[RFC PATCH 0/4] efi: implement generic compressed boot support

[Ard Biesheuvel]

Relatively modern architectures such as arm64 or RISC-V don't implement
a self-decompressing kernel, and leave it up to the bootloader to
decompress the compressed image before executing it. Not doing so makes
sense for bare metal boot, as it essentially duplicates a lot of fiddly
preparation work to create a 1:1 mapping and set up the C runtime, and
to discover or infer where DRAM lives from device trees or other
firmware tables.

For EFI boot, the situation is a bit different: the EFI entrypoint is
called with a 1:1 mapping covering all of DRAM already active, and with
a stack, a heap, a memory map and boot services to load and start
images. This means it is rather trivial to implement a
self-decompressing wrapper for EFI boot in a generic manner, and reuse
it across architectures that implement EFI boot.

The downside is that two signatures are needed for UEFI secure boot: one
for the decompressed image and one for the compressed images, as they
are both PE/COFF EFI executables that are launched by the firmware. But
that is actually the whole point, so it is simply the other side of the
same coin.

Related discussion going on here:
https://lore.kernel.org/all/20220430090518.3127980-22-chenhuacai@loongson.cn/

Cc: Arnd Bergmann <arnd at arndb.de>
Cc: Huacai Chen <chenhuacai at loongson.cn>

Ard Biesheuvel (4):
  efi: stub: add prototypes for load_image and start_image
  efi: stub: split off printk() routines
  efi: stub: implement generic EFI zboot
  arm64: efi: enable generic EFI compressed boot

 arch/arm64/Makefile                            |   5 +
 arch/arm64/boot/Makefile                       |  17 ++-
 drivers/firmware/efi/Kconfig                   |   6 +
 drivers/firmware/efi/libstub/Makefile          |   4 +-
 drivers/firmware/efi/libstub/efi-stub-helper.c | 141 -----------------
 drivers/firmware/efi/libstub/efistub.h         |   8 +-
 drivers/firmware/efi/libstub/printk.c          | 158 ++++++++++++++++++++
 drivers/firmware/efi/libstub/zboot-header.S    |  88 +++++++++++
 drivers/firmware/efi/libstub/zboot.c           |  86 +++++++++++
 drivers/firmware/efi/libstub/zboot.lds         |  44 ++++++
 10 files changed, 412 insertions(+), 145 deletions(-)
 create mode 100644 drivers/firmware/efi/libstub/printk.c
 create mode 100644 drivers/firmware/efi/libstub/zboot-header.S
 create mode 100644 drivers/firmware/efi/libstub/zboot.c
 create mode 100644 drivers/firmware/efi/libstub/zboot.lds

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2.30.2