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

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Date:   Fri, 10 Aug 2018 16:13:11 +0100
From:   David Howells <dhowells@...hat.com>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     dhowells@...hat.com, "Eric W. Biederman" <ebiederm@...ssion.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, "Theodore Y. Ts'o" <tytso@....edu>,
        Miklos Szeredi <miklos@...redi.hu>
Subject: Re: BUG: Mount ignores mount options

Andy Lutomirski <luto@...capital.net> wrote:

> > /dev/loop0 /root/loop0-noacl-noquota-nouser_xattr ext4 rw,relatime,nouser_xattr,noacl 0 0
> > /dev/loop0 /root/loop0-acl-quota-user_xattr ext4 rw,relatime,nouser_xattr,noacl 0 0
> 
> To make sure I understand correctly: the problem is that the second mount
> ignored the options because the device was already mounted, right?
> 
> For the new API, I think the only remotely sane approach is to refuse to
> mount or init or whatever you call it an already mounted bdev. If user code
> genuinely needs to bind-mount an existing mount that is known only by its
> bdev, we can add a specific API just for that.

I'm adding some flags to fsopen() to allow userspace to say whether it wants
no sharing, same parameters-only sharing or anything-goes sharing (as now).

I'm also adding a flag whereby userspace can forbid anyone else from sharing a
new superblock it has just set up.

David