linux-kernel - Re: BUG: Mount ignores mount options

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Date:   Fri, 10 Aug 2018 13:00:22 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     "Theodore Y. Ts'o" <tytso@....edu>
Cc:     David Howells <dhowells@...hat.com>, viro@...iv.linux.org.uk,
        John Johansen <john.johansen@...onical.com>,
        Tejun Heo <tj@...nel.org>, selinux@...ho.nsa.gov,
        Paul Moore <paul@...l-moore.com>,
        Li Zefan <lizefan@...wei.com>, linux-api@...r.kernel.org,
        apparmor@...ts.ubuntu.com,
        Casey Schaufler <casey@...aufler-ca.com>, fenghua.yu@...el.com,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Eric Biggers <ebiggers@...gle.com>,
        linux-security-module@...r.kernel.org,
        Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        Johannes Weiner <hannes@...xchg.org>,
        Stephen Smalley <sds@...ho.nsa.gov>,
        tomoyo-dev-en@...ts.sourceforge.jp, cgroups@...r.kernel.org,
        torvalds@...ux-foundation.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, Miklos Szeredi <miklos@...redi.hu>
Subject: Re: BUG: Mount ignores mount options

"Theodore Y. Ts'o" <tytso@....edu> writes:

> On Fri, Aug 10, 2018 at 04:11:31PM +0100, David Howells wrote:
>> 
>> Yes.  Since you *absolutely* *insist* on this being fixed *right* *now* *or*
>> *else*, I'm working up a set of additional patches to give userspace the
>> option of whether they want no sharing; sharing, but only with exactly the
>> same parameters; or to ignore the parameter differences and just accept
>> sharing of what's already already mounted (ie. the current behaviour).
>
> But there's no way to support "no sharing", at least not in the
> general case.  A file system can only be mounted once, and without
> file system support, there's no way for a file system to be mounted
> with the bsddf or minixdf mount simultaneously.
>
> Even *with* file system support, there's no way today for the VFS to
> keep track of whether a pathname resolution came through one
> mountpoint or another, so I can't do something like this:
>
> 	mount /dev/sdXX -o casefold /android-data
> 	mount /dev/sdXX -o nocasefold /android-data-2
>
> Which is a pity, since if we could we could much more easily get rid
> of the horror which is Android's wrapfs...
>
> So if the file system has been mounted with one set of mount options,
> and you want to try to mount it with a conflicting set of mount
> options and you don't want it to silently ignore the mount options,
> the *only* thing we can today is to refuse the mount and return an
> error.  
>
> I'm not sure Eric would really consider that an improvement for the
> container use case....

I think I would consider it an improvement.  I keep running into cases
where the mount options differed and something was done silently and
that causes problems.

Eric