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    Subject[PATCH v2 2/2] x86: paravirt: make native_save_fl extern inline
    Date
    native_save_fl() is marked static inline, but by using it as
    a function pointer in arch/x86/kernel/paravirt.c, it MUST be outlined.

    paravirt's use of native_save_fl() also requires that no GPRs other than
    %rax are clobbered.

    Compilers have different heuristics which they use to emit stack guard
    code, the emittance of which can break paravirt's callee saved assumption
    by clobbering %rcx.

    Marking a function definition extern inline means that if this version
    cannot be inlined, then the out-of-line version will be preferred. By
    having the out-of-line version be implemented in assembly, it cannot be
    instrumented with a stack protector, which might violate custom calling
    conventions that code like paravirt rely on.

    The semantics of extern inline has changed since gnu89. This means that
    folks using GCC versions >= 5.1 may see symbol redefinition errors at
    link time for subdirs that override KBUILD_CFLAGS (making the C standard
    used implicit) regardless of this patch. This has been cleaned up
    earlier in the patch set, but is left as a note in the commit message
    for future travelers.

    Reports:
    https://lkml.org/lkml/2018/5/7/534
    https://github.com/ClangBuiltLinux/linux/issues/16

    Discussion:
    https://bugs.llvm.org/show_bug.cgi?id=37512
    https://lkml.org/lkml/2018/5/24/1371

    Thanks to the many folks that participated in the discussion.

    Debugged-by: Alistair Strachan <astrachan@google.com>
    Debugged-by: Matthias Kaehlcke <mka@chromium.org>
    Reported-by: Sedat Dilek <sedat.dilek@gmail.com>
    Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
    Suggested-by: H. Peter Anvin <hpa@zytor.com>
    Suggested-by: Tom Stellar <tstellar@redhat.com>
    Tested-by: Sedat Dilek <sedat.dilek@gmail.com>
    ---
    No changes in v2, but here's the discussion of v1:
    https://patchwork.kernel.org/patch/10444075/

    arch/x86/include/asm/irqflags.h | 2 +-
    arch/x86/kernel/Makefile | 1 +
    arch/x86/kernel/irqflags.S | 26 ++++++++++++++++++++++++++
    3 files changed, 28 insertions(+), 1 deletion(-)
    create mode 100644 arch/x86/kernel/irqflags.S

    diff --git a/arch/x86/include/asm/irqflags.h b/arch/x86/include/asm/irqflags.h
    index 89f08955fff7..c4fc17220df9 100644
    --- a/arch/x86/include/asm/irqflags.h
    +++ b/arch/x86/include/asm/irqflags.h
    @@ -13,7 +13,7 @@
    * Interrupt control:
    */

    -static inline unsigned long native_save_fl(void)
    +extern inline unsigned long native_save_fl(void)
    {
    unsigned long flags;

    diff --git a/arch/x86/kernel/Makefile b/arch/x86/kernel/Makefile
    index 02d6f5cf4e70..8824d01c0c35 100644
    --- a/arch/x86/kernel/Makefile
    +++ b/arch/x86/kernel/Makefile
    @@ -61,6 +61,7 @@ obj-y += alternative.o i8253.o hw_breakpoint.o
    obj-y += tsc.o tsc_msr.o io_delay.o rtc.o
    obj-y += pci-iommu_table.o
    obj-y += resource.o
    +obj-y += irqflags.o

    obj-y += process.o
    obj-y += fpu/
    diff --git a/arch/x86/kernel/irqflags.S b/arch/x86/kernel/irqflags.S
    new file mode 100644
    index 000000000000..937bd08c3f79
    --- /dev/null
    +++ b/arch/x86/kernel/irqflags.S
    @@ -0,0 +1,26 @@
    +/* SPDX-License-Identifier: GPL-2.0 */
    +
    +#include <asm/asm.h>
    +#include <asm/export.h>
    +#include <linux/linkage.h>
    +
    +/*
    + * unsigned long native_save_fl(void)
    + */
    +ENTRY(native_save_fl)
    + pushf
    + pop %_ASM_AX
    + ret
    +ENDPROC(native_save_fl)
    +EXPORT_SYMBOL(native_save_fl)
    +
    +/*
    + * void native_restore_fl(unsigned long flags)
    + * %rdi: flags
    + */
    +ENTRY(native_restore_fl)
    + push %_ASM_DI
    + popf
    + ret
    +ENDPROC(native_restore_fl)
    +EXPORT_SYMBOL(native_restore_fl)
    --
    2.17.1.1185.g55be947832-goog
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