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Message-ID: <CAKwvOdk7pR5dK0ynxPOWHVYVWLMF1CUn6c=_GvpF-80YHNhQEQ@mail.gmail.com>
Date: Mon, 21 Sep 2020 08:53:39 -0700
From: Nick Desaulniers <ndesaulniers@...gle.com>
To: Sami Tolvanen <samitolvanen@...gle.com>
Cc: Masahiro Yamada <masahiroy@...nel.org>, Will Deacon <will@...nel.org>,
Peter Zijlstra <peterz@...radead.org>, Steven Rostedt <rostedt@...dmis.org>,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>, "Paul E. McKenney" <paulmck@...nel.org>,
Kees Cook <keescook@...omium.org>,
clang-built-linux <clang-built-linux@...glegroups.com>,
Kernel Hardening <kernel-hardening@...ts.openwall.com>,
linux-arch <linux-arch@...r.kernel.org>,
Linux ARM <linux-arm-kernel@...ts.infradead.org>,
Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>,
linux-pci@...r.kernel.org,
"maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>, Arvind Sankar <nivedita@...m.mit.edu>
Subject: Re: [PATCH v3 03/30] x86/boot/compressed: Disable relocation relaxation
It looks like this just hit mainline over the weekend. FWIW. EOM.
On Fri, Sep 18, 2020 at 1:14 PM Sami Tolvanen <samitolvanen@...gle.com> wrote:
>
> From: Arvind Sankar <nivedita@...m.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@...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/
>
> Reported-by: Nick Desaulniers <ndesaulniers@...gle.com>
> Signed-off-by: Arvind Sankar <nivedita@...m.mit.edu>
> Tested-by: Nick Desaulniers <ndesaulniers@...gle.com>
> Tested-by: Sedat Dilek <sedat.dilek@...il.com>
> Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>
> Cc: stable@...r.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
>
--
Thanks,
~Nick Desaulniers
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