bootstd: Support for distro specific EFI folders

Sat Dec 9 23:42:13 CET 2023

On Sat, Dec 9, 2023 at 4:26 PM Shantur Rathore <i at shantur.com> wrote:
>
> Hi Peter,
>
> On Wed, Dec 6, 2023 at 12:16 PM Peter Robinson <pbrobinson at gmail.com> wrote:
> >
> > On Sun, Nov 19, 2023 at 6:55 PM Mark Kettenis <mark.kettenis at xs4all.nl> wrote:
> > >
> > > > Date: Sat, 18 Nov 2023 23:52:11 +0100
> > > > From: Heinrich Schuchardt <heinrich.schuchardt at canonical.com>
> > >
> > > Hi Heinrich,
> > >
> > > > On 11/18/23 22:28, Shantur Rathore wrote:
> > > > > Hi Heinrich,
> > > > >
> > > > > On Fri, Nov 17, 2023 at 3:12 PM Heinrich Schuchardt
> > > > > <heinrich.schuchardt at canonical.com> wrote:
> > > > >>
> > > > >> On 11/16/23 19:45, Shantur Rathore wrote:
> > > > >>> On Thu, Nov 16, 2023 at 6:15 PM Heinrich Schuchardt
> > > > >>> <heinrich.schuchardt at canonical.com> wrote:
> > > > >>>>
> > > > >>>> On 11/16/23 17:52, Shantur Rathore wrote:
> > > > >>>>> Hi Simon,
> > > > >>>>>
> > > > >>>>> Currently bootstd - bootmethod_efi only looks for the default
> > > > >>>>> bootxx64.efi in /EFI/boot folder only.
> > > > >>>>> Generally many distros end up putting their bootloaders in
> > > > >>>>> EFI/<distro> folders like EFI/ubuntu and EFI/debian etc.
> > > > >>>>>
> > > > >>>>> In x86 worlds, the NVRAM is modified and new boot entries are added to
> > > > >>>>> support these but in the U-boot world the NVRAM variables are
> > > > >>>>> read-only.
> > > > >>>>
> > > > >>>> I guess you are referring to UEFI boot options. These typically are not
> > > > >>>> stored in non-volatile RAM but on a SPI flash device.
> > > > >>>>
> > > > >>>
> > > > >>> Thanks for correcting me.
> > > > >>>
> > > > >>>>>
> > > > >>>>> What would be the best way to implement this?
> > > > >>>>>
> > > > >>>>> I was thinking of having a "efi_prefixes" environment variable which
> > > > >>>>> can be set to "ubuntu debian centos" etc and bootmethod_efi can try
> > > > >>>>> all of them. Will bootmethod_efi be able to support multiple entries (
> > > > >>>>> thinking of multiboot ) ?
> > > > >>>>
> > > > >>>> On my laptop I have:
> > > > >>>>
> > > > >>>> EFI/Microsoft/Boot/bootmgr.efi
> > > > >>>> EFI/Microsoft/Boot/memtest.efi
> > > > >>>> EFI/Boot/bootx64.efi
> > > > >>>> EFI/Boot/fbx64.efi
> > > > >>>> EFI/Boot/mmx64.efi
> > > > >>>> EFI/debian/shimx64.efi
> > > > >>>> EFI/debian/grubx64.efi
> > > > >>>> EFI/debian/mmx64.efi
> > > > >>>> EFI/debian/fbx64.efi
> > > > >>>> EFI/ubuntu/grubx64.efi
> > > > >>>> EFI/ubuntu/shimx64.efi
> > > > >>>> EFI/ubuntu/mmx64.efi
> > > > >>>>
> > > > >>>> Obviously each installed operating system provides multiple EFI binaries
> > > > >>>> and non uses the fallback file name BOOT<ARCH>.EFI. A value "ubuntu
> > > > >>>> debian centos" would not be able to describe which file you are looking
> > > > >>>> for.
> > > > >>>>
> > > > >>>> We already have the U-Boot command line eficonfig and efidebug commands
> > > > >>>> to set up UEFI boot options which can describe which EFI binary to load
> > > > >>>> and which command line to pass to it. These are considered by the
> > > > >>>> existing boot flows.
> > > > >>>
> > > > >>> So, I am building a new RockPro64 based cluster and using Canonical
> > > > >>> MAAS to set them up automatically, booting them up using DHCP and
> > > > >>> installing them over the network.
> > > > >>> I configured an Armbian image using Packer to be compatible with MAAS
> > > > >>> and it happily installs it. As part of installation process, a
> > > > >>> grub-install is run which installs the grub efi,
> > > > >>> this EFI ends up in EFI/debian instead of expected EFI/boot.
> > > > >>> To be able to make it boot, I have to add commands to move it to
> > > > >>> EFI/boot. I am trying to find a way in U-Boot that would allow me to
> > > > >>> skip this step.
> > > > >>> With eficonfig if I understand correctly, it would need manual
> > > > >>> intervention to create boot entries.
> > > > >>>
> > > > >>>>
> > > > >>>> If you are installing the shim-signed package on Ubuntu, the EFI boot
> > > > >>>> option for Ubuntu is set up by EFI/BOOT/BOOT<ARCH>.EFI using the content
> > > > >>>> of EFI/ubuntu/BOOT<ARCH>.CSV. This is done before ExitBootServices() and
> > > > >>>> therefore should work with current U-Boot.
> > > > >>>>
> > > > >>>> Patches are pending upstream to make EFI variables writable from Linux
> > > > >>>> if they are stored in the RPMB partition in the eMMC. See this series:
> > > > >>>>
> > > > >>>> https://lore.kernel.org/linux-efi/20231107054057.1893-2-masahisa.kojima@linaro.org/
> > > > >>>>
> > > > >>>
> > > > >>> Would it be possible to save it in SPI Flash as the U-Boot environment ?
> > > > >>
> > > > >> Currently this is not supported by U-Boot.
> > > > >>
> > > > >> The U-Boot environment variables can be stored in lots of different
> > > > >> places SPI flash is only one of these. But none of these locations is
> > > > >> protected from OS access which would be preferable for UEFI variables
> > > > >> for security reasons.
> > > > >>
> > > > >> To support boards without eMMC the right way forward would be writing a
> > > > >> new implementation of the OP-TEE standalone MM which writes the
> > > > >> variables to SPI flash instead of the RPMB partition and ensures that
> > > > >> the SPI flash' MMIO registers are protected against access from the
> > > > >> non-secure world.
> > > > >
> > > > > Thanks for explaining this to me.
> > > > > This seems like a long way to go, for now what would be an acceptable
> > > > > solution, some options are
> > > > >
> > > > > 1. Allow to set a space separated efi_prefixes (e.g. "boot ubuntu
> > > > > debian") variable which is ready by bootmeth_efi and used as
> > > > > efi/<efi_prefix> instead of efi/boot.
> > > > > 2. Improve bootmeth_efi to find all bootxxxx.efi in efi/ folder and
> > > > > present all them as bootflows to bootstd.
> > > >
> > > > As mentioned in a prior mail ubuntu/bootxxxx.efi and debian/bootxxxx.efi
> > > > don't exist. I would prefer not to add any distro specific stuff in
> > > > upstream U-Boot. Instead we will continue to drive what Linaro has
> > > > suggested to improve U-Boot EFI variables support in Linux.
> > >
> > > I agree that adding hacks like this is not a good idea.
> > >
> > > The Linaro approach that involves OP-TEE makes for a fairly complex
> > > solution.  And there are plenty of boards that have neither eMMC nor
> > > SPI flash.  If Secure Boot is not a requirement (and I'd argue that
> > > for most "hobbyist" boards it isn't) storing the EFI variables in a
> > > file on the ESP (as implemented by the CONFIG_EFI_VARIABLE_FILE_STORE
> > > Kconfig option) is a workable alternative.  And this is actually what
> > > the rockpro64-rk3399_defconfig enables.
> > >
> > > I noticed that the latest EBBR attempts to standardize this:
> > >
> > >   https://arm-software.github.io/ebbr/index.html#document-chapter5-variable-storage
> > >
> > > Not sure what the status here is.  But if the Linux kernel folks
> > > accept that alternative implementations for runtime EFI variable
> > > access are a thing, then a method that modifies the file would be
> > > possible as well.  Or maybe it is good enough to implement support for
> > > this in the efivar library.
> > >
> > > That said, Linux distros probably should install their EFI bootloader
> > > as EFI/BOOT/BOOTxxxx.EFI if that file doesn't exist yet.  Some Linux
> > > distros already do this.  This would make the distro work "out of the
> > > box" on a lot more boards.
> >
> > I believe the EFI/BOOT/BOOTxxxx.EFI is the EFI fallback mechanism (not
> > sure it that's the proper spec name but it's what I've heard it
> > referred to) and tha's the way Fedora generally works to boot devices
> > that don't have working set variable where the boot entry can be
> > updated. I had assumed that was part of the standard as, at least in
> > Fedora, that has always been there and explicitly the way we made UEFI
> > work on U-Boot from the outset.
>
> This is definitely the standard for Removable Storage as per the spec.
> Section 3.5.1.1 Removable Media Boot Behavior - Link [0]
>
> [0] - https://uefi.org/sites/default/files/resources/UEFI_Spec_2_10_Aug29.pdf
>
> But the spec isn't stopping the firmware to perform scan to be able to list
> the possible bootable EFIs, in-fact it says firmware can do special things as in
> Section  3.4.3 Boot Option Variables Default Boot Behavior
>
> @Simon Glass - If one was to implement such a scanning behaviour (
> knowing it might not be
> merged ), how should one go about this?
>
> As I understand, the bootmeth_efi is expected to provide only 1
> bootflow for a partition, what would
> be the best way for bootmeth_efi to provide multiple bootflows for
> each partition.
>
> Kind regards,
> Shantur

Throwing spanner in the works, with Unified Kernel Image coming in,
it will be required to implement the scanning for EFI files.

https://github.com/uapi-group/specifications/blob/main/specs/boot_loader_specification.md#locating-boot-entries

Kind regards,
Shantur