Commit 09e43968 authored by Arvind Sankar's avatar Arvind Sankar Committed by Ingo Molnar

x86/boot/compressed: Disable relocation relaxation

The x86-64 psABI [0] 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
  to
	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
  relocations.

Nick Desaulniers reports [1,2]:

  "A recent change [3] 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 [4] 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
unconditionally.

[0] https://gitlab.com/x86-psABIs/x86-64-ABI/-/blob/master/x86-64-ABI/linker-optimization.tex#L65
[1] https://lore.kernel.org/lkml/20200807194100.3570838-1-ndesaulniers@google.com/
[2] https://github.com/ClangBuiltLinux/linux/issues/1121
[3] https://reviews.llvm.org/rGc41a18cf61790fc898dcda1055c3efbf442c14c0
[4] https://lore.kernel.org/lkml/20200731202738.2577854-1-nivedita@alum.mit.edu/Reported-by: default avatarNick Desaulniers <ndesaulniers@google.com>
Signed-off-by: default avatarArvind Sankar <nivedita@alum.mit.edu>
Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
Tested-by: default avatarNick Desaulniers <ndesaulniers@google.com>
Tested-by: default avatarSedat Dilek <sedat.dilek@gmail.com>
Acked-by: default avatarArd Biesheuvel <ardb@kernel.org>
Reviewed-by: default avatarNick Desaulniers <ndesaulniers@google.com>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20200812004308.1448603-1-nivedita@alum.mit.edu
parent 76366050
...@@ -43,6 +43,8 @@ KBUILD_CFLAGS += -Wno-pointer-sign ...@@ -43,6 +43,8 @@ KBUILD_CFLAGS += -Wno-pointer-sign
KBUILD_CFLAGS += $(call cc-option,-fmacro-prefix-map=$(srctree)/=) KBUILD_CFLAGS += $(call cc-option,-fmacro-prefix-map=$(srctree)/=)
KBUILD_CFLAGS += -fno-asynchronous-unwind-tables KBUILD_CFLAGS += -fno-asynchronous-unwind-tables
KBUILD_CFLAGS += -D__DISABLE_EXPORTS 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__ KBUILD_AFLAGS := $(KBUILD_CFLAGS) -D__ASSEMBLY__
GCOV_PROFILE := n GCOV_PROFILE := n
......
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