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

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Message-Id: <5A7CC3BF-5F4E-4624-A129-4BD0434D0747@amacapital.net>
Date:   Thu, 12 Jul 2018 17:24:36 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     David Howells <dhowells@...hat.com>
Cc:     "Theodore Y. Ts'o" <tytso@....edu>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Lutomirski <luto@...nel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Linux API <linux-api@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Jann Horn <jannh@...gle.com>
Subject: Re: [PATCH 24/32] vfs: syscall: Add fsopen() to prepare for superblock creation [ver #9]



> On Jul 12, 2018, at 5:03 PM, David Howells <dhowells@...hat.com> wrote:
> 
> Andy Lutomirski <luto@...capital.net> 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.