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    SubjectRe: [PATCH v2] x86/boot/compressed: Disable relocation relaxation
    On Tue, Aug 11, 2020 at 5:43 PM 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>

    LGTM

    > ---
    > 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
    >


    --
    Thanks,
    ~Nick Desaulniers

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