Proposed Enhancements to the EFI booting

Fri Aug 11 19:48:04 UTC 2017

On Fri, Aug 11, 2017 at 1:40 PM, Mark Johnston <markj at freebsd.org> wrote:

> On Thu, Aug 10, 2017 at 10:03:57PM -0600, Warner Losh wrote:
> > Greetings,
> >
> > I've been circulating a document (the latest version you can find here
> > https://docs.google.com/document/d/1aK9IqF-60JPEbUeSAUAkYjF2W_
> 8EnmczFs6RqCT90Jg/edit?usp=sharing
> > )
> >
> > I'm opening it up for general comments now. I intend to turn this into a
> > full spec, including the bits in the UEFI standard that I reference by
> > pointer, and move to implementation soon once any issues are resolved.
> >
> > Warner
> >
> > FreeBSD UEFI boot protocol
> >
> > This document outlines the enhancements to the UEFI Boot Manager
> Protocol,
> > as outlined in Version 2.6 of the UEFI spec, chapter 3. It assumes that
> you
> > are familiar with that chapter and though a comprehensive doc with our
> > additions folded into it would be good, it hasn’t been produced.
> > Current Algorithm
> >
> > Boot1.efi currently searches for /boot/loader.efi using a fairly
> > complicated algorithm.
> >
> > First, we look for any ZFS pool that has a /boot/loader.efi on it,
> > consistent with its boot environment settings. If we find that, we use
> it.
> >
> > Second, we search all of the partitions on the device that we booted
> > boot1.efi off of for a UFS partition that has a /boot/loader.efi we can
> > load. If we find that, we use it.
> >
> > Third, we search all other devices for a partition that is UFS formatted
> > and has a /boot/loader.efi we can load.
> >
> > There’s some problems with this algorithm. It’s not possible to specify a
> > partition to boot from. We boot from the first /boot/loader.efi that’s
> > found, even if multiple partitions on a drive have one. Second, it’s very
> > non-deterministic. The second step was added to add some determinism to
> the
> > process so if you plugged in a FreeBSD 11.0R release USB stick and
> > rebooted, you would reboot to the installed system and not the USB
> > installer. It doesn’t use the ad-hoc method for identifying which FreeBSD
> > partition to boot off of that we use for BIOS boots since in the UEFI
> > runtime environment it’s tricky to get the attribute flags of a
> partition.
> > Finally, it doesn’t use the standard UEFI boot manager protocol.
> > Proposed Algorithm
> >
> > First, the FreeBSD UEFI Boot Loader UUID shall be
> > cfee69ad-a0de-47a9-93a8-f63106f8ae99 (below as FreeBSD:). The UEFI
> defined
> > global variable UUID (EFI_GLOBAL_VARIABLE) is
> > 8BE4DF61-93CA-11d2-AA-0D-00-E0-98-03-2B-8C (below as UEFI:)
> >
> > Boot1.efi will find what to use with the following algorithm:
> >
> >
> >    1.
> >
> >    If the UEFI boot manager passes an optional parameter to boot1.efi,
> then
> >    parse it as if it were a EFI Device Path to find the next stage boot
> loader
> >    to use. If we can use that boot loader, we’re done. Otherwise, ignore
> it.
> >    2.
> >
> >    Get the value of UEFI:BootCurrent. This will be a 4 digit hex number
> >    XXXX. If FreeBSD:BootLoaderXXXX exists, and specifies a file we can
> load,
> >    use it. This is in the form of a EFI device path (binary). If it is a
> File
> >    path that’s ZFS:yyyy, then boot from the Boot environment for ZFS pool
> >    yyyy. If :yyyy is omitted, use the first ZFS pool you find.
> >    3.
> >
> >    If a ZFS pool exists with a bootable environment, boot it.
> >    4.
> >
> >    If a partition with a known filesystem exists on the same device as
> >    boot1.efi was loaded from, and it contains /boot/loader.efi, use it.
> >    5.
> >
> >    Set FreeBSD:BootFailedXXXX to the reason for the failure and return
> >    Failure to UEFI (so UEFI goes to the next item in the list)
> >
> >
> > If FreeBSD:BootArgumentsXXXX exists, parse it like ASCII text and pass it
> > to the boot  loader found in steps 1-4 above. Otherwise pass nothing.
> >
> > If the FreeBSD:Update variable is set, the rc system will update the boot
> > order so that UEFI:BootCurrent is at the start of UEFI:BootOrder. This is
> > currently done in /etc/rc.d/gptupdate. I propose a /etc/rc.d/efiupdate to
> > do this, and to report all the BootFailedXXXX variables ala gptupdate.
> >
> > Discussion of points raised on IRC:
> >
> > FreeBSD currently implements a ‘boot once’ option that’s similar to
> > ‘BootNext’ but happens only once. In UEFI, BootNext almost implements
> this.
> > The BIOS loads that option, then deletes the BootNext env variable.
> Unless
> > the OS does something the make it permanent, this is the same as
> FreeBSD’s
> > boot once (because the order then reverts to BootOrder). I think that
> > BootFailed needs to be implemented with the extra protocol outlined in
> step
> > 5.
>
> There is also a "bootme" GPT flag that is currently ignored when booting
> using boot1.efi. We use it to avoid in-place upgrades: there are two
> root filesystems, only one of which is mounted during boot. Upgrades are
> done by installing the to-version of the OS to the inactive root
> filesystem, flipping the "bootme" flag off on the active partition,
> flipping it on on the inactive partition, and rebooting.
>

Correct. EFI Boot Manager has a concept of profiles being active or not
active. That's how this functionality will be implemented. There's no
standardized notion of a partition being bootable or not. That's a FreeBSD
extension. same with bootnext and bootfail flags.

So upgrades would become flipping the boot order in UEFI:BootOrder and/or
toggling active on the two UEFI:BootXXXX variables. It just changes the
command you need to issue from gpart <mumble> to efibootmgr <mumble>.

> How should this interact with step 4 of your algorithm? Should we prefer
> a GPT "bootme" entry over a filesystem on the same device as boot1.efi?

No. We shouldn't. That duplicates functionality, and the flags bits we need
to make these decisions are a pain to get to in boot1.efi. They are FreeBSD
specific, but won't be carried forward since better functionality already
exists in the BootOrder and BootXXX variables.

This is much more flexible as well, since it allows you to have multiple
profiles that point to the same partition, but have different parameters,
only one of which is active at any time, which is impossible with the old
GPT scheme.

Warner

>
> > Open Issues:
> >
> > Allan Jude has raised some issues about ZFS. ZFS has it’s own way to deal
> > with all this and he’s curious how this fits into that. EFI doesn’t know
> > about ZFS, so at most we can only define what happens when boot1.efi
> takes
> > over. In that case, I proposed to him that we do what we do now, and
> > selection of what ZFS thing can be done either as I proposed, or via the
> > zfsbootcfg process if nothing further than ZFS is specified (or
> > automatically selected).
> >
> > How does loader.efi fit for people that load it directly?
> >
> > Do we want to enable / disable the automatic looking for boot locations
> > separately from what to boot being explicitly specified or not? I’m
> leaning
> > against.
> >
> > Can nextboot -k foo be implemented with this? My first notion is that it
> > would FreeBSD:KernelXXXX to specify the kernel and FreeBSD:NextKernelXXXX
> > which would load that kernel and then unset FreeBSD:NextKernelXXXX. Ditto
> > for FreeBSD:KernelFlagsXXXX and FreeBSD:NextKernelFlagsXXXX. nextboot(8)
> > would set all these. Maybe FreeBSD:LoaderXXXX and FreeBSD:NextLoaderXXXX
> > too...
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