, the d_automount interface has no control of these flags at all.
Whatever it sets will be over-written by do_add_mount.

Thanks,
NeilBrown