Re: [PATCH 24/32] vfs: syscall: Add fsopen() to prepare for superblock creation [ver #9]

[Andy Lutomirski]

> On Jul 12, 2018, at 5:03 PM, David Howells <dhowells@xxxxxxxxxx> wrote:
> 
> Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> 
>>>> I tend to think that this *should* fail using the new API.  The semantics
>>>> of the second mount request are bizarre at best.
>>> 
>>> You still have to support existing behaviour lest you break userspace.
>>> 
>> 
>> I assume the existing behavior is that a bind mount is created?  If so, the
>> new mount(8) tool could do it in user code.
> 
> You have a race there.
> 
> Also you can't currently directly create a bind mount from userspace as you
> can only bind from another path point - which you may not be able to access
> (either by permission failure or because it's not in your mount namespace).
> 

Are you trying to preserve the magic bind semantics with the new API?  If you are, I think it should be by explicit opt in only. Otherwise you risk having your shiny new way to specify fs options get ignored when the magic bind mount happens.