Re: [PATCH 1/1] mnt: add support for non-rootfs initramfs

[Ignat Korchagin]

Sorry, forgot to follow up in this thread. I've reposted v2 patches
with documentation.

Regards,
Ignat

On Mon, Mar 30, 2020 at 8:03 PM Randy Dunlap <rdunlap@xxxxxxxxxxxxx> wrote:
>
> On 3/30/20 6:14 AM, Ignat Korchagin wrote:
> > The main need for this is to support container runtimes on stateless Linux
> > system (pivot_root system call from initramfs).
> >
> > Normally, the task of initramfs is to mount and switch to a "real" root
> > filesystem. However, on stateless systems (booting over the network) it is just
> > convenient to have your "real" filesystem as initramfs from the start.
> >
> > This, however, breaks different container runtimes, because they usually use
> > pivot_root system call after creating their mount namespace. But pivot_root does
> > not work from initramfs, because initramfs runs form rootfs, which is the root
> > of the mount tree and can't be unmounted.
> >
> > One workaround is to do:
> >
> >   mount --bind / /
> >
> > However, that defeats one of the purposes of using pivot_root in the cloned
> > containers: get rid of host root filesystem, should the code somehow escapes the
> > chroot.
> >
> > There is a way to solve this problem from userspace, but it is much more
> > cumbersome:
> >   * either have to create a multilayered archive for initramfs, where the outer
> >     layer creates a tmpfs filesystem and unpacks the inner layer, switches root
> >     and does not forget to properly cleanup the old rootfs
> >   * or we need to use keepinitrd kernel cmdline option, unpack initramfs to
> >     rootfs, run a script to create our target tmpfs root, unpack the same
> >     initramfs there, switch root to it and again properly cleanup the old root,
> >     thus unpacking the same archive twice and also wasting memory, because
> >     the kernel stores compressed initramfs image indefinitely.
> >
> > With this change we can ask the kernel (by specifying nonroot_initramfs kernel
> > cmdline option) to create a "leaf" tmpfs mount for us and switch root to it
> > before the initramfs handling code, so initramfs gets unpacked directly into
> > the "leaf" tmpfs with rootfs being empty and no need to clean up anything.
> >
> > This also bring the behaviour in line with the older style initrd, where the
> > initrd is located on some leaf filesystem in the mount tree and rootfs remaining
> > empty.
> >
> > Signed-off-by: Ignat Korchagin <ignat@xxxxxxxxxxxxxx>
> > ---
> >  fs/namespace.c | 47 +++++++++++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 47 insertions(+)
>
> Hi,
> Please document "nonroot_initramfs" in
> Documentation/admin-guide/kernel-parameters.txt.
>
> > diff --git a/fs/namespace.c b/fs/namespace.c
> > index 85b5f7bea82e..a1ec862e8146 100644
> > --- a/fs/namespace.c
> > +++ b/fs/namespace.c
> > @@ -3701,6 +3701,49 @@ static void __init init_mount_tree(void)
> >       set_fs_root(current->fs, &root);
> >  }
> >
> > +#if IS_ENABLED(CONFIG_TMPFS)
> > +static int __initdata nonroot_initramfs;
> > +
> > +static int __init nonroot_initramfs_param(char *str)
> > +{
> > +     if (*str)
> > +             return 0;
> > +     nonroot_initramfs = 1;
> > +     return 1;
> > +}
> > +__setup("nonroot_initramfs", nonroot_initramfs_param);
> > +
> > +static void __init init_nonroot_initramfs(void)
> > +{
> > +     int err;
> > +
> > +     if (!nonroot_initramfs)
> > +             return;
> > +
> > +     err = ksys_mkdir("/root", 0700);
> > +     if (err < 0)
> > +             goto out;
> > +
> > +     err = do_mount("tmpfs", "/root", "tmpfs", 0, NULL);
> > +     if (err)
> > +             goto out;
> > +
> > +     err = ksys_chdir("/root");
> > +     if (err)
> > +             goto out;
> > +
> > +     err = do_mount(".", "/", NULL, MS_MOVE, NULL);
> > +     if (err)
> > +             goto out;
> > +
> > +     err = ksys_chroot(".");
> > +     if (!err)
> > +             return;
> > +out:
> > +     printk(KERN_WARNING "Failed to create a non-root filesystem for initramfs\n");
> > +}
> > +#endif /* IS_ENABLED(CONFIG_TMPFS) */
> > +
> >  void __init mnt_init(void)
> >  {
> >       int err;
> > @@ -3734,6 +3777,10 @@ void __init mnt_init(void)
> >       shmem_init();
> >       init_rootfs();
> >       init_mount_tree();
> > +
> > +#if IS_ENABLED(CONFIG_TMPFS)
> > +     init_nonroot_initramfs();
> > +#endif
> >  }
> >
> >  void put_mnt_ns(struct mnt_namespace *ns)
> >
>
> thanks.
> --
> ~Randy
>