Re: [PATCHv2 1/5] VFS: DazukoFS, stackable-fs, file access control

[John Ogness <dazukocode@ogness.net>]

On 2009-02-12, Jan Engelhardt <jengelh@medozas.de> wrote:
>>+DazukoFS is typically mounted on top of an existing directory. For example,
>>+to stack DazukoFS on top of the /opt directory, the following mount(8)
>>+command can be given:
>>+
>>+# mount -t dazukofs /opt /opt
> [...]
>>+# mount -t dazukofs /opt /mnt
>>+
>>+When accessing files within /mnt, you will be accessing files in /opt
>>+(through DazukoFS). When accessing files directly in /opt, DazukoFS will not
>>+be involved.
>>+
>>+THIS HAS POTENTIAL PROBLEMS!
>
> This ought to be done better in light of the problem that one
> could issue `mount -t dazukofs /mnt /opt`
> In kernel code, I'm thinking something like
>
> static int foofs_dothis(...)
> {
> 	buf = call d_path on sb->s_root
> 	call namei_lookup on buf
> 	use lookup's resulting dentry as source directory
> }
>
> This assumes at least that namei_lookup will not return the new SB's
> dentry for the given path.
> Iff that works out somehow, the 'device' part in the mount command
> line is practically ignored like it is for like procfs.

If we want to forbid users from being able to do this, I think it
would be simpler just to fail the mount if it is _not_ specified for
direct stacking. This forces users to use a syntax that makes it very
clear what they are doing.

Since mounting to a different directory could cause problems, I think
it will be good to forbid it. I will make this change for the next
patch series.

John Ogness