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Date:   Sat, 15 Aug 2020 17:49:03 +0200
From:   Sedat Dilek <sedat.dilek@...il.com>
To:     Arvind Sankar <nivedita@...m.mit.edu>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Fangrui Song <maskray@...gle.com>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        e5ten.arch@...il.com,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Ard Biesheuvel <ardb@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        LKML <linux-kernel@...r.kernel.org>, stable@...r.kernel.org
Subject: Re: [PATCH v2] x86/boot/compressed: Disable relocation relaxation

On Wed, Aug 12, 2020 at 2:43 AM Arvind Sankar <nivedita@...m.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@...PCREL(%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@...PCREL(%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@...m.mit.edu>
> Reported-by: Nick Desaulniers <ndesaulniers@...gle.com>
> Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>
> Tested-by: Nick Desaulniers <ndesaulniers@...gle.com>
> Cc: stable@...r.kernel.org
> Signed-off-by: Arvind Sankar <nivedita@...m.mit.edu>

Thanks for the patch.

Tested-by: Sedat Dilek <sedat.dilek@...il.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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