Re: [PATCH] x86/boot/compressed: Disable relocation relaxation for non-pie link

[Nick Desaulniers <ndesaulniers@google.com>]

On Tue, Aug 11, 2020 at 10:36 AM Arvind Sankar <nivedita@alum.mit.edu> wrote:
>
> 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.
>
> [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/
>
> Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
> Reported-by: Nick Desaulniers <ndesaulniers@google.com>
> Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
> Tested-by: Nick Desaulniers <ndesaulniers@google.com>
> Cc: stable@vger.kernel.org # 4.19.x

Thanks Arvind, good write up.  Just curious about this stable tag, how
come you picked 4.19?  I can see boot failures in our CI for x86+LLD
back to 4.9.  Can we amend that tag to use `# 4.9`? I'd be happy to
help submit backports should they fail to apply cleanly.
https://travis-ci.com/github/ClangBuiltLinux/continuous-integration/builds/179237488

> ---
>  arch/x86/boot/compressed/Makefile | 6 ++++++
>  1 file changed, 6 insertions(+)
>
> diff --git a/arch/x86/boot/compressed/Makefile b/arch/x86/boot/compressed/Makefile
> index 3962f592633d..c5449bea58ec 100644
> --- a/arch/x86/boot/compressed/Makefile
> +++ b/arch/x86/boot/compressed/Makefile
> @@ -62,6 +62,12 @@ KBUILD_LDFLAGS += $(shell $(LD) --help 2>&1 | grep -q "\-z noreloc-overflow" \
>  endif
>  LDFLAGS_vmlinux := -T
>
> +# Disable relocation relaxation if not linking as PIE
> +ifeq ($(filter -pie,$(KBUILD_LDFLAGS)),)
> +KBUILD_CFLAGS += $(call as-option, -Wa$(comma)-mrelax-relocations=no)
> +KBUILD_AFLAGS += $(call as-option, -Wa$(comma)-mrelax-relocations=no)
> +endif
> +
>  hostprogs      := mkpiggy
>  HOST_EXTRACFLAGS += -I$(srctree)/tools/include
>
> --
> 2.26.2
>


-- 
Thanks,
~Nick Desaulniers