Re: [PATCH v2] x86/boot/compressed: Disable relocation relaxation

[Nick Desaulniers <ndesaulniers@google.com>]

Hi Ingo,
I saw you picked up Arvind's other series into x86/boot.  Would you
mind please including this, as well?  Our CI is quite red for x86...

EOM

On Sat, Aug 15, 2020 at 8:49 AM Sedat Dilek <sedat.dilek@gmail.com> wrote:
>
> On Wed, Aug 12, 2020 at 2:43 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. 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/
> >
> > 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
> > Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
>
> Thanks for the patch.
>
> Tested-by: Sedat Dilek <sedat.dilek@gmail.com>
>
> - Sedat -
>
> [1] https://github.com/ClangBuiltLinux/linux/issues/1120#issuecomment-674409705
>
> > ---
> >  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
> > --
> > 2.26.2
> >
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-- 
Thanks,
~Nick Desaulniers