Re: BUG: Mount ignores mount options

From: ebiederm@xmission.com (Eric W. Biederman)
To: "Theodore Y. Ts'o" <tytso@mit.edu>
Cc: David Howells <dhowells@redhat.com>,
	viro@zeniv.linux.org.uk,
	John Johansen <john.johansen@canonical.com>,
	Tejun Heo <tj@kernel.org>,
	selinux@tycho.nsa.gov, Paul Moore <paul@paul-moore.com>,
	Li Zefan <lizefan@huawei.com>,
	linux-api@vger.kernel.org, apparmor@lists.ubuntu.com,
	Casey Schaufler <casey@schaufler-ca.com>,
	fenghua.yu@intel.com,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Eric Biggers <ebiggers@google.com>,
	linux-security-module@vger.kernel.org,
	Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>,
	Johannes Weiner <hannes@cmpxchg.org>,
	Stephen Smalley <sds@tycho.nsa.gov>,
	tomoyo-dev-en@lists.sourceforge.jp, cgroups@vger.kernel.org,
	torvalds@linux-foundation.org, linux-fsdevel@vger.kernel.org,
	linux-kernel@vger.kernel.org, Miklos Szeredi <miklos@szeredi.hu>
Subject: Re: BUG: Mount ignores mount options
Date: Fri, 10 Aug 2018 13:00:22 -0500	[thread overview]
Message-ID: <87mutunlu1.fsf@xmission.com> (raw)
In-Reply-To: <20180810153902.GH21087@thunk.org> (Theodore Y. Ts'o's message of "Fri, 10 Aug 2018 11:39:02 -0400")

"Theodore Y. Ts'o" <tytso@mit.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