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    SubjectRe: [PATCH v3 12/17] arm64: reserve x18 from general allocation with SCS
    On Thu, Oct 31, 2019 at 9:47 AM <samitolvanen@google.com> wrote:
    >
    > Reserve the x18 register from general allocation when SCS is enabled,
    > because the compiler uses the register to store the current task's
    > shadow stack pointer. Note that all external kernel modules must also be
    > compiled with -ffixed-x18 if the kernel has SCS enabled.

    Inline/out-of-line assembly will also need to be careful not to
    accidentally overwrite the current task's shadow stack pointer.

    Without automated checking for that, we'll have to rely on a
    "gentlemen's promise" for code reviewers, though we'd likely hit such
    a regression with Android+mainline.

    Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>

    >
    > Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
    > ---
    > arch/arm64/Makefile | 4 ++++
    > 1 file changed, 4 insertions(+)
    >
    > diff --git a/arch/arm64/Makefile b/arch/arm64/Makefile
    > index 2c0238ce0551..ef76101201b2 100644
    > --- a/arch/arm64/Makefile
    > +++ b/arch/arm64/Makefile
    > @@ -72,6 +72,10 @@ stack_protector_prepare: prepare0
    > include/generated/asm-offsets.h))
    > endif
    >
    > +ifeq ($(CONFIG_SHADOW_CALL_STACK), y)
    > +KBUILD_CFLAGS += -ffixed-x18
    > +endif
    > +
    > ifeq ($(CONFIG_CPU_BIG_ENDIAN), y)
    > KBUILD_CPPFLAGS += -mbig-endian
    > CHECKFLAGS += -D__AARCH64EB__
    > --
    > 2.24.0.rc0.303.g954a862665-goog
    >


    --
    Thanks,
    ~Nick Desaulniers

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