Re: [PATCH v3 3/3] x86: paravirt: make native_save_fl extern inline

[Nick Desaulniers <ndesaulniers@google.com>]

On Thu, Jun 7, 2018 at 11:32 AM Nick Desaulniers
<ndesaulniers@google.com> wrote:
>
> 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: Arnd Bergmann <arnd@arndb.de>
> Suggested-by: H. Peter Anvin <hpa@zytor.com>
> Suggested-by: Tom Stellar <tstellar@redhat.com>
> Tested-by: Sedat Dilek <sedat.dilek@gmail.com>
> ---
>  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..ddeeaac8adda
> --- /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)
> + * %eax/%rdi: flags
> + */
> +ENTRY(native_restore_fl)
> +       push %_ASM_ARG1
> +       popf
> +       ret
> +ENDPROC(native_restore_fl)
> +EXPORT_SYMBOL(native_restore_fl)
> --
> 2.17.1.1185.g55be947832-goog
>

Probably should have mentioned in the notes/cover letter that this was
rebased on hpa's patch (2/3) in this series. As a change from v2.

-- 
Thanks,
~Nick Desaulniers