linux-kernel - Re: [PATCH 24/32] vfs: syscall: Add fsopen() to prepare for superblock creation [ver #9]

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Date:   Thu, 12 Jul 2018 21:08:42 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Al Viro <viro@...IV.linux.org.uk>,
        David Howells <dhowells@...hat.com>, linux-api@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, torvalds@...ux-foundation.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 24/32] vfs: syscall: Add fsopen() to prepare for
 superblock creation [ver #9]

On Thu, Jul 12, 2018 at 11:30:32AM -0700, Andy Lutomirski wrote:
> 
> > On Jul 12, 2018, at 11:03 AM, Greg KH <gregkh@...uxfoundation.org> wrote:
> > 
> >> On Thu, Jul 12, 2018 at 06:20:24PM +0100, Al Viro wrote:
> >>> On Thu, Jul 12, 2018 at 07:15:05PM +0200, Greg KH wrote:
> >>>> On Tue, Jul 10, 2018 at 11:44:09PM +0100, David Howells wrote:
> >>>> Provide an fsopen() system call that starts the process of preparing to
> >>>> create a superblock that will then be mountable, using an fd as a context
> >>>> handle.  fsopen() is given the name of the filesystem that will be used:
> >>>> 
> >>>>    int mfd = fsopen(const char *fsname, unsigned int flags);
> >>>> 
> >>>> where flags can be 0 or FSOPEN_CLOEXEC.
> >>>> 
> >>>> For example:
> >>>> 
> >>>>    sfd = fsopen("ext4", FSOPEN_CLOEXEC);
> >>>>    write(sfd, "s /dev/sdb1"); // note I'm ignoring write's length arg
> >>>>    write(sfd, "o noatime");
> >>>>    write(sfd, "o acl");
> >>>>    write(sfd, "o user_attr");
> >>>>    write(sfd, "o iversion");
> >>>>    write(sfd, "o ");
> >>>>    write(sfd, "r /my/container"); // root inside the fs
> >>>>    write(sfd, "x create"); // create the superblock
> >>> 
> >>> Ugh, creating configfs again in a syscall form?  I know people love
> >>> file descriptors, but can't you do this with a configfs entry instead if
> >>> you really want to do this type of thing from userspace in this type of
> >>> "style"?
> >>> 
> >>> Why reinvent the wheel again?
> >> 
> >> The damn thing REALLY, REALLY depends upon the fs type.  How would
> >> you map it on configfs?
> > 
> > /sys/kernel/config/fs/ext4/ would work, right?  Each fs "type" would be
> > listed there.
> > 
> > Anyway, the whole "write a bunch of options and then do a 'create'" is
> > exactly the way configfs works.  Why not use that?
> > 
> > 
> 
> How do you mount configfs in the first place?  And how do you use this
> in a mount namespace without a private configfs instance or where you
> don’t want configfs mounted?--

Ok, fair enough, I missed the part where this is going to replace
mount(2).  Although you could just use mount(2) to mount configfs on a
mount point in the initramfs image and then go from there at boot time :)

/me runs away...