[PATCH] efi/zboot: Limit compression options to GZIP and ZSTD

[PATCH] efi/zboot: Limit compression options to GZIP and ZSTD

[Ard Biesheuvel]

From: Ard Biesheuvel <ardb@kernel.org>

For historical reasons, the legacy decompressor code on various
architectures supports 7 different compression types for the compressed
kernel image.

EFI zboot is not a compression library museum, and so the options can be
limited to what is likely to be useful in practice:

- GZIP is tried and tested, and is still one of the fastest at
  decompression time, although the compression ratio is not very high;
  moreover, Fedora is already shipping EFI zboot kernels for arm64 that
  use GZIP, and QEMU implements direct support for it when booting a
  kernel without firmware loaded;

- ZSTD has a very high compression ratio (although not the highest), and
  is almost as fast as GZIP at decompression time.

Reducing the number of options makes it less of a hassle for other
consumers of the EFI zboot format (such as QEMU today, and kexec in the
future) to support it transparently without having to carry 7 different
decompression libraries.

Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Peter Jones <pjones@redhat.com>
Cc: Pingfan Liu <piliu@redhat.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
 drivers/firmware/efi/Kconfig                |  4 ----
 drivers/firmware/efi/libstub/Makefile.zboot | 18 ++++++------------
 2 files changed, 6 insertions(+), 16 deletions(-)

diff --git a/drivers/firmware/efi/Kconfig b/drivers/firmware/efi/Kconfig
index e312d731f4a3..5fe61b9ab5f9 100644
--- a/drivers/firmware/efi/Kconfig
+++ b/drivers/firmware/efi/Kconfig
@@ -76,10 +76,6 @@ config EFI_ZBOOT
 	bool "Enable the generic EFI decompressor"
 	depends on EFI_GENERIC_STUB && !ARM
 	select HAVE_KERNEL_GZIP
-	select HAVE_KERNEL_LZ4
-	select HAVE_KERNEL_LZMA
-	select HAVE_KERNEL_LZO
-	select HAVE_KERNEL_XZ
 	select HAVE_KERNEL_ZSTD
 	help
 	  Create the bootable image as an EFI application that carries the
diff --git a/drivers/firmware/efi/libstub/Makefile.zboot b/drivers/firmware/efi/libstub/Makefile.zboot
index 65ffd0b760b2..48842b5c106b 100644
--- a/drivers/firmware/efi/libstub/Makefile.zboot
+++ b/drivers/firmware/efi/libstub/Makefile.zboot
@@ -12,22 +12,16 @@ quiet_cmd_copy_and_pad = PAD     $@
 $(obj)/vmlinux.bin: $(obj)/$(EFI_ZBOOT_PAYLOAD) FORCE
 	$(call if_changed,copy_and_pad)
 
-comp-type-$(CONFIG_KERNEL_GZIP)		:= gzip
-comp-type-$(CONFIG_KERNEL_LZ4)		:= lz4
-comp-type-$(CONFIG_KERNEL_LZMA)		:= lzma
-comp-type-$(CONFIG_KERNEL_LZO)		:= lzo
-comp-type-$(CONFIG_KERNEL_XZ)		:= xzkern
-comp-type-$(CONFIG_KERNEL_ZSTD)		:= zstd22
-
 # in GZIP, the appended le32 carrying the uncompressed size is part of the
 # format, but in other cases, we just append it at the end for convenience,
 # causing the original tools to complain when checking image integrity.
-# So disregard it when calculating the payload size in the zimage header.
-zboot-method-y                         := $(comp-type-y)_with_size
-zboot-size-len-y                       := 4
+comp-type-y				:= gzip
+zboot-method-y				:= gzip
+zboot-size-len-y			:= 0
 
-zboot-method-$(CONFIG_KERNEL_GZIP)     := gzip
-zboot-size-len-$(CONFIG_KERNEL_GZIP)   := 0
+comp-type-$(CONFIG_KERNEL_ZSTD)		:= zstd
+zboot-method-$(CONFIG_KERNEL_ZSTD)	:= zstd22_with_size
+zboot-size-len-$(CONFIG_KERNEL_ZSTD)	:= 4
 
 $(obj)/vmlinuz: $(obj)/vmlinux.bin FORCE
 	$(call if_changed,$(zboot-method-y))
-- 
2.47.0.338.g60cca15819-goog

Re: [PATCH] efi/zboot: Limit compression options to GZIP and ZSTD

[Gerd Hoffmann]

On Fri, Dec 06, 2024 at 03:25:00PM +0100, Ard Biesheuvel wrote:
> From: Ard Biesheuvel <ardb@kernel.org>
> 
> For historical reasons, the legacy decompressor code on various
> architectures supports 7 different compression types for the compressed
> kernel image.
> 
> EFI zboot is not a compression library museum, and so the options can be
> limited to what is likely to be useful in practice:
> 
> - GZIP is tried and tested, and is still one of the fastest at
>   decompression time, although the compression ratio is not very high;
>   moreover, Fedora is already shipping EFI zboot kernels for arm64 that
>   use GZIP, and QEMU implements direct support for it when booting a
>   kernel without firmware loaded;
> 
> - ZSTD has a very high compression ratio (although not the highest), and
>   is almost as fast as GZIP at decompression time.
> 
> Reducing the number of options makes it less of a hassle for other
> consumers of the EFI zboot format (such as QEMU today, and kexec in the
> future) to support it transparently without having to carry 7 different
> decompression libraries.
> 
> Cc: Gerd Hoffmann <kraxel@redhat.com>
> Cc: Peter Jones <pjones@redhat.com>
> Cc: Pingfan Liu <piliu@redhat.com>
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>

Acked-by: Gerd Hoffmann <kraxel@redhat.com>

take care,
  Gerd