[syslinux] Bootloader data in /boot vs package systems (and atomic updates)

[Gene Cumm]

On Tue, Apr 22, 2014 at 6:58 PM, Colin Walters <walters at verbum.org> wrote:
> Hi,
>
> I'm working on https://wiki.gnome.org/Projects/OSTree which is a
> general-purpose update system for Linux-based operating systems.  It isn't a
> package system itself, but operates at the same level.
>
> A primary goal of OSTree is fully atomic updates, and the "swapped chroots"
> model works well for most of userspace and the kernel/initramfs.
>
> However, syslinux relies on loadable modules in /boot.  At present in Fedora
> at least, the RPM package updates these, and it has a %post to ensure that
> when a newer version of extlinux is installed, we update both the boot
> modules and the MBR.

I think you mean the VBR since no Syslinux variant lives in the MBR
and on Linux, the MBR requires manual installation (outside executing
one of the installer binaries like linux/syslinux or
extlinux/extlinux; paths relative to source tree root in versions
before 6.00)

> For more background, see:
> https://bugzilla.redhat.com/show_bug.cgi?id=1090083
>
> In constrat, GRUB2 keeps its modules in /usr/lib, and grub-install *copies*
> them to /boot at the same time that the MBR is updated.

Does the installation of an updated package of GRUB2 trigger an
installation of modules during install on Fedora or does the
installation of an updated package require manual installation every
time?  Or is this perhaps a configuration?

The alternative is an additional script that would run the installer
binary extlinux and copy *.c32 at the same time.

> I like this model more as it allows me to decouple the delivery of updated
> bootloader data with its actual installation, and ensure that when I do want
> to update the bootloader, it can happen at a safe(r) time.

This would also be a distinct departure from past behavior.

> Would there be any interest in patches for extlinux to support this model?
>
> Any thoughts on the general topic?

Unlike GRUB*, Syslinux will never be in the MBR and the post-MBR
sectors (aka hidden sectors or reserved sectors).  GRUB* can live
inside the partition entirely to allow a standard MBR but this is
dependent on file system and version.

-- 
-Gene