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 11:30:32 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Greg KH <gregkh@...uxfoundation.org>
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 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?