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Subject[PATCH 6/8] efi/libstub/arm: add opt-in Kconfig option for the DTB loader
Date
There are various ways a platform can provide a device tree binary
to the kernel, with different levels of sophistication:
- ideally, the UEFI firmware, which is tightly coupled with the
platform, provides a device tree image directly as a UEFI
configuration table, and typically permits the contents to be
manipulated either via menu options or via UEFI environment
variables that specify a replacement image,
- GRUB for ARM has a 'devicetree' directive which allows a device
tree image to be loaded from any location accessible to GRUB, and
supersede the one provided by the firmware,
- the EFI stub implements a dtb= command line option that allows a
device tree image to be loaded from a file residing in the same
file system as the one the kernel image was loaded from.

The dtb= command line option was never intended to be more than a
development feature, to allow the other options to be implemented
in parallel. So let's make it an opt-in feature that is disabled
by default, but can be re-enabled at will.

Note that we already disable the dtb= command line option when we
detect that we are running with UEFI Secure Boot enabled.

Acked-by: Leif Lindholm <leif.lindholm@linaro.org>
Reviewed-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
---
drivers/firmware/efi/Kconfig | 12 ++++++++++++
drivers/firmware/efi/libstub/arm-stub.c | 7 ++++---
2 files changed, 16 insertions(+), 3 deletions(-)

diff --git a/drivers/firmware/efi/Kconfig b/drivers/firmware/efi/Kconfig
index 781a4a337557..fc1cb2961d5b 100644
--- a/drivers/firmware/efi/Kconfig
+++ b/drivers/firmware/efi/Kconfig
@@ -87,6 +87,18 @@ config EFI_RUNTIME_WRAPPERS
config EFI_ARMSTUB
bool

+config EFI_ARMSTUB_DTB_LOADER
+ bool "Enable the DTB loader"
+ depends on EFI_ARMSTUB
+ help
+ Select this config option to add support for the dtb= command
+ line parameter, allowing a device tree blob to be loaded into
+ memory from the EFI System Partition by the stub.
+
+ The device tree is typically provided by the platform or by
+ the bootloader, and so this option is mostly for development
+ purposes only.
+
config EFI_BOOTLOADER_CONTROL
tristate "EFI Bootloader Control"
depends on EFI_VARS
diff --git a/drivers/firmware/efi/libstub/arm-stub.c b/drivers/firmware/efi/libstub/arm-stub.c
index 01a9d78ee415..c98b1856fc3d 100644
--- a/drivers/firmware/efi/libstub/arm-stub.c
+++ b/drivers/firmware/efi/libstub/arm-stub.c
@@ -202,9 +202,10 @@ unsigned long efi_entry(void *handle, efi_system_table_t *sys_table,
* 'dtb=' unless UEFI Secure Boot is disabled. We assume that secure
* boot is enabled if we can't determine its state.
*/
- if (secure_boot != efi_secureboot_mode_disabled &&
- strstr(cmdline_ptr, "dtb=")) {
- pr_efi(sys_table, "Ignoring DTB from command line.\n");
+ if (!IS_ENABLED(CONFIG_EFI_ARMSTUB_DTB_LOADER) ||
+ secure_boot != efi_secureboot_mode_disabled) {
+ if (strstr(cmdline_ptr, "dtb="))
+ pr_efi(sys_table, "Ignoring DTB from command line.\n");
} else {
status = handle_cmdline_files(sys_table, image, cmdline_ptr,
"dtb=",
--
2.17.1
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