linux-kernel - Re: [PATCH RESEND] init/initramfs.c: make initramfs support pivot

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Date:   Fri, 21 May 2021 08:41:55 +0800
From:   Menglong Dong <menglong8.dong@...il.com>
To:     Luis Chamberlain <mcgrof@...nel.org>
Cc:     Jan Kara <jack@...e.cz>, Jens Axboe <axboe@...nel.dk>,
        hare@...e.de, gregkh@...uxfoundation.org, tj@...nel.org,
        Menglong Dong <dong.menglong@....com.cn>, song@...nel.org,
        neilb@...e.de, Andrew Morton <akpm@...ux-foundation.org>,
        wangkefeng.wang@...wei.com, f.fainelli@...il.com, arnd@...db.de,
        Barret Rhoden <brho@...gle.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        mhiramat@...nel.org, Steven Rostedt <rostedt@...dmis.org>,
        Kees Cook <keescook@...omium.org>, vbabka@...e.cz,
        Alexander Potapenko <glider@...gle.com>, pmladek@...e.com,
        Chris Down <chris@...isdown.name>, ebiederm@...ssion.com,
        jojing64@...il.com, LKML <linux-kernel@...r.kernel.org>,
        palmerdabbelt@...gle.com, linux-fsdevel@...r.kernel.org,
        Alexander Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH RESEND] init/initramfs.c: make initramfs support pivot_root

Hello!

Thanks for your reply!

On Fri, May 21, 2021 at 5:41 AM Luis Chamberlain <mcgrof@...nel.org> wrote:
>
> Can't docker instead allow to create containers prior to creating
> your local docker network namespace? Not that its a great solution,
> but just worth noting.
>

That's a solution, but I don't think it is feasible. Users may create many
containers, and you can't make docker create all the containers first
and create network namespace later, as you don't know if there are any
containers to create later.

> >
> >  struct file_system_type rootfs_fs_type = {
> >       .name           = "rootfs",
> > -     .init_fs_context = rootfs_init_fs_context,
> > +     .init_fs_context = ramfs_init_fs_context,
>
> Why is this always static now? Why is that its correct
> now for init_mount_tree() always to use the ramfs context?

Because the root mount in init_mount_tree() is not used as rootfs any more.
In do_populate_rootfs(), I mounted a second rootfs, which can be ramfs or
tmpfs, and that's the real rootfs for initramfs. And I call this root
as 'user_root',
because it is created for user space.

int __init mount_user_root(void)
{
       return do_mount_root(user_root->dev_name,
                            user_root->fs_name,
                            root_mountflags,
                            root_mount_data);
 }

In other words, I moved the realization of 'rootfs_fs_type' here to
do_populate_rootfs(), and fixed this 'rootfs_fs_type' with
ramfs_init_fs_context, as it is a fake root now.

Now, the rootfs that user space used is separated with the init_task,
and that's exactly what a block root file system does.

Thanks!
Menglong Dong