Re: [PATCH 11/11] btrfs: use automount to bind-mount all subvol roots.

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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



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