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

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Message-ID: <20180811021704.GE6515@ZenIV.linux.org.uk>
Date:   Sat, 11 Aug 2018 03:17:04 +0100
From:   Al Viro <viro@...IV.linux.org.uk>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     David Howells <dhowells@...hat.com>,
        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

On Sat, Aug 11, 2018 at 02:58:15AM +0100, Al Viro wrote:
> On Fri, Aug 10, 2018 at 08:05:44PM -0500, Eric W. Biederman wrote:
> 
> > All I proposed was that we distinguish between a first mount and an
> > additional mount so that userspace knows the options will be ignored.
> 
> For pity sake, just what does it take to explain to you that your
> notions of "first mount" and "additional mount" ARE HEAVILY FS-DEPENDENT
> and may depend upon the pieces of state userland (especially in container)
> simply does not have?
> 
> One more time, slowly:
> 
> mount -t nfs4 wank.example.org:/foo/bar /mnt/a
> mount -t nfs4 wank.example.org:/baz/barf /mnt/b
> 
> yield the same superblock.  Is anyone who mounts something over NFS
> required to know if anybody else has mounted something from the same
> server, and if so how the hell are they supposed to find that out,
> so that they could decide whether they are creating the "first" or
> "additional" mount, whatever that might mean in this situation?
> 
> And how, kernel-side, is that supposed to be handled by generic code
> of any description?  
> 
> While we are at it,
> mount -t nfs4 wank.example.org:/foo/bar -o wsize=16384 /mnt/c
> is *NOT* the same superblock as the previous two.

s/as the previous two/as in the previous two cases/, that is - the first two
examples yield one superblock, this one - another.