[PATCH v2 7/7] arm: efi: add PE/COFF debug table to EFI header

[Ard Biesheuvel <ard.biesheuvel-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org>]

This updates the PE/COFF header to emit the absolute path to the
decompressor vmlinux ELF file into a so-called NB10 Codeview entry.
This is hugely helpful when debugging the firmware->stub handover.

Cc: Russell King <linux-I+IVW8TIWO2tmTQ+vhA3Yw@public.gmane.org>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org>
---
 arch/arm/boot/compressed/Makefile     |  4 ++
 arch/arm/boot/compressed/efi-header.S | 47 ++++++++++++++++++++
 2 files changed, 51 insertions(+)

diff --git a/arch/arm/boot/compressed/Makefile b/arch/arm/boot/compressed/Makefile
index d50430c40045..6b978bdbac3e 100644
--- a/arch/arm/boot/compressed/Makefile
+++ b/arch/arm/boot/compressed/Makefile
@@ -196,3 +196,7 @@ AFLAGS_hyp-stub.o := -Wa,-march=armv7-a
 
 $(obj)/hyp-stub.S: $(srctree)/arch/$(SRCARCH)/kernel/hyp-stub.S
 	$(call cmd,shipped)
+
+ifeq ($(CONFIG_EFI)$(CONFIG_DEBUG_INFO),yy)
+AFLAGS_head.o += -DVMLINUX_PATH="\"$(realpath $(obj)/vmlinux)\""
+endif
diff --git a/arch/arm/boot/compressed/efi-header.S b/arch/arm/boot/compressed/efi-header.S
index a14516f7a27e..e902414e90b6 100644
--- a/arch/arm/boot/compressed/efi-header.S
+++ b/arch/arm/boot/compressed/efi-header.S
@@ -98,6 +98,11 @@ extra_header_fields:
 		.quad	0				@ CertificationTable
 		.quad	0				@ BaseRelocationTable
 
+#ifdef CONFIG_DEBUG_INFO
+		.long	efi_debug_table - start		@ DebugTable
+		.long	efi_debug_table_size
+#endif
+
 section_table:
 		.ascii	".text\0\0\0"
 		.long	__pecoff_code_size		@ VirtualSize
@@ -127,6 +132,48 @@ section_table:
 
 		.set	section_count, (. - section_table) / 40
 
+#ifdef CONFIG_DEBUG_INFO
+		/*
+		 * The debug table is referenced via its Relative Virtual
+		 * Address (RVA), which is only defined for those parts of
+		 * the image that are covered by a section declaration. Since
+		 * this header is not covered by any section, the debug table
+		 * must be emitted elsewhere. So stick it in the .rodata
+		 * section instead.
+		 *
+		 * Note that the EFI debug entry itself may legally have a
+		 * zero RVA, which means we can simply put it right after the
+		 * section headers.
+		 */
+		.section	".rodata", #alloc
+
+		.align	2
+efi_debug_table:
+		// EFI_IMAGE_DEBUG_DIRECTORY_ENTRY
+		.long	0				@ Characteristics
+		.long	0				@ TimeDateStamp
+		.short	0				@ MajorVersion
+		.short	0				@ MinorVersion
+		.long	IMAGE_DEBUG_TYPE_CODEVIEW	@ Type
+		.long	efi_debug_entry_size		@ SizeOfData
+		.long	0				@ RVA
+		.long	efi_debug_entry - start		@ FileOffset
+
+		.set	efi_debug_table_size, . - efi_debug_table
+		.previous
+
+efi_debug_entry:
+		// EFI_IMAGE_DEBUG_CODEVIEW_NB10_ENTRY
+		.ascii	"NB10"				@ Signature
+		.long	0				@ Unknown
+		.long	0				@ Unknown2
+		.long	0				@ Unknown3
+
+		.asciz	VMLINUX_PATH
+
+		.set	efi_debug_entry_size, . - efi_debug_entry
+#endif
+
 		.align	12
 __efi_start:
 		.text
-- 
2.9.3