diff mbox series

[v3,03/30] x86/boot/compressed: Disable relocation relaxation

Message ID 20200918201436.2932360-4-samitolvanen@google.com (mailing list archive)
State New, archived
Headers show
Series Add support for Clang LTO | expand

Commit Message

Sami Tolvanen Sept. 18, 2020, 8:14 p.m. UTC 
From: Arvind Sankar <nivedita@alum.mit.edu>

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: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Tested-by: Nick Desaulniers <ndesaulniers@google.com>
Tested-by: Sedat Dilek <sedat.dilek@gmail.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Cc: stable@vger.kernel.org
---
 arch/x86/boot/compressed/Makefile | 2 ++
 1 file changed, 2 insertions(+)

Comments

Nick Desaulniers Sept. 21, 2020, 3:53 p.m. UTC | #1 
It looks like this just hit mainline over the weekend. FWIW. EOM.

On Fri, Sep 18, 2020 at 1:14 PM Sami Tolvanen <samitolvanen@google.com> wrote:
>
> From: Arvind Sankar <nivedita@alum.mit.edu>
>
> 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: Nick Desaulniers <ndesaulniers@google.com>
> Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
> Tested-by: Nick Desaulniers <ndesaulniers@google.com>
> Tested-by: Sedat Dilek <sedat.dilek@gmail.com>
> Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
> Cc: stable@vger.kernel.org
> ---
>  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
> --- a/arch/x86/boot/compressed/Makefile
> +++ b/arch/x86/boot/compressed/Makefile
> @@ -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
> --
> 2.28.0.681.g6f77f65b4e-goog
>
diff mbox series

Patch

diff --git a/arch/x86/boot/compressed/Makefile b/arch/x86/boot/compressed/Makefile
index 3962f592633d..ff7894f39e0e 100644
--- a/arch/x86/boot/compressed/Makefile
+++ b/arch/x86/boot/compressed/Makefile
@@ -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