[PATCH v3 0/2] efi/arm64: add vmlinux link to PE/COFF debug table

[PATCH v3 0/2] efi/arm64: add vmlinux link to PE/COFF debug table

[Ard Biesheuvel]

Much like GNU debug links, PE/COFF binaries can carry the path on the build
host to the binary containing the debugging symbols. Since the kernel binary
is loaded by UEFI at an a priori unknown offset in the 1:1 mapping of physical
memory, having this information is useful for the debugger automation to find
the file and the offset, and load the symbols automatically.

So if we have debugging symbols to begin with (CONFIG_DEBUG_INFO=y), add the
absolute path to vmlinux to the PE/COFF debug table.

v3: move the EFI debug table itself into the EFI header, which removes the
    need for externally visible symbols or changes to the linker script (#2)
    update commit log to explain that the debug support table is defined
    in the UEFI spec, and is thus not EDK2/PI specific

v2: rebase onto v4.10-rc
    use strip rather than objcopy for first pass (#1)
    move debug table inside #ifdef CONFIG_EFI region in head.S (#2)

Note that these patches can be merged independently, although they are
both required to get the functionality.

Ard Biesheuvel (2):
  efi: libstub: Preserve .debug sections after absolute relocation check
  efi: arm64: Add vmlinux debug link to the Image binary

 arch/arm64/kernel/Makefile            |  4 ++
 arch/arm64/kernel/head.S              | 46 +++++++++++++++++++-
 drivers/firmware/efi/libstub/Makefile | 24 ++++++----
 3 files changed, 65 insertions(+), 9 deletions(-)

-- 
2.7.4

[PATCH v3 1/2] efi: libstub: Preserve .debug sections after absolute relocation check

[Ard Biesheuvel]

The build commands for the ARM and arm64 EFI stubs strip the .debug
sections and other sections that may legally contain absolute relocations,
in order to inspect the remaining sections for the presence of such
relocations.

This leaves us without debugging symbols in the stub for no good reason,
considering that these sections are omitted from the kernel binary anyway,
and that these relocations are thus only consumed by users of the ELF
binary, such as debuggers.

So move to 'strip' for performing the relocation check, and if it succeeds,
invoke objcopy as before, but leaving the .debug sections in place. Note
that these sections may refer to ksymtab/kcrctab contents, so leave those
in place as well.

Cc: Matt Fleming <matt@codeblueprint.co.uk>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
---
 drivers/firmware/efi/libstub/Makefile | 24 +++++++++++++-------
 1 file changed, 16 insertions(+), 8 deletions(-)

diff --git a/drivers/firmware/efi/libstub/Makefile b/drivers/firmware/efi/libstub/Makefile
index d564d25df8ab..33e0e2f1a730 100644
--- a/drivers/firmware/efi/libstub/Makefile
+++ b/drivers/firmware/efi/libstub/Makefile
@@ -11,7 +11,7 @@ cflags-$(CONFIG_X86)		+= -m$(BITS) -D__KERNEL__ -O2 \
 				   -mno-mmx -mno-sse
 
 cflags-$(CONFIG_ARM64)		:= $(subst -pg,,$(KBUILD_CFLAGS))
-cflags-$(CONFIG_ARM)		:= $(subst -pg,,$(KBUILD_CFLAGS)) -g0 \
+cflags-$(CONFIG_ARM)		:= $(subst -pg,,$(KBUILD_CFLAGS)) \
 				   -fno-builtin -fpic -mno-single-pic-base
 
 cflags-$(CONFIG_EFI_ARMSTUB)	+= -I$(srctree)/scripts/dtc/libfdt
@@ -60,7 +60,7 @@ CFLAGS_arm64-stub.o 		:= -DTEXT_OFFSET=$(TEXT_OFFSET)
 extra-$(CONFIG_EFI_ARMSTUB)	:= $(lib-y)
 lib-$(CONFIG_EFI_ARMSTUB)	:= $(patsubst %.o,%.stub.o,$(lib-y))
 
-STUBCOPY_FLAGS-y		:= -R .debug* -R *ksymtab* -R *kcrctab*
+STUBCOPY_RM-y			:= -R *ksymtab* -R *kcrctab*
 STUBCOPY_FLAGS-$(CONFIG_ARM64)	+= --prefix-alloc-sections=.init \
 				   --prefix-symbols=__efistub_
 STUBCOPY_RELOC-$(CONFIG_ARM64)	:= R_AARCH64_ABS
@@ -68,17 +68,25 @@ STUBCOPY_RELOC-$(CONFIG_ARM64)	:= R_AARCH64_ABS
 $(obj)/%.stub.o: $(obj)/%.o FORCE
 	$(call if_changed,stubcopy)
 
+#
+# Strip debug sections and some other sections that may legally contain
+# absolute relocations, so that we can inspect the remaining sections for
+# such relocations. If none are found, regenerate the output object, but
+# this time, use objcopy and leave all sections in place.
+#
 quiet_cmd_stubcopy = STUBCPY $@
-      cmd_stubcopy = if $(OBJCOPY) $(STUBCOPY_FLAGS-y) $< $@; then	\
-		     $(OBJDUMP) -r $@ | grep $(STUBCOPY_RELOC-y)	\
-		     && (echo >&2 "$@: absolute symbol references not allowed in the EFI stub"; \
-			 rm -f $@; /bin/false); else /bin/false; fi
+      cmd_stubcopy = if $(STRIP) --strip-debug $(STUBCOPY_RM-y) -o $@ $<; \
+		     then if $(OBJDUMP) -r $@ | grep $(STUBCOPY_RELOC-y); \
+		     then (echo >&2 "$@: absolute symbol references not allowed in the EFI stub"; \
+			   rm -f $@; /bin/false); 			  \
+		     else $(OBJCOPY) $(STUBCOPY_FLAGS-y) $< $@; fi	  \
+		     else /bin/false; fi
 
 #
 # ARM discards the .data section because it disallows r/w data in the
 # decompressor. So move our .data to .data.efistub, which is preserved
 # explicitly by the decompressor linker script.
 #
-STUBCOPY_FLAGS-$(CONFIG_ARM)	+= --rename-section .data=.data.efistub \
-				   -R ___ksymtab+sort -R ___kcrctab+sort
+STUBCOPY_FLAGS-$(CONFIG_ARM)	+= --rename-section .data=.data.efistub
+STUBCOPY_RM-$(CONFIG_ARM)	+= -R ___ksymtab+sort -R ___kcrctab+sort
 STUBCOPY_RELOC-$(CONFIG_ARM)	:= R_ARM_ABS
-- 
2.7.4

[PATCH v3 2/2] efi: arm64: Add vmlinux debug link to the Image binary

[Ard Biesheuvel]

When building with debugging symbols, take the absolute path to the
vmlinux binary and add it to the special PE/COFF debug table entry.
This allows a debug EFI build to find the vmlinux binary, which is
very helpful in debugging, given that the offset where the Image is
first loaded by EFI is highly unpredictable.

On implementations of UEFI that choose to implement it, this
information is exposed via the EFI debug support table, which is a UEFI
configuration table that is accessible both by the firmware at boot time
and by the OS at runtime, and lists all PE/COFF images loaded by the
system.

The format of the NB10 Codeview entry is based on the definition used
by EDK2, which is not entirely unreasonable given that our implementation
of the PE/COFF header targets UEFI specifically, and is not for general
consumption of PE/COFF compatible systems.

Cc: Matt Fleming <matt@codeblueprint.co.uk>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
---
 arch/arm64/kernel/Makefile |  4 ++
 arch/arm64/kernel/head.S   | 46 +++++++++++++++++++-
 2 files changed, 49 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/kernel/Makefile b/arch/arm64/kernel/Makefile
index 7d66bbaafc0c..6dbc0e5527f5 100644
--- a/arch/arm64/kernel/Makefile
+++ b/arch/arm64/kernel/Makefile
@@ -55,3 +55,7 @@ obj-y					+= $(arm64-obj-y) vdso/ probes/
 obj-m					+= $(arm64-obj-m)
 head-y					:= head.o
 extra-y					+= $(head-y) vmlinux.lds
+
+ifeq ($(CONFIG_EFI)$(CONFIG_DEBUG_INFO),yy)
+AFLAGS_head.o += -DVMLINUX_PATH="\"$(shell readlink -f $(objtree)/vmlinux)\""
+endif
diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
index 4b1abac3485a..3942d2af3f67 100644
--- a/arch/arm64/kernel/head.S
+++ b/arch/arm64/kernel/head.S
@@ -149,7 +149,7 @@ extra_header_fields:
 	.quad	0				// SizeOfHeapReserve
 	.quad	0				// SizeOfHeapCommit
 	.long	0				// LoaderFlags
-	.long	0x6				// NumberOfRvaAndSizes
+	.long	(section_table - .) / 8		// NumberOfRvaAndSizes
 
 	.quad	0				// ExportTable
 	.quad	0				// ImportTable
@@ -158,6 +158,11 @@ extra_header_fields:
 	.quad	0				// CertificationTable
 	.quad	0				// BaseRelocationTable
 
+#ifdef CONFIG_DEBUG_INFO
+	.long	efi_debug_table - _head		// DebugTable
+	.long	efi_debug_table_size
+#endif
+
 	// Section table
 section_table:
 
@@ -195,6 +200,45 @@ section_table:
 	.short	0		// NumberOfLineNumbers  (0 for executables)
 	.long	0xe0500020	// Characteristics (section flags)
 
+#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 .init.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.
+	 */
+	__INITRODATA
+	.align	2
+efi_debug_table:
+	// EFI_IMAGE_DEBUG_DIRECTORY_ENTRY
+	.long	0			// Characteristics
+	.long	0			// TimeDateStamp
+	.short	0			// MajorVersion
+	.short	0			// MinorVersion
+	.long	2			// Type == EFI_IMAGE_DEBUG_TYPE_CODEVIEW
+	.long	efi_debug_entry_size	// SizeOfData
+	.long	0			// RVA
+	.long	efi_debug_entry - _head	// FileOffset
+
+	.set	efi_debug_table_size, . - efi_debug_table
+	.previous
+
+efi_debug_entry:
+	// EFI_IMAGE_DEBUG_CODEVIEW_NB10_ENTRY
+	.long	0x3031424E		// Signature
+	.long	0			// Unknown
+	.long	0			// Unknown2
+	.long	0			// Unknown3
+
+	.asciz	VMLINUX_PATH
+
+	.set	efi_debug_entry_size, . - efi_debug_entry
+#endif
+
 	/*
 	 * EFI will load .text onwards at the 4k section alignment
 	 * described in the PE/COFF header. To ensure that instruction
-- 
2.7.4