On 06/10/2020 05:35, Heinrich Schuchardt wrote:
Am 6. Oktober 2020 00:37:58 MESZ schrieb Grant Likely grant.likely@arm.com:
On 03/10/2020 09:51, Heinrich Schuchardt wrote:
Hello Ilias, hello Christian,
with commit ec80b4735a59 ("efi_loader: Implement FileLoad2 for
initramfs
loading") Ilias provided the possibility to specify a device path (CONFIG_EFI_INITRD_FILESPEC) from which an initial RAM disk can be served via the EFI_FILE_LOAD2_PROTOCOL.
Ard extended the Linux EFI stub to allow loading the initial RAM disk via the EFI_FILE_LOAD2_PROTOCOL with the utmost priority.
With commit ecc7fdaa9ef1 ("bootm: Add a bootm command for type IH_OS_EFI") Cristian enabled signed FIT images that contain a device tree and a UEFI binary (enabled by CONFIG_BOOTM_EFI=y).
In the DTE calls we have discussed that it is unfortunate that we do
not
have a method to validate initial RAM images in the UEFI context.
To me it would look like a good path forward to combine the two
ideas:
- Let the signed FIT image (of type IH_OS_EFI) contain a RAM disk
- Pass location and size to the UEFI subsystem and serve them via the EFI_FILE_LOAD2_PROTOCOL.
We could also extend the bootefi command to be callable as
bootefi $kernel_addr_r $ramdisk_addr_r:$filesize $fdt_addr_rlike the booti command to serve an initial RAM disk.
What are your thoughts?
Hi Heinrich,
I've got concerns about this approach. Even though it uses the UEFI infrastructure, images deployed in this way are U-Boot specific and won't ever be applicable on EDK2 or other UEFI implementations.
However there is another way to approach it which I think Francois touched on. If instead a UEFI stub was added to the FIT image, in the same way that the kernel has a UEFI stub, then the logic of decoding the FIT and choosing the correct DTB & initrd can be part of the image and it becomes applicable to any UEFI implementation. It would also address
Ard's concern of loading the FIT into memory, and then copying due to the EFI_FILE_LOAD2 path. The FIT stub would already know the image is in RAM, that is is reserved correctly, and just pass the correct addresses
to the kernel as part of the normal boot flow.
Signing would also be taken care of because the whole FIT can be signed, and that signature would be checked when it gets loaded.
Thoughts?
The gain of a fit image in U-Boot used for calling the Linux kernel via the EFI stub vs calling the legacy entry point comes down to providing the EFI_RNG_PROTOCOL to be used for KASLR.
I agree with that, but that is not my concern.
My concern is that the FIT image format will only be supported by U-Boot. Other UEFI implementations do not implement it.
On the other hand, adding a UEFI Stub to the FIT image format makes it a generic solution that can be used by any UEFI implementation. This would be separate from the linux kernel's UEFI stub, and should only deal with choosing the appropriate kernel/initrd/dtb from the FIT and then calling into the kernel's stub to actually boot the kernel.
For initrd a stub UEFI binary will work. But if you want to provide a kernel specific dtb with the same stub binary it will require a new service for device-tree fixups.
Devicetree fixups indeed needs to be solved. I would propose registering a new protocol for fixups. If the protocol is present, then stub can call it. If not, then the DTB from the fit should be used unmodified.
g.