linux-kernel - Re: [RFD] A mount api that notices previous mounts

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Date:   Wed, 30 Jan 2019 18:43:06 +0100
From:   Karel Zak <kzak@...hat.com>
To:     David Howells <dhowells@...hat.com>
Cc:     "Eric W. Biederman" <ebiederm@...ssion.com>,
        linux-api@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, Al Viro <viro@...IV.linux.org.uk>,
        Miklos Szeredi <miklos@...redi.hu>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        util-linux@...r.kernel.org, Andy Lutomirski <luto@...capital.net>
Subject: Re: [RFD] A mount api that notices previous mounts

On Wed, Jan 30, 2019 at 01:01:54PM +0000, David Howells wrote:
> Karel Zak <kzak@...hat.com> wrote:
> 
> > It seems more elegant is to ask for Nth option as expected by fsinfo().
> 
> More elegant yes, but there's an issue with atomiticity[*].  I'm in the
> process of switching to something that returns you a single buffer with all
> the options in, but each key and each value is preceded by a length count.

Sounds good, for me is important to avoid all the split/join
operations with the strings.

> The reasons for not using separator characters are:
> 
>  (1) There's no separator char that cannot validly occur within an option[**].

Yes, it's pretty common for selinux mount options where "," is
used within an option, so mount options string looks like

    'context="system_u:object_r:tmp_t:s0:c127,c456",noexec'

and I have doubts all the parses in userspace are compatible with this
use case...

>  (2) Makes it possible to return binary values if we need to.

Yes.

> [**] Oh, and look at cifs where you can *change* the separator char during
>      option parsing ("sep=<char>").

No comment :-)

    Karel

-- 
 Karel Zak  <kzak@...hat.com>
 http://karelzak.blogspot.com