arm64/efi: isolate EFI stub from the kernel proper

Since arm64 does not use a builtin decompressor, the EFI stub is built
into the kernel proper. So far, this has been working fine, but actually,
since the stub is in fact a PE/COFF relocatable binary that is executed
at an unknown offset in the 1:1 mapping provided by the UEFI firmware, we
should not be seamlessly sharing code with the kernel proper, which is a
position dependent executable linked at a high virtual offset.

So instead, separate the contents of libstub and its dependencies, by
putting them into their own namespace by prefixing all of its symbols
with __efistub. This way, we have tight control over what parts of the
kernel proper are referenced by the stub.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Matt Fleming <matt.fleming@intel.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/kernel/efi-entry.S

@@ -29,7 +29,7 @@
 	 * we want to be. The kernel image wants to be placed at TEXT_OFFSET
 	 * from start of RAM.
 	 */
-ENTRY(efi_stub_entry)
+ENTRY(entry)
 	/*
 	 * Create a stack frame to save FP/LR with extra space
 	 * for image_addr variable passed to efi_entry().
@@ -86,8 +86,8 @@ ENTRY(efi_stub_entry)
 	 * entries for the VA range of the current image, so no maintenance is
 	 * necessary.
 	 */
-	adr	x0, efi_stub_entry
-	adr	x1, efi_stub_entry_end
+	adr	x0, entry
+	adr	x1, entry_end
 	sub	x1, x1, x0
 	bl	__flush_dcache_area
 
@@ -120,5 +120,5 @@ efi_load_fail:
 	ldp	x29, x30, [sp], #32
 	ret
 
-efi_stub_entry_end:
-ENDPROC(efi_stub_entry)
+entry_end:
+ENDPROC(entry)