[PATCH v3 03/30] x86/boot/compressed: Disable relocation relaxation
From: Sami Tolvanen
Date: Fri Sep 18 2020 - 16:14:52 EST
From: Arvind Sankar <nivedita@xxxxxxxxxxxx>
The x86-64 psABI  specifies special relocation types
(R_X86_64_[REX_]GOTPCRELX) for indirection through the Global Offset
Table, semantically equivalent to R_X86_64_GOTPCREL, which the linker
can take advantage of for optimization (relaxation) at link time. This
is supported by LLD and binutils versions 2.26 onwards.
The compressed kernel is position-independent code, however, when using
LLD or binutils versions before 2.27, it must be linked without the -pie
option. In this case, the linker may optimize certain instructions into
a non-position-independent form, by converting foo@GOTPCREL(%rip) to $foo.
This potential issue has been present with LLD and binutils-2.26 for a
long time, but it has never manifested itself before now:
- LLD and binutils-2.26 only relax
movq foo@GOTPCREL(%rip), %reg
leaq foo(%rip), %reg
which is still position-independent, rather than
mov $foo, %reg
which is permitted by the psABI when -pie is not enabled.
- gcc happens to only generate GOTPCREL relocations on mov instructions.
- clang does generate GOTPCREL relocations on non-mov instructions, but
when building the compressed kernel, it uses its integrated assembler
(due to the redefinition of KBUILD_CFLAGS dropping -no-integrated-as),
which has so far defaulted to not generating the GOTPCRELX
Nick Desaulniers reports [1,2]:
A recent change  to a default value of configuration variable
(ENABLE_X86_RELAX_RELOCATIONS OFF -> ON) in LLVM now causes Clang's
integrated assembler to emit R_X86_64_GOTPCRELX/R_X86_64_REX_GOTPCRELX
relocations. LLD will relax instructions with these relocations based
on whether the image is being linked as position independent or not.
When not, then LLD will relax these instructions to use absolute
addressing mode (R_RELAX_GOT_PC_NOPIC). This causes kernels built with
Clang and linked with LLD to fail to boot.
Patch series  is a solution to allow the compressed kernel to be
linked with -pie unconditionally, but even if merged is unlikely to be
backported. As a simple solution that can be applied to stable as well,
prevent the assembler from generating the relaxed relocation types using
the -mrelax-relocations=no option. For ease of backporting, do this
Reported-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
Signed-off-by: Arvind Sankar <nivedita@xxxxxxxxxxxx>
Tested-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
Tested-by: Sedat Dilek <sedat.dilek@xxxxxxxxx>
Reviewed-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
arch/x86/boot/compressed/Makefile | 2 ++
1 file changed, 2 insertions(+)
diff --git a/arch/x86/boot/compressed/Makefile b/arch/x86/boot/compressed/Makefile
index 3962f592633d..ff7894f39e0e 100644
@@ -43,6 +43,8 @@ KBUILD_CFLAGS += -Wno-pointer-sign
KBUILD_CFLAGS += $(call cc-option,-fmacro-prefix-map=$(srctree)/=)
KBUILD_CFLAGS += -fno-asynchronous-unwind-tables
KBUILD_CFLAGS += -D__DISABLE_EXPORTS
+# Disable relocation relaxation in case the link is not PIE.
+KBUILD_CFLAGS += $(call as-option,-Wa$(comma)-mrelax-relocations=no)
KBUILD_AFLAGS := $(KBUILD_CFLAGS) -D__ASSEMBLY__
GCOV_PROFILE := n