linux-kernel - Re: [RFC PATCH 2/3] add statmnt(2) syscall

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Message-ID: <20230918-bruchfest-erliegen-2bff785bf978@brauner>
Date:   Mon, 18 Sep 2023 15:05:46 +0200
From:   Christian Brauner <brauner@...nel.org>
To:     Ian Kent <raven@...maw.net>
Cc:     Sargun Dhillon <sargun@...gun.me>,
        Miklos Szeredi <mszeredi@...hat.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-api@...r.kernel.org, linux-man@...r.kernel.org,
        linux-security-module@...r.kernel.org, Karel Zak <kzak@...hat.com>,
        David Howells <dhowells@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <christian@...uner.io>,
        Amir Goldstein <amir73il@...il.com>
Subject: Re: [RFC PATCH 2/3] add statmnt(2) syscall

On Mon, Sep 18, 2023 at 07:36:39AM +0800, Ian Kent wrote:
> 
> On 18/9/23 02:18, Sargun Dhillon wrote:
> > On Wed, Sep 13, 2023 at 9:25 AM Miklos Szeredi <mszeredi@...hat.com> wrote:
> > > Add a way to query attributes of a single mount instead of having to parse
> > > the complete /proc/$PID/mountinfo, which might be huge.
> > > 
> > > Lookup the mount by the old (32bit) or new (64bit) mount ID.  If a mount
> > > needs to be queried based on path, then statx(2) can be used to first query
> > > the mount ID belonging to the path.
> > > 
> > > Design is based on a suggestion by Linus:
> > > 
> > >    "So I'd suggest something that is very much like "statfsat()", which gets
> > >     a buffer and a length, and returns an extended "struct statfs" *AND*
> > >     just a string description at the end."
> > > 
> > > The interface closely mimics that of statx.
> > > 
> > > Handle ASCII attributes by appending after the end of the structure (as per
> > > above suggestion).  Allow querying multiple string attributes with
> > > individual offset/length for each.  String are nul terminated (termination
> > > isn't counted in length).
> > > 
> > > Mount options are also delimited with nul characters.  Unlike proc, special
> > > characters are not quoted.
> > > 
> > Thank you for writing this patch. I wish that this had existed the many times
> > I've written parsers for mounts files in my life.
> > 
> > What do you think about exposing the locked flags, a la what happens
> > on propagation of mount across user namespaces?
> 
> Which flags do you mean?

When you propagate mounts across mount+user namespaces a subset of
(security sensitive) mount attributes become locked. This information is
currently only available via internal flags but not in any way
explicitly exposed to userspace.

There's a proposal to extend mount_setattr(2) to explicitly allow
locking flags but that would mean a new set of mount attr flags.

So until the format of that is determined and settled this should be
kept out of statmount().