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Subject: Re: BUG: Mount ignores mount options
To: "Theodore Y. Ts'o" <tytso@mit.edu>,
        David Howells <dhowells@redhat.com>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
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        Stephen Smalley <sds@tycho.nsa.gov>,
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        linux-kernel@vger.kernel.org, Miklos Szeredi <miklos@szeredi.hu>
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 <22361.1533913891@warthog.procyon.org.uk> <20180810153902.GH21087@thunk.org>
Cc: Casey Schaufler <casey@schaufler-ca.com>
From: Casey Schaufler <casey@schaufler-ca.com>
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Date: Fri, 10 Aug 2018 08:55:51 -0700
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On 8/10/2018 8:39 AM, Theodore Y. Ts'o wrote:
> 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....
>
> 						- Ted
>
> P.S.  And as Al has pointed out, this would require special, per-file
> system support to determine whether the mount options are conflicting
> or not....

This extends to LSMs that support mount options (SELinux and Smack)
as well.