arm64/efi: efistub: jump to 'stext' directly, not through the header

After the EFI stub has done its business, it jumps into the kernel by
branching to offset #0 of the loaded Image, which is where it expects
to find the header containing a 'branch to stext' instruction.

However, the UEFI spec 2.1.1 states the following regarding PE/COFF
image loading:
"A UEFI image is loaded into memory through the LoadImage() Boot
Service. This service loads an image with a PE32+ format into memory.
This PE32+ loader is required to load all sections of the PE32+ image
into memory."

In other words, it is /not/ required to load parts of the image that are
not covered by a PE/COFF section, so it may not have loaded the header
at the expected offset, as it is not covered by any PE/COFF section.

So instead, jump to 'stext' directly, which is at the base of the
PE/COFF .text section, by supplying a symbol 'stext_offset' to
efi-entry.o which contains the relative offset of stext into the Image.
Also replace other open coded calculations of the same value with a
reference to 'stext_offset'

Acked-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Roy Franz <roy.franz@linaro.org>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>

1 files changed, 2 insertions, 1 deletions

diff --git a/arch/arm64/kernel/efi-entry.S b/arch/arm64/kernel/efi-entry.S
index 619b1dd7bcde..a0016d3a17da 100644
--- a/arch/arm64/kernel/efi-entry.S
+++ b/arch/arm64/kernel/efi-entry.S
@@ -61,7 +61,8 @@ ENTRY(efi_stub_entry)
 	 */
 	mov	x20, x0		// DTB address
 	ldr	x0, [sp, #16]	// relocated _text address
-	mov	x21, x0
+	ldr	x21, =stext_offset
+	add	x21, x0, x21
 
 	/*
 	 * Flush dcache covering current runtime addresses