linux-kernel - Re: [PATCH 4/9] Implement fsopen() to prepare for a mount

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Message-ID: <1494355229.2659.15.camel@redhat.com>
Date:   Tue, 09 May 2017 14:40:29 -0400
From:   Jeff Layton <jlayton@...hat.com>
To:     Karel Zak <kzak@...hat.com>, David Howells <dhowells@...hat.com>
Cc:     viro@...iv.linux.org.uk, linux-fsdevel@...r.kernel.org,
        linux-nfs@...r.kernel.org, linux-kernel@...r.kernel.org,
        mszeredi@...hat.com
Subject: Re: [PATCH 4/9] Implement fsopen() to prepare for a mount

On Thu, 2017-05-04 at 15:34 +0200, Karel Zak wrote:
> On Thu, May 04, 2017 at 02:06:51PM +0100, David Howells wrote:
> > Karel Zak <kzak@...hat.com> wrote:
> > > The very basic mount(2) problem is that you have to parse
> > > /proc/self/mountinfo to get information about the mounted filesystem.
> > > It seems that your read() is also one way communication.
> > > 
> > > What we really need is to have a way how to specify *what* you want to
> > > read. The error message is not enough, I want to know the finally used
> > > mount options, mount ID, etc. It would be nice to have something like
> > > 
> > > 
> > >    fsmount(mfd, AT_FDCWD, "/mnt", 0);
> > > 
> > >    write(mfd, "o");
> > >    read(mfd, ....);     // read mount options
> > > 
> > >    write(mdf, "i");
> > >    read(mfd, ....);     // read mount ID
> > > 
> > > 
> > > but it seems ugly. Maybe introduce another function like 
> > > 
> > >     fsinfo(mdf, "o", buf, bufsz)
> > > 

Not sure that it's any prettier, but you could use an ioctl to switch
between different read requests:

ioctl(mfd, FSOPEN_READ_OPTIONS);
read(mfd, ....);

ioctl(mfd, FSOPEN_READ_MOUNT_ID);
read(mfd, ....);

> > > to get mount options (etc.) and to avoid separate write & read.
> > 
> > What is it you're trying to do?  Just read back the state of the new mount?
> 
>  ...read back the state of the new mount, because for example mount
>  options can be modified by FS driver. It would be also nice to have
>  API to get state of arbitrary mount without parsing mountinfo (the
>  file is huge on some systems).
> 


-- 
Jeff Layton <jlayton@...hat.com>