Re: [RFC][PATCH 0/76] vfs: 'views' for filesystems with more than one root

[Amir Goldstein <amir73il@gmail.com>]

On Thu, Jun 7, 2018 at 12:19 AM, Jeff Mahoney <jeffm@suse.com> wrote:
[...]
>>
>> FYI, the Overlayfs file/inode mapping is about to change with many
>> VFS hacks queued for removal, so stay tuned.
>>
>> [...]
>
> I have to admit I'm curious how this will work.  I've heard rumor of
> using overlayfs inodes and calling the underlying file system's
> inode_operations.  If part of that work removes the danger of overlayfs
> inode numbers colliding with xino mode, I'm definitely interested.

See https://marc.info/?l=linux-fsdevel&m=152760014530531&w=2

It doesn't remove the need to maintain a unique and persistent inode
number namespace in overlayfs. It just reduces exposure of the underlying
inode to VFS.

[...]

>>> - Audit.  As it happens, most of audit has a path or file that can be
>>> used.  We do run into problems with fsnotify.  fsnotify_move is called
>>> from vfs_rename which turns into a can of worms pretty quickly.
>>>
>>
>> Can you please elaborate on that problem.
>> Do you mean when watching a directory for changes, you need to
>> be able to tell in which fs_view the directory inode that is being watched?
>
> I was investigating whether Dave's suggestion of using a vfsmount was
> feasible.  When following the audit call graph up, I found
> fsnotify_move, called by vfs_rename.  Piping a vfsmount into the vfs_*
> operations has historically been rejected by Al (see Apparmor
> discussions, among others), and with good reason.  The file system
> implementation shouldn't care about where it's mounted.  While piping it
> into vfs_rename doesn't seem too invasive, it's called by various file
> systems' ->rename callback, so then we're stuck piping vfsmounts into
> inode_operations, which is what Al's been wanting to avoid for years.
>

Yes, I've heard about this objection, thought didn't have a reference.

[...]
>>
>> I have an interest of solving another problem.
>> In VFS operations where only inode is available, I would like to be able to
>> report fsnotify events (e.g. fsnotify_move()) only in directories under a
>> certain subtree root. That could be achieved either by bind mount the subtree
>> root and passing vfsmount into vfs_rename() or by defining an fs_view on the
>> subtree and mounting that fs_view.
>
> I'm not sure there's a lot of overlap, but I expect that this will end
> up running into the same review feedback that Al gave during the
> AppArmor merge: vfsmounts have no business at the lower level and you
> can get the same behavior by hooking in at a higher level.  See
> security_path_* vs security_inode_* for how that was resolved.
>
> What you're talking about isn't really what we had in mind for the
> fs_view.  In our case, it sits between the inode and superblock, which
> would be at too low a level for determining whether an inode is under a
> certain subtree.  In any event, wouldn't you need a path instead of an
> inode to do what you're proposing?
>

Not if the fs_view could have a root that is different than sb root
then I could attach fsnotify mark to fs_view instead of say vfsmount
and I have the information I need inside fsnotify_move().

Thanks,
Amir.