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

[RFC PATCH 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 patch 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.

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              | 34 +++++++++++++++++++-
 arch/arm64/kernel/image.h             |  3 ++
 drivers/firmware/efi/libstub/Makefile | 19 ++++++++---
 4 files changed, 54 insertions(+), 6 deletions(-)

-- 
2.7.4

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

[Ard Biesheuvel]

The build commands for the ARM and arm64 EFI stubs strips the .debug
sections and other sections that my 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,
given that these sections are omitted from the kernel binary, and that
these relocations are thus only interpreted by the debugger.

So if the relocation check succeeds, invoke objcopy again, but this time,
leave the .debug sections in place. Note that these sections may refer
to ksymtab/kcrctab contents, so leave those in place as well.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
---
 drivers/firmware/efi/libstub/Makefile | 19 ++++++++++++++-----
 1 file changed, 14 insertions(+), 5 deletions(-)

diff --git a/drivers/firmware/efi/libstub/Makefile b/drivers/firmware/efi/libstub/Makefile
index c06945160a41..66584f173123 100644
--- a/drivers/firmware/efi/libstub/Makefile
+++ b/drivers/firmware/efi/libstub/Makefile
@@ -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			:= -R .debug* -R *ksymtab* -R *kcrctab*
 STUBCOPY_FLAGS-$(CONFIG_ARM64)	+= --prefix-alloc-sections=.init \
 				   --prefix-symbols=__efistub_
 STUBCOPY_RELOC-$(CONFIG_ARM64)	:= R_AARCH64_ABS
@@ -68,11 +68,20 @@ STUBCOPY_RELOC-$(CONFIG_ARM64)	:= R_AARCH64_ABS
 $(obj)/%.stub.o: $(obj)/%.o FORCE
 	$(call if_changed,stubcopy)
 
+#
+# This calls objcopy twice: the first time it includes STUBCOPY_RM, and inspects
+# the result to ensure that the actual code itself does not contain any absolute
+# references. If this succeeds, the objcopy is performed a second time, but this
+# time the .debug and other sections are retained, given that we know that the
+# absolute relocations they may contain are harmless.
+#
 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 $(OBJCOPY) $(STUBCOPY_FLAGS-y) $(STUBCOPY_RM) $< $@; \
+		     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
-- 
2.7.4

[RFC PATCH 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.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
---
 arch/arm64/kernel/Makefile |  4 +++
 arch/arm64/kernel/head.S   | 34 +++++++++++++++++++-
 arch/arm64/kernel/image.h  |  3 ++
 3 files changed, 40 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/kernel/Makefile b/arch/arm64/kernel/Makefile
index 7d66bbaafc0c..1def85599fdb 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					+= $(head-y) vmlinux.lds
+
+ifneq ($(CONFIG_DEBUG_INFO),)
+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 427f6d3f084c..2f3fbd966de7 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:
 
@@ -206,6 +211,33 @@ section_table:
 efi_header_end:
 #endif
 
+#ifdef CONFIG_DEBUG_INFO
+	__INITDATA
+
+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	efi_debug_entry_offset	// RVA
+	.long	efi_debug_entry_offset	// FileOffset
+
+ENTRY(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
+	.set	efi_debug_table_size, . - efi_debug_table
+#endif
+
 	__INIT
 
 	/*
diff --git a/arch/arm64/kernel/image.h b/arch/arm64/kernel/image.h
index c7fcb232fe47..ad5406f011c2 100644
--- a/arch/arm64/kernel/image.h
+++ b/arch/arm64/kernel/image.h
@@ -116,6 +116,9 @@ __efistub__end			= KALLSYMS_HIDE(_end);
 __efistub__edata		= KALLSYMS_HIDE(_edata);
 __efistub_screen_info		= KALLSYMS_HIDE(screen_info);
 
+#ifdef CONFIG_DEBUG_INFO
+efi_debug_entry_offset = efi_debug_entry - _text;
+#endif
 #endif
 
 #endif /* __ASM_IMAGE_H */
-- 
2.7.4

RE: [RFC PATCH 1/2] efi: libstub: preserve .debug sections after absolute relocation check

[Cohen, Eugene]

I was literally just looking at this.  I originally commented out the removal of debug (which I though was really annoying) but ran into the absolute relocations warning for symbols that it need not apply to.  Your two-pass solution is a nice one.

> -----Original Message-----
> From: Ard Biesheuvel [mailto:ard.biesheuvel@linaro.org]
> Sent: Wednesday, October 19, 2016 5:41 AM
> To: linux-efi@vger.kernel.org; linux-arm-kernel@lists.infradead.org
> Cc: mark.rutland@arm.com; leif.lindholm@linaro.org; Cohen, Eugene
> <eugene@hp.com>; matt@codeblueprint.co.uk; Ard Biesheuvel
> <ard.biesheuvel@linaro.org>
> Subject: [RFC PATCH 1/2] efi: libstub: preserve .debug sections after absolute
> relocation check
> 
> The build commands for the ARM and arm64 EFI stubs strips the .debug
> sections and other sections that my 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,
> given that these sections are omitted from the kernel binary, and that
> these relocations are thus only interpreted by the debugger.
> 
> So if the relocation check succeeds, invoke objcopy again, but this time,
> leave the .debug sections in place. Note that these sections may refer
> to ksymtab/kcrctab contents, so leave those in place as well.
> 
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>

Reviewed-by: Eugene Cohen <eugene@hp.com>

> ---
>  drivers/firmware/efi/libstub/Makefile | 19 ++++++++++++++-----
>  1 file changed, 14 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/firmware/efi/libstub/Makefile
> b/drivers/firmware/efi/libstub/Makefile
> index c06945160a41..66584f173123 100644
> --- a/drivers/firmware/efi/libstub/Makefile
> +++ b/drivers/firmware/efi/libstub/Makefile
> @@ -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			:= -R .debug* -R *ksymtab* -R *kcrctab*
>  STUBCOPY_FLAGS-$(CONFIG_ARM64)	+= --prefix-alloc-sections=.init \
>  				   --prefix-symbols=__efistub_
>  STUBCOPY_RELOC-$(CONFIG_ARM64)	:= R_AARCH64_ABS
> @@ -68,11 +68,20 @@ STUBCOPY_RELOC-$(CONFIG_ARM64)	:=
> R_AARCH64_ABS
>  $(obj)/%.stub.o: $(obj)/%.o FORCE
>  	$(call if_changed,stubcopy)
> 
> +#
> +# This calls objcopy twice: the first time it includes STUBCOPY_RM, and inspects
> +# the result to ensure that the actual code itself does not contain any absolute
> +# references. If this succeeds, the objcopy is performed a second time, but this
> +# time the .debug and other sections are retained, given that we know that the
> +# absolute relocations they may contain are harmless.
> +#
>  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 $(OBJCOPY) $(STUBCOPY_FLAGS-y) $(STUBCOPY_RM) $<
> $@; \
> +		     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
> --
> 2.7.4